Lagoons, estuaries and deltas

I TA L I A N H A B I TAT S

23

I TA L I A N H A B I TAT S

M I N I S T E R O D E L L’ A M B I E N T E E D E L L A T U T E L A D E L T E R R I T O R I O E D E L M A R E

M U S E O F R I U L A N O D I S T O R I A N AT U R A L E · C O M U N E D I U D I N E

Lagoons, estuaries and deltasBoundaries between the sea and rivers

Italian habitatsItalian Ministry of the Environment and Territorial Protection / Ministero dell’Ambiente e della Tutela delTerritorio e del MareFriuli Museum of Natural History / Museo Friulano di Storia Naturale · Comune di Udine

Scientific coordinatorsAlessandro Minelli · Sandro Ruffo · Fabio Stoch

Editorial committeeAldo Cosentino · Alessandro La Posta · Carlo Morandini · Giuseppe Muscio

“Lagoons, estuaries and deltas · Boundaries between the sea and rivers”edited by Alessandro Minelli

TextsAnnelore Bezzi · Mauro Bon · Francesco Bracco · Daniele Curiel · Francesca Delli Quadri ·Giorgio Fontolan · Gilberto Gandolfi · Alessandro Minelli · Andrea Rismondo · Francesco Scarton ·Margherita Solari · Davide Tagliapietra · Marco Ulliana · Mariacristina Villani

In collaboration withMaria Manuela Giovannelli · Antonella Miola · Marco Sigovini

English translationAlison Garside · Gabriel Walton

IllustrationsRoberto Zanella

Graphic designFurio Colman

PhotographsNevio Agostini 7, 15, 21, 82, 87, 88, 143, 145 · Archive Museo Friulano di Storia Naturale 81, 84, 85,90, 107/2, 148 · Archive Naturmedia 86, 137 · Mauro Bon 66, 132 · Massimo Buccheri 80 ·Marco Cantonati 32 · Compagnia Generale Ripreseaeree 12, 13 · Ettore Contarini 101/2, 105 ·Giuseppe Corriero 58 · Daniele Curiel 24, 25, 26, 27, 28, 29, 30, 31 · Ulderica Da Pozzo 9, 11, 16, 19,23, 135, 144 · Vitantonio Dell’Orto 6, 20 ,41, 74, 75, 77, 112, 115, 116, 118, 119, 120, 121, 123, 124,126, 127, 128, 129, 130, 131, 133, 134 · Dario Ersetti 22, 39, 79, 147 · Giorgio Fontolan 10, 18 ·Francesco Scarton 117, 122, 142 · Paolo Glerean 138 · Giuseppe Muscio 76 · Nicola Novarini 113, 114 ·Arnaldo Piccinini 65, 68, 69, 70, 67 · Andrea Rismondo 33, 34, 35, 36, 37 · Lorenzo Seitz 40, 46, 47, 48, 56 ·Marco Sigovini 50, 51 · Fabio Stoch 14, 59, 60, 61, 72, 140 · Egidio Trainito 42, 44, 45, 52, 53, 54, 62, 63,64, 71, 73 · Marco Uliana 83, 91, 92, 93, 94, 95, 96,97, 98, 99, 100, 101/1, 102, 103, 104, 106, 107/1,108, 109, 110, 111, 139 · Roberto Valle 125 · Augusto Vigna Taglianti 57 · Francesco Zaramella 49, 141

© 2009 Museo Friulano di Storia Naturale, Udine, ItalyAll rights reserved.No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form orby any means, without the prior permission in writing of the publishers.

ISBN 88 88192 46 8ISSN 1724-6539

Cover photo: Lagoon of Grado and Marano (Friuli, photo: U. Da Pozzo)

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Annelore Bezzi · Francesca Delli Quadri · Giorgio Fontolan

Submerged vegetation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Daniele Curiel · Andrea Rismondo

Aquatic invertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Davide Tagliapietra · Alessandro Minelli

Fishes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63Gilberto Gandolfi

Terrestrial vegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75Francesco Bracco · Mariacristina Villani

Terrestrial invertebrates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93Marco Uliana · Alessandro Minelli

Terrestrial vertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113Mauro Bon · Francesco Scarton

Conservation and management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135Annelore Bezzi · Mauro Bon · Francesco Bracco · Francesca Delli Quadri ·Gilberto Gandolfi · Mariacristina Villani

Teaching suggestions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145Margherita Solari

Select bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153

List of species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

ContentsItalian habitats

6TheMediterraneanmaquis

24Italianhabitats

12Aridmeadows

18Mountainconifer forests

2Springs andspringwatercourses

3Woodlandsof the PoPlain

4Sand dunesand beaches

5Mountainstreams

1Caves andkarsticphenomena

8Brackishcoastal lakes

9Mountainpeat-bogs

10Realms ofsnow and ice

11Pools, pondsandmarshland

7Sea cliffs androckycoastlines

14High-altitudelakes

15Beech forestsof theApennines

16The pelagicdomain

17Volcaniclakes

13Rocky slopesand screes

20Subterraneanwaters

21Rivers andriverinewoodlands

22Marine bio-constructions

23Lagoons,estuariesand deltas

19Seagrassmeadows

■ Foreword

Italy is a country with more than 7500km of various types of coastline. Fromthe geomorphological viewpoint, oneof the most interesting aspects of thesecoasts is the relationship between thesea and the rivers that discharge into it,modelling as they do the coastline bydistributing freshwater and sediments.The sea acts with tides, waves andlong-term fluctuations in level (eustatism). This volume of “Italian Habitats”illustrates three environments which result from interactions between riversand the sea: deltas, lagoons and estuaries. The vegetation and fauna ofsubmerged environments are the subject of the first chapters, followed bydescriptions of above-water environments.

■ Deltas

When a river flows into the sea, the speed of its current is drastically reduced,causing the sediment it carries to be deposited. If the energy of the waves is notsufficient to remove the deposited material completely, an accumulation forms,partly emerging and partly submerged, which is called a delta. This term derivesfrom the triangular shape of many sedimentary bodies of this type, being similarto the capital letter of the same name in the Greek alphabet. In reality, themorphology and dimensions of a delta may vary widely, and in most cases differsignificantly from the ideal model of the Nile Delta, which was the originalinspiration for the name. Each delta is subdivided into a delta plain, delta frontand pro-delta. The plain is a flat area, situated just above sea level, crossed bybranching river channels (distributaries) edged by natural banks. Extremelyheterogeneous morphological elements meet here that result in the developmentof diversified habitats. Fluvial processes are prevalent in the upper part; during

7IntroductionANNELORE BEZZI · FRANCESCA DELLI QUADRI · GIORGIO FONTOLAN

Po Delta (between Emilia Romagna and Veneto)

Mouth of the river Bevano (Emilia Romagna)

spates, large amounts of fine sedimentescape from the channel beds andaccumulate on the delta plain; thebanks of the channels rise, but are alsofrequently breached, with the formationof breach fans and the consequentdeposition of sand outside the channel.Processes due to marine dynamics actin the lower part of the delta plain andthe morphologies of coastal types meet,with frequent mixing of fresh and sea

waters: lagoons, bays, sandbanks, tidal flats, abandoned strips of beach, andsand dunes. Sediments may be very heterogeneous in this zone, depending onthe environment and how they were deposited.The delta front is the shallower part of the submerged delta, mainly composedof sandy sediment. This is the most active part of the deltaic structure, and themost interesting from the morphodynamic point of view. It is here that riverwater loses speed as it is discharged, and is consequently less capable ofmaintaining transported sediments in suspension. These then accumulate instructures called mouth bars (loss of competence).The mouth bars assume different shapes according to the hydrodynamicbehaviour of the river at the point where it enters the receiving basin, i.e, the sea.The way in which river water expands into sea water, i.e., through turbulent mixingor as a floating body of water, depends not only on the speed of the current andslope of the seabed, but mainly on the density distribution of the water column.This distribution is generally influenced by the seasons, with alternating high andlow regimes. The density distribution of the water column may be:● homopycnal: when the waters are well mixed, with similar densities both atthe surface and on the bottom;● hypopycnal: when river water is less dense than that of the sea, because ofa low sediment load. It tends to form a surface layer until some distance fromthe mouth, gradually losing its load of suspended sediments, which settle. Thiscondition is common when there is little water in the river;● hyperpycnal: typical of full spate regimes, when river water is denser thanthat of the sea and tends to move in contact with the bottom.The terminal part of the deltaic structure that develops towards the sea is calledthe pro-delta. This is the deepest part, originating from finer materialstransported out to sea and deposited in layers on the seabed. In cross-section,this sediment accumulation is typically wedge-shaped and represents a

significant historical and geological archive. The sediments that form it contain arecord of the environmental and anthropic changes that have taken place in theriver basin from which the sediments originated.According to a classical scheme, deltas are classified on the basis of shape,which is the result of the combined action of the sediment load carried by theriver, the energy of wave motion, and the influence of tides. The Mississippi, in theGulf of Mexico, is a typical example of a river-dominated delta, in which the river’scontribution dominates the physical modelling operated by the sea. This type ofdelta develops in conditions of shallow water, where currents, waves and tidesare very weak, and therefore in a sheltered gulf or sea into which the river flowscarrying large quantities of water and sediments. In these conditions, the deltatends to advance rapidly towards the sea (progradation). The result is a delta plainwith a very uneven coastline, with tongues of emerging land with the typicalelongated (digitate) shape jutting out into the sea, like a bird’s foot. These areproduced by the rapid seaward advance of the distributaries, confined by naturalbanks, which erode the sandbars previously deposited at the mouth. In Italy, themost obvious examples of this type of delta are the mouths of the Po and Isonzo.Instead, the São Francisco, in Brazil, is the prototype of the wave-dominateddelta, with its classic cuspidate shape. The morphology is characterised by asmall number of distributary channels, from the mouth of which a very regularcoastline departs, forming two lateral wings. The action of waves and currents

8 9

Mouth of the river Isonzo (Friuli Venezia Giulia)

WAVE-DOMINATED

DELTAS

RIVER

TIDESWAVES

Elongated form

Estuary

Cus

pida

tefo

rm

Elon

gate

dfo

rm

Loba

tefo

rm

RIVER-DOMINATED

DELTAS

TIDE-DOMINATED

DELTAS

Classification diagram of different delta types

Roman times. Their growth was subsequently very rapid (around 7 km in 2500years), due to sediments arriving from the drainage basins which were rapidlybeing deforested by man. On the delta of the Ombrone in particular, there arebetween 12 and 16 main sand belts, some of which are still separated bypools, known as “chiari”.The history of the evolution of the Po is extremely complex, as the river hasbeen considerably influenced by human interventions aimed at regulation of itsdistributor channels. According to descriptions by Pliny and Polybius, therewere three main distributaries during the 2nd-5th centuries A.D., forming thesame number of cuspidate deltas. In more recent times, with a hydrographicalnetwork completely modified by man, the so-called modern delta has beenformed, now extending for about 730 km2 and more similar in shape to that ofdigitate deltas. This evolution was caused by the increased sediment transportand raising of the banks of the distributor channels by the hand of man, whichhas caused their rapid progress towards the sea.The general reduction in river sediment loads during the latter half of the lastcentury has recently led to general erosion of the Italian deltas and adjacentcoasts. However, in this respect, it should be emphasised that the shoreline onboth sides of a river mouth is, by its very nature, extremely ephemeral andsubject to sudden variations both seawards and landwards, in response to highor low discharges of river water and high tides.

11along the coast prevail over the other components, carrying fine sedimentsout to sea and redistributing sand along the shore to form the typical beaches.The Italian examples are extremely varied, consisting of almost all the deltason the Tyrrhenian coast (rivers Arno, Ombrone, Tiber and Volturno) and theTagliamento on the Adriatic. In points where the discharges are very limited,the river mouth may become obstructed by accumulated sediments, whichare only removed on the occasion of full spates. Examples are the mouths ofthe seasonal streams which discharge into the Ionian Sea in Basilicata (riversCavone, Bradano, Basento) and Calabria (Torbido, Precariti, Barruca, Novito),and, in Sicily, the Simeto in the Gulf of Catania.The growth phases of cuspidate deltas are marked by belts of sand or coastaldunes separated by depressions which sometimes contain marshes or pools. Insome cases, these survive over long periods of time, and may then be clearlyidentified at the sides of the active delta as testimony to its evolutionary history.An example is the delta of the Tagliamento, which formed about 2000 yearsago, in which eight belts of dunes can be identified, separated by depressions.The Arno and Ombrone share a similar evolutionary history: initially (betweenabout 6000 and 25,000 years ago), they formed their own plains by filling widelagoon inlets, into which they flowed with internal deltas. These lobes are stillvisible today on satellite images as drier areas. Once they had filled in theirlagoons, they began to discharge directly into the sea only during Etruscan-

10

Mouth of the Allaro, a seasonal stream in Calabria Mouth of the Piave (Veneto)

Annelore Bezzi · Francesca Delli Quadri · Giorgio FontolanTypes of deltas and estuaries

IsonzoLong sandbarsedge the seaoutlet of the onlydistributor channelof the river Isonzo,characterised by aweak regimen ofwaves and shallowbottoms(scale 1:100,000).

PoThe modern deltaof the Po, whichextends forapproximately 730km2, has a similarshape to that ofdigitate deltas, withvarious distributorchannels flowingout to sea with theirloads of sediment(scale 1:500,000).

OmbroneOwing to the lack ofurbanisation in thearea, the sandy beltsof the cuspidatedelta of the Ombroneare still clearlyvisible, equallyaligned on eitherside of the rivermouth as testimonyto the differentphases of growth(scale 1:100,000).

TagliamentoThe strong currentsalong the shorelinehave caused thecuspidate delta ofthe Tagliamento tobecome asymmetric.The dune cordons thatreveal the formation ofthe nearby land overthe last 2000 years,have been obliteratedby farming(scale 1:200,000).

CavoneThe currents thattransport sandalong the shorepartially close themouth of this smallriver on the Ioniancoast of Basilicata.The emergingsandbars are onlyeroded when theriver is in full spate(scale 1:25,000).

TacinaA typical image ofthe Ionian coast ofCalabria: themeagre discharge ofthe Tacina seasonalstream is notsufficient to removethe sedimentcompletely, whichtends to obstructthe mouth(scale 1:25,000).

12 13

lagoons and estuaries. An estuary might be more properly indicated as “thelowest part of a river valley flooded by the sea, subject to tidal fluctuations andthe meeting of fresh and salt waters”.Seawater, therefore, may enter watercourses or correspond to an area ofmixing, depending on how well tidal currents contrast river ones. Accordingto this definition, given the limited tidal ranges of the Mediterranean, it is noteasy to identify typical estuaries on a large scale in Italy. The Italian coasts areclassified in the micro-tidal category (i.e., a tidal range of less than 2 m),ranges generally being less than 30 cm. This means that, compared with theAtlantic coasts of France and Northern Spain, which have tidal ranges ofmore than 4 m, the phenomena caused by tides are much less obvious alongItalian coasts.There are no large rivers with estuaries in Italy. Smaller watercourses with lowsediment loads may form a confined but significant estuary environment insemi-closed basins, examples being the lagoons of Grado-Marano, Caorle andVenice, and the smaller inlets of Scardovari, Canarin and Goro in the Po Delta.An example of a river mouth on the coast of the open sea with estuarinefeatures is the river Magra, which flows into the Ligurian Sea: hydrographicalstudies of its last stretch have identified stratification, water with salinitycomparable to that of seawater being found permanently on the bottom,which reaches as much as 5-7 km from the mouth when the river is low.

15■ Estuaries

When tides are dominant, river mouths assume a typical longitudinal funnel-shaped configuration, often with banks elongated in the direction of the river.The sea may penetrate hundreds of kilometres upstream, and the main physicalprocess is mixing of fresh and salt waters to create estuaries.In the oceanographic literature, the term estuary has a more general meaning.UNESCO uses Pritchard’s definition: “a semi-closed coastal body of water thathas free communication with the open sea and within which the seawaterdilutes significantly with the freshwater arriving from the surface hydrographicalnetwork”. In other words, it is the mixing of masses of fresh and salt water thatgives rise to estuarine conditions within semi-closed basins. It is thus commonto apply the name estuary to internal lagoon areas (such as the Lagoon ofVenice and its estuary), where the arrival of a sufficient number of watercoursesdilutes the marine characteristics of the basin and generates a marked salinitygradient between the edges of the lagoon and tidal inlets.Following this principle, the salt wedge may periodically intrude into the riversdischarging into the lagoon, extending the estuary environment to the finalstretches of these watercourses.The above definition is too generic for any more precise distinction of thedifferent geomorphological contexts, especially in the distinction between

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Mouth of the Mignone (Latium): when the river is low, the water stagnates and does not reach the sea Area of the Goro inlet, Po Delta (Emilia Romagna)

■ Lagoons

In Italy, the northern Adriatic is characterised by abnormal tidal excursions, withaverage spring tides of 1.1-1.2 m which, occurring as they do along aprevalently low-lying coast and affected by limited wave motion, gives rise totypical morphologies and habitats.These conditions, together with the significant sediment loads of the majorAlpine rivers, led to the formation of the large lagoons of Grado-Marano andVenice.Those in the northern Adriatic are therefore the only Italian cases of truelagoons, meaning areas of semi-enclosed sea due to the presence ofpeninsulas or systems of barrier islands and regulated by the ebb and flow oftides through one or more channels of communication with the sea (tidal inlets).In fact, no stretch of water should be called a “lagoon” which is sited inenvironments with practically no tide, such as the Ligurian, Tyrrhenian andIonian coasts.The northern Adriatic lagoons originated 5000-6000 years ago, when the seaflooded vast alluvial plains, and created large bays bordered by rapidly formingdeltas. The sediments from the rivers Isonzo, Tagliamento, Piave and Adigewere progressively transported along the shoreline, until they were distributedin the form of lidos, thus separating the large bays from the sea (then regulatedby the tidal ebb and flow) and forming the lagoons.In greater detail, according to studies of deep sediments, the central part ofthe Lagoon of Venice began to form around 6000 years ago, whereas in thenorthern sector of the lagoon and the delta of the river Adige, the first lagoonaldeposits appear to date back to slightly earlier times. The first dune belts ofthe coastal system of the Piave, further to the north, date back to just over

1716

Thatched fishermen’s huts in the Lagoon of Grado-Marano

Secondarychannel

High tide level Channel

Sand bank Tidal creekShoulder Mudflat

Tidal level

Levels

High tide, maximum

High tide, average

Mean sea level

Low tide, average

Low tide, maximum

Cross-section of a lagoon

5000 years ago. Lagoon sedimentsfound at depths of 6 metres are about6000 years old.In the sector between the Piave andthe Tagliamento, lagoon sedimentsdeposited on the Pleistocene plain aredated to around 5000 B.C. Furthereast, the marine ingression creating theLagoon of Marano goes back tobetween 5500 and 4200 years ago.The Lagoon of Grado appears to havebeen formed much more recently, as testified by the presence of the Romans inthe area, and can be traced to the process of easterly migration of the mouth ofthe Isonzo.The natural factors contributing towards the formation and maintenance oflagoons are, in the first place, those linked to the hydrodynamic system, whichis closely regulated by the tidal action: the hydrographical network of a lagoonis composed of a dense criss-crossing of channels of different widths anddepths, which allow the masses of water that enter through the tidal inlets toexpand within the basin. Each individual inlet serves a lagoon basin, separatedfrom the adjacent one by a watershed, an imaginary line representing the pointof separation between the masses of water entering the lagoon from twoneighbouring inlets. Between the watershed of two adjacent basins, themasses of water only mix in exceptional cases (e.g., strong winds), so thewaters that flow in through an inlet as the tide rises, drain out through the sameinlet as it falls.The network of lagoon channels can be subdivided into the larger mainchannels, which develop from the area of the tidal inlet towards the interior.They are often the relicts of the ancient fluvial system preceding the marineingression, and are in some cases connected with the tributary watercourses ofthe lagoon itself. Secondary channels are smaller and shallower, they branchout from the main ones, and distribute and receive the waters from thesurrounding land.During the period of spring tides, when the tidal range is at its peak, the waterthat flows through the lagoon inlets reaches its highest speeds, even fasterthan 1 m/s, and rapidly spreads through the main channels and then throughthe secondary ones, until it reaches the tidal flats. As the system of channelsgradually multiplies, water speed falls to a few cm/sec. When the water reachesthe tidal flats, the speed of the current again sharply diminishes, due to lateral

19Tides

Tides are caused by the force ofattraction that the Moon and, to alesser extent the Sun, exerts on theEarth. For this reason, the phases ofthe moon during a lunar month (29.5days) give rise to significantdifferences in tidal ranges.During the phase of the new moon, letus imagine that the Moon lies on a linelinking the Sun and the Earth and isthus not visible. At full moon, it isentirely visible, because it is situatedon the same alignment but on the sideopposite the Sun.During these two lunar phases, calledsyzygy, the attractions exerted by bothSun and Moon are combined, and sothe differences between high and lowtides are at their maximum (“springtides”).Vice versa, when the Moon, Earth andSun are at right angles to one another,the neap phase occurs (only half theMoon is visible), and the tides have asmall range (“neap tides”).

Annelore Bezzi · Francesca Delli Quadri · Giorgio Fontolan

lunar solar combined

SPRING TIDE

TIDAL DATA IN RELATION TO PHASES OF THE MOON

NEAP TIDE

livel

lo d

ell’a

cqua

(m)

2

1

0

0 2 4 6 8 10 12 14 16 18 20days

SPRING TIDE NEAP TIDE

Sun

Earth

Moon

new quarter full

Torcello in the Lagoon of Venice (Veneto)

The Lagoon of Grado-Marano (Friuli VeneziaGiulia) during low (top) and high (bottom) tide.

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expansion of water masses. Thisprocess is repeated in the oppositedirection during the ebb tide: the mainchannel acts as a collector, all thesecondary channels distribute on thetidal flats, and the lagoon waters flowout towards the sea, reaching theirmaximum speed at the tidal inlets.This is the hydrodynamic model whichis responsible for the arrival, transportand deposition of sediments inside thelagoon, giving rise to the variousmorphologies found in the basin.The typical morphologies thatcharacterise the northern Adriaticcoastal lagoons are grouped in threemorphological zones, according totheir position with respect to sea level:● the shoals, islands, beaches andinternal coast of the lagoon are above the average level of high tides;● the main channels and tidal inlets are below the average level of low tides;● the intertidal areas, i.e., those lying between the average levels of low tidesand high tides, which are periodically submerged. Typical examples are thetidal flats, i.e., the shallow lagoon beds and secondary channels, which emergewhen low tides discharge water towards the sea.In lagons, islands are portions of land never submerged by high tides. They mayconstitute relicts of ancient beaches, or have originated from the accumulationof sediment transported by rivers.Shoals are the most characteristic landforms of lagoon environments. They aretabular raised areas, mainly composed of silty-clay sediments, which are onlysubmerged by water in exceptional cases. Shoals have irregular topographyand may contain saltpans, which receive an exchange of water through small,tortuous channels (tidal creeks), especially during high tides.Because of the complexity of the processes causing the formation of lagoons(distribution of freshwater and materials from rivers, marine ingressions, andlater drops in water level), shoals are classified on the basis of their differentorigins:● shoals of ancient river edges, or ones located at the sides of watercourseswhich still discharge into the lagoon, with a characteristic long, narrow shape;

2120

Water exchange in a lagoon occurs through small channels (tidal creeks), mainly during high tides

Sandbanks may be stabilised by vegetation(“Zangheri” sandbank meadow, EmiliaRomagna)

Tidal flats are lagoon beds that onlyemerge at low tide, especially springtides, and are not therefore suitable forcolonisation by pioneer plants. Theyslope very gently (with gradients of onlya few dozen centimetres per kilometre)and have a system of secondarychannels discharging towards the mainchannel.These channels are meandering and afew metres wide; their minimum levelcoincides with the average of the lowspring tides, so their maximum depth(slightly more than a metre) depends onthe tidal range and coincides with thepoint of confluence with the mainchannel.Lastly, some of the most typicalenvironments in Italian lagoons are thefish-farm basins, exploited since early times for rearing and capturing fish.These often original basins are generally surrounded by embankments thatseparate them from the main lagoon, in order to protect them from the effectsof tides and to regulate the entry of brackish and freshwaters from thehinterland. They currently occupy vast portions of the Lagoon of Venice andthat of Grado-Marano and, to a lesser extent, those of Caorle and Caleri.As regards sediments, there is generally a progressive reduction in particle sizefrom the tidal inlets towards more internal areas. These variations are usuallyassociated with the energy level of the environment: lower where the basin isdeeper, more sheltered and further from the sea; higher close to the tidal inlets,in the immediate vicinity of the channels, and in areas where the water isshallower and the bed is affected by the wind, waves and sea currents. Sandysediments are deposited near the tidal inlets, often forming characteristicmorphologies known as flow-tide delta (on the lagoon side) and ebb-tide delta(on the seaward side).Sand becomes progressively finer along the main channels and further in,especially on the tidal flats; fine sand is progressively replaced by mud, whichbecomes dominant towards the interior of the lagoon, to become exclusive, inits finest fraction, closest to the internal lagoon coast, as well as to thewatershed.

23● shoals composed of the still emerging part of an ancient coastal plain invad-ed by brackish waters: they are located on the edge of the lagoon towards dryland, and their sediments contain a thick layer of deposits of continental origin,overlain by a thin layer of lagoonal type;● residual shoals of a marshy environment, with layers of peat in the sedi-ments: they were formed by river alluvial deposits in the lagoon basin which,following emersion, became covered by marsh vegetation. After river watershad been diverted they no longer received sufficient amounts of sediment toimpede their rapid sinking, mainly due to compaction of the peat;● channel shoals: these are in equilibrium, with sufficient sedimentation tocompensate for slow sinking. Their shape is typically concave, with a lower-lying central area; the side bordering the channel is generally steep, whereasthe other slopes gently towards a marsh or tidal flat. Channel shoals form at thepoints of convergence of lagoonal flows and in the zones of watershed where,in ideal conditions, the resulting water velocity is almost zero. These sandbanksgrow in height because the waters that flow over them during high spring tides,slowed by vegetation, release part of their solid load, and expand due to theslowing down that sea currents of different intensity and direction exert recipro-cally.For some authors, this last type constitutes a shoal in the strict sense - theresult of the deposition of sediments arriving exclusively from the sea.

22

Mouth of the Agri (Basilicata)

Sandbanks, mudflats and channels in theLagoon of Grado-Marano

■ Foreword

In optimal environmental conditions, thedistribution of brackish water vegetationdifferentiates between that part of thebody of water directly influenced by thesea and the more landward one, whichis affected by confinement of waters orsometimes by freshwater. With distancefrom the points of exchange with thesea, submerged vegetation becomessimplified, because, as the type of sediments alters, from prevalently sandy tosilty and becomes more subject to re-suspension, the morphological andphysico-chemical characteristics of the water also change.Most of the phytobenthos of lagoons is typically of marine origin, with speciesthat are capable of tolerating marked reductions in salinity. This dominance ofmarine species is also due to the fact that the typical organisms of internalwaters are rarely capable of adapting to salinity higher than 5‰.When the arrival of freshwater is limited, salinity becomes less important forregulating the presence and abundance of submerged vegetation, whereaswater transparency, sedimentation, hydrodynamics and the availability ofnutrients become more so. As these environments are shallow (sometimes lessthan 40-50 cm), water turbulence due to weather - or human activities, such asmollusc-farming - may easily trigger events of re-suspension that reduce watertransparency and limit compaction of sediments.When dystrophic conditions change with season, few variations occur in thestructure of the communities, and the resilience of the plant communities allowstypical conditions to be restored within a reasonable time. If dystrophic conditionspersist for more than one year, submerged vegetation substantially modifies, withthe gradual substitution of species with long life-cycles and high ecological value(such as seagrasses, brown algae like Cystoseira barbata and Fucus virsoides) byspecies with short life-cycles (such as the Ulvaceae or Cyanophyceae).

25Submerged vegetationDANIELE CURIEL · ANDREA RISMONDO

Sargassum in the Lagoon of Venice

Chaetomorpha

Chaetomorpha aerea and C. linum(perhaps conspecifics) are veryfrequent. In the summer months theyare often mixed with green algae of thegenus Cladophora (e.g., C. sericea, C.albida, C. rupestris, etc.) In the 1980sand early 1990s, all the lagoons in thenorthern Adriatic were afflicted byblooms of these green macroalgae, theprevalent species in terms of extentand biomass being Ulva laetevirens.Recorded since the 1940s-50s in muddy areas of the Po Delta, in the Valli diComacchio and Lagoon of Venice, but today much rarer and limited only tohollows, are the subspherical-shaped thalli, 3-8 cm long, of the green algaValonia aegagropila. These macroalgae may form extensive, uniform,continuous coverings up to 30-40 cm thick, and with a wet biomass of morethan 10 kg/m2. In fish-farm basins where salinity is maintained constant atvalues slightly above 10‰, partly in order to favour hunting, green algaebelonging to the genera Chara and Lamprothamnion (L. papulosum) are foundon the bottoms, mixed with the seagrasses Nanozostera noltii or Ruppia (R.maritima and R. spiralis) or with the green alga Valonia. These macroalgae areoften not taken into consideration by marine algologists because they are onlyfound in transition environments and adjacent flowing waters.Coherent substrates in these brackish environments host a few macroalgaewhich would have difficulty in living on loose substrates because of their need toanchor themselves to something solid. Both in vivified areas and in internal areasclose to the transition to brackish waters, on the highest levels which are onlywet during high tides (at the limit between the intertidal and supralittoral areas),are green algae of the genus Blidingia (B. minima, B. ramifera), which areresistant to emersion. The height of the tides and wave motion caused byshipping may raise the colonised level by up to 20-30 cm. Below this, startingfrom average sea level, in the intertidal zone, a filamentous distichous green alga(Bryopsis plumosa) may be observed, followed, but only in vivified environmentslike the Lagoon of Venice, by the green alga Codium fragile subsp.tomentosoides, with its cylindrical dichotomous branching thallus. This alga, anative of the Pacific Ocean and Asiatic seas, has been present in the northernAdriatic since the 1970s-80s. Although viewed as one of the most invasivealgae, because of its ability to adapt to differing conditions of salinity andtemperature, only isolated thalli are found, at least in the Lagoon of Venice.

■ The macroalgae: green algae

Compared with the marine environment, where red algae prevail, green algaeare the best-represented plants in the transition zone, especially in terms ofabundance. These algae are generally euryecious and find optimal conditionsin the shallows of internal and confined areas where water exchange is limitedand nutrients are plentiful. In the transition environments of the northernAdriatic, the most abundant and best-known alga is Ulva laetevirens, with itswidened leaf thallus, commonly known as sea lettuce. Other species of thegenus Ulva (U. rotunda, U. curvata) or Gayralia oxysperma (morphologicallysimilar but with a single layer of cells in the lamina) never reach the distributionand biomass levels of U. laetevirens.Another very widespread algal component is that of the Enteromorpha species,a group of green algae recently unified with the genus Ulva. These green algae,with tubular simple or branched thalli that may measure from a few centimetresto 50-60 cm, live in eutrophic environments. The most common species includeU. intestinalis, U. clathrata and U. flexuosa. In summer months, the gasespresent in the cavities and products of photosynthesis raise the thalli from thebed, forming drifts floating on the surface of the water.During the summer, especially in the internal waters of fish-farm basins or inassociation with seagrasses, uniform carpets of the green filamentous algae

2726

Ulva laetevirens

The green alga Valonia aegagropila

■ Red algae

As in the marine environment, red algaeare a numerically conspicuous part ofthe aquatic vegetation in transitionenvironments, especially in those morehighly vivified, like the lagoons ofVenice and Grado-Marano.Being ecologically sensitive, especiallyto any reduction in salinity and increasein turbidity (natural or induced), redalgae are limited in the fish-farm ponds,the inlets in the piallasse (marshy areas)between Romagna and the Veneto ortowards the interior shores of the largelagoons of Venice and Grado-Marano.Only more tolerant species, which canadapt to waters rich in nutrients andsediments arriving from the rivers, can survive. On loose beds, the speciestypical of brackish water include those of the genus Gracilaria (G.bursapastoris, G. armata and G. gracilis). Gracilariopsis longissima (previouslyknown as Gracilaria verrucosa) formed extensive macroalgae covers in the1980s, alone or in association with Ulva.Today, the phenomenon of algal bloom appears to be more limited in the twolarge lagoons of the northern Adriatic, but reappears almost annually in the smallto medium-sized transition environments in the Po Delta, although less intenselythan in the past. With currents and winds, the thalli of Gracilaria / Gracilariopsisroll around on the bottom and accumulate in sheltered areas, where they mayproliferate.In summer months, in the Lagoons of Venice and Grado-Marano, with theirreduced hydrodynamics, more or less extensive covers of Spyridia filamentosaare to be found.Mainly in the spring and summer months, on loose bottoms without macroalgalcover, natural coherent substrates such as the valves of molluscs (Tapes,Cerastoderma, Cassostrea or serpulid tubes), or artificial substrates (stones,shingle) favour the presence of red algae with pulvini (cushion-likeenlargements), 5-10 cm tall.Among the most eye-catching are the branched filamentous forms ofCeramium (the dark-red C. virgatum, or C. diaphanum, with alternating light

29Alien species

For more than a decade now, severalspecies endemic to other seas have beenfound on both soft and firm substratesin transition environments in theMediterranean. They have arrived thanksto the voluntary and involuntary actions ofman. Until the Suez Canal was opened in1869, the introduction of new species intothe Mediterranean occurred accidentallythrough the Strait of Gibraltar. With theopening of the Suez Canal, the Red Seaand the Mediterranean, having remainedseparated for more than 10 million years,were put into communication again.For hydrological and ecological reasons,migrations are prevalently towards theMediterranean. In the last 20-30 years,this slow but continual introductionof species has become insignificantcompared with the much faster pace withwhich man has favoured the introductionof new species along the coasts of theMediterranean. Transition environments,historically the sites of importanteconomic activities (fishing, shipping andport structures, etc.), are consequentlythe main points at which new speciesarrive. If they adapt to the newenvironmental conditions, they can spreaduntil they eventually reach an equilibriumwith native species. The spread of others,which do not find optimal conditions inthe new environment is limited, or theydisappear. In the Mediterranean, thetwo most striking examples of transitionenvironments affected by many alienspecies, are the Lagoon of Thau inSouthern France and the Lagoon ofVenice. In the latter, more than 20 specieshave been recorded on the substratesoff Chioggia and Venice since the early1990s, and, because they are foundoutside their potential distribution area,they are correctly defined as alien species

or NIS - non-indigenous species. Therehave only been a few records for some ofthem (e.g., Sorocarpus sp., Prasiola sp.),probably because they have not adaptedto the new and complex lagoonconditions. Instead, others have spreadover the years and become integratedwith native species (e.g., Antithamnionnipponicum, Grateloupia turuturu,Codium fragile subsp. tomentosoides).Yet others have spread rapidly andbecome dominant in the macroalgalcommunity, with widespread diffusion andhigh abundance (Sargassum muticum,Undaria pinnatifida, Polysiphoniamorrowii). A very unusual case, whichnow involves various lagoons of theMediterranean, the Atlantic coasts ofFrance and the northern seas, is that ofthe two brown algae, Sargassum andUndaria. Although having different life-cycles - Sargassum being perennial andUndaria annual - in the climate of theNorthern Adriatic these two species havethalli observable from October andNovember, which disappear betweenJune and July. Attempts have been madeto eradicate them in various locations, sofar with little success. Both species haveextremely efficient dispersal systems fortheir millions of spores, which can betaken great distances by tides - and evenfragments of thalli have high viability.

Daniele Curiel · Andrea Rismondo

Gracilaria

Undaria

28

31and dark bands) and the Antithamnion (A. cruciatum, and, in the Lagoon ofVenice, A. nipponicum, a native of Oriental seas). Among the branchedlaminar forms, Nitophyllum punctatum and Radicilingua thysanorhizans areespecially striking.Conspicuous numbers of red algae are also to be found on the leaves ofseagrasses, especially Cymodocea nodosa. On the oldest (external) leaves, arich community of medium-small species develops, including the filamentousforms Ceramium, Antithamnion, Chondria capillaris or Chondria dasyphylla, andvarious species of the genus Polysiphonia. The red encrusting coralline algae ofthe genus Hydrolithon and Pneophyllum fragile may also be abundant,especially in the months of scarce or no growth of leaves.In the Lagoon of Grado-Marano, but to an even greater extent in that of Venice,there are many artificial coherent substrates (stones, shores of the islands,signalling devices for shipping, etc.), where species typical of these substratesmay be observed. Above the average tidal level, Gelidium pusillum andGigartina acicularis are frequent, with their branched filamentous thalli firmlyanchored to the substrate and able to resist emersion for some hours. Belowthe average tidal level are species of the genus Polysiphonia (P. morrowii, aspecies native to the Oriental seas, P. harvey and P. denudata) and, lower down,Rhodymenia ardissonei, a sciophilous (shade-loving) alga, indicative of the lackof light in these environments caused by water turbidity.

■ Brown algae

Brown algae do not find ideal conditionsfor growth in transition environments.On the coherent substrates of the twolargest lagoons, species of ecologicalvalue can only be found in areas moreaffected by seawater distribution, withFucus virsoides (a species of borealaffinity, present in the Mediterraneanonly in the northern Adriatic), Cystoseirabarbata or Petalonia fascia.Dictyota dichotoma var. dichotoma andScytosiphon lomentaria may also beobserved in more internal areas.The microscopic species of the generaMyrionema, Leptonematella and thefilamentous forms Ectocarpus andHinksia are typically found on the leaves of the seagrasses. The latter are alsothe only brown macroalgae which, in the presence of nutrient availability andreduced hydrodynamics, may also flourish on loose substrates. In the Lagoonof Venice, this occurs mainly amongst the seagrasses when, following theirsummer proliferation, the filamentous thalli of Ectocarpus become entangledwith the seagrass leaves, forming larger and larger clumps.Two brown algae, Undaria pinnatifida and Sargassum muticum, which areendemic to Far Eastern seas, have been found on artificial solid substrates inthe Lagoon of Venice since the early 1990s. The large size of the thallus andthe abundances these species are able to reach render them highlycompetitive with native species.Mention should also be made of the algae of the genus Vaucheria. In the deepbeds of the lagoons of Venice and Grado-Marano, there has been aproliferation of these macroalgae in the past decade; unlike all the otherspecies, they do not need a firm substrate to anchor themselves because theygrow partly immersed in sandy-muddy sediments. Vaucheria dichotoma var.marina, today identified as V. submarina, is more common and has beenrecorded since 1800, but V. piloboloides has also recently been found in theLagoon of Venice. These macroalgae play an important role in compacting thesediment layer and greatly reducing re-suspension of sediments duringperiods of water turbulence.

30

Polysiphonia morrowii

Fucus virsoides

33■ Seagrasses

Lagoon beds may be covered byvarying densities of meadows ofseagrass, a small but important groupof flowering plants which are fullydescribed in the Italian Habitats volume“Seagrass Meadows”.Posidonia oceanica, Zostera marina,Nanozostera noltii, Cymodocea nodosaand two species of the genus Ruppiaare present in the Adriatic.The success of these macrophytes inthe underwater environment of lagoonsis due to the various morphological andphysiological adaptations which theyhave had to develop to survive so well.These include adaptation to the saltyenvironment and to submersion (hydrophytic habitus), the capacity to resistthe action of waves and tidal currents, and hydrophilic pollination anddissemination.

Phythoplankton

The highly variable environmentalconditions of brackish-waterenvironments, in terms of salinity level(10-40‰), temperature (sometimesexceeding 30 °C), turbidity (Secchi diskvalues up to 10-30 cm) and dissolvedoxygen (from anoxic to super-saturatedconditions), mean that onlyphytoplankton with a wide ecologicalrange and high quali-quantitativevariability during the course of the yearcan survive. As well as abiotic factors,an important role is also played by bioticfactors, such as the presence orabsence of herbivorous predators.For clearer understanding of variabilityover the year, in the Lagoon of Venicein the early years of the 21st centurythere have been minima of 1x105

cells/litre in winter and peaks of 19x106

cells/litre in spring-summer.The most abundant taxa in all transitionenvironments are the Bacillariophyceaeand Pyrrophyta, more commonly knownas diatoms and dinoflagellates. Diatomshave a silicified cell wall (frustule)formed of two interlocking valves.In the Lagoon of Venice, the mostcommon and abundant diatoms belong

to the genera Chaetoceros, Cocconeis,Nitzschia, Thalassiosira andSkeletonema and, among the formswhich constitute mucilaginous tubulesand are also epiphytes of seagrasses,Navicula and Melosira. Species of thegenera Prorocentrum and Gyrodiniumare frequent among the dinoflagellates.In the Lagoon of Grado-Marano,the diatoms of the genera Synedra,Navicula and Rhizosolenia are frequentand abundant. Among thedinoflagellates are Exuviella,Gymnodinium and Prorocentrum.In the brackish-water lagoons of EmiliaRomagna, diatoms of the generaNitzschia, Thalassiosira, Skeletonemaand Melosira are common.Of the dinoflagellates, those of thegenera Prorocentrum, Peridinium orGlenodinium have often producedlarge-scale algal blooms.The microalgae belonging to thechlorophytes (green algae) are lessabundant, but no less important.They include species belonging to theVolvocales (Dunaniella being typicalof hyper-saline basins) andChlorococcales (with Chlorella).

Daniele Curiel · Andrea Rismondo

Posidonia oceanica

Nitzschia (approx. 2000x)

Seagrass distribution in the Adriatic Sea

SPECIES GEOGRAPHICAL DISTRIBUTION

Posidonia oceanica Gulf of Trieste, including Grado coastWestern Istrian coast Venetian coast (dead mattes)

Zostera marina Western Istrian coastGulf of TriesteLagoons of Venice and Grado-MaranoLagoon areas of Po Delta

Nanozostera noltii Istrian bays and sheltered areasWestern Istrian coastGulf of TriesteLagoons of Venice and Grado-MaranoLagoon areas of Po Delta

Cymodocea nodosa Istrian bays and sheltered areasLagoons of Venice and Grado-MaranoScattered along lidos of Jesolo, Eraclea and Caorle

Ruppia sp.pl. Shoal and drainage areas of northern Adriatic lagoons in general

32

Cocconeis (approx. 2000x)

34 35The main factors regulating thepresence and distribution patterns ofseagrasses on the beds of coastallagoons are, in order of importance:particle-size distribution of sediments,water turbidity, temperature patterns,local hydrodynamics and resulting waterexchange, eutrophication and theconsequent presence of macroalgae.The quality of waters and sedimentsand the hydro-morphological factorsof basins tend to favour one or other ofthe existing species.In the Adriatic coastal lagoons, Ruppiais present in a variety of micro-environments. These cannot beprecisely characterised in terms ofsalinity, because the genus is to befound not only in the fish-farm ponds of Venice and Grado-Marano and insome habitats with very low salinity, but also in pools nearer the sea, whichare subjected to high salt concentrations.In the northern Adriatic lagoons, Nanozostera noltii is a typical species ofmarshes and tidal flats, the more internal and confined areas subjected tocycles of emersion and with lower salinity than that of the sea.However, in the larger and morphologically more complicated basins, such asthe Lagoon of Venice, the species may have more than one phenotype,sometimes colonising tidal flats with low, dense carpets and thus ensuring themorphological protection of these systems.At other times it forms extensive meadows on the deeper beds, where itgrows much larger and with longer leaves. In the northern Adriatic, it issubstantially homogeneous, being found both in the Lagoon of Venice and inthat of Grado-Marano, on the same morphological systems and therefore ontidal flats and on the edges of deeper channels, on clearly silty beds. Veryrare and extremely localised in the other, smaller, Adriatic basins, it has beenreported close to the sluices or entry channels of waters such as those atComacchio.Since the late 1990s, N. noltii has been undergoing a highly generalisedprocess of retreat from the areas of sandbanks - particularly marked in theLagoon of Venice. This has many causes - such as the general increase inNanozostera noltii

Ruppia

turbidity and prolonged periods of emersion during very hot sunny periods -but the intensified erosion processes, especially in the lagoons suffering fromsevere human pressures, should not be underestimated.The larger Zostera marina, with leaves 6-7 mm wide and up to a metre long,lives almost always submerged in more turbulent and less confined waters,preferring bottoms with silty sand.Its distribution area covers truly lagoonal waters, as in the lagoons of Veniceand Grado-Marano, where it forms extensive meadows. In the Po Delta andthe brackish lagoons along the Romagna coast, its populations are rarer andmore scattered, mainly in the channels linked with the sea.However, even truly marine waters cannot be excluded, because along thecoast east of Grado, near the mouth of the river Primero, which has noartificial protection and is characterised by entirely natural beds, there is awhole alternation of sandbars and depressions which is widely colonised byZ. marina. Alternating with Cymodocea nodosa, it most probably takesadvantage of the mixed sandy-silty sediments and the presence ofdischarging freshwater.Zostera marina is the most euryhaline of the species described here,tolerating salinity of between 5‰ and that of seawater; for this reason,paradoxically, it may spread towards the coastline, and in any case in thedirections of flows of freshwater, more than Nanozostera noltii can.

3736 In the lagoons of Venice and Grado-Marano, N. noltii forms large vegetatedbeds that extend from vivified sites to others which are partly confined. Itavoids areas subject to water stagnation, as the upper limit of the favourabletemperature range for this macrophyte is around 25-27°C.Cymodocea nodosa, with a dense, robust root system, is found close to inletswhere sediments are predominantly sandy and water exchange is medium tohigh. A stenohaline species, it cannot resist conditions of high turbidity and,although it grows at depths of up to 10 metres along the Istrian coast, it is notfound deeper than 4 metres near the lagoon inlets, on occasional deltasandbars and in the other Adriatic coastal lakes like those in Apulia. C.nodosa is a species of sub-tropical origin and is highly seasonal, especially inthe northern Adriatic lagoons, where the temperature may drop below zero inwinter and rise above 30°C in summer.In winter, the macrophyte uses a strategy of conservation of the root systemand hypogeal (buried) stalks, where sugar is stored, and this may lead to theloss of all its leaves. Only with the marked rise in temperature, usually in May,is there rapid and intense re-growth which, in the space of a few days, givesrise to dense new meadows of brilliant green.At the inlets, on coastal sandbars, and in the brackish coastal lakes of thesouthern Adriatic, C. nodosa retains its leaf apparatus in the winter months,although it may be reduced, degraded and covered by epiphytes.

Foglie di Zostera marina Cymodocea nodosa

39

Deltas, estuaries and lagoons aretransitional coastal ecosystemscharacterised by gradients of manyimportant environmental factors, suchas water exchange, salinity, sedimentstructure, turbidity and nutrient load.The transition gradient is generallyperpendicular to the coastline, from thesea landwards, with consequentlyprogressively changing habitats andbiological communities.

■ Variations in biodiversity

Gradually penetrating a transitional ecosystem, there is a progressive decline inthe number of species. This phenomenon occurs in both directions - from thesea towards the land, and vice versa - and biodiversity reaches its minimum inthe area where river and sea meet.The accumulation of organic matter in areas subject to slow exchange andhydrodynamics plays a key role in modulating the availability of oxygen, whichbecomes a strongly limiting factor that increases the vulnerability ofbiocoenoses. In Mediterranean lagoons, where the contribution of freshwater islower and the seawater is less diluted, the reduction in the number of speciesalong the sea-land axis is mainly attributed to the hydrology of the basins andsediment properties, and only secondarily to salinity.Tides greatly accelerate these exchanges, and their cyclical nature generatescharacteristic models of time-space distribution. The periods of emersion andsubmersion dictated by the tidal regime are other important factors affectingbiological communities in transition environments.Most of the species which live in these areas are of marine origin, so that thedegree of connection with the sea greatly influences the recruitment ofspecies which require a larval stage involving marine dispersal, and this has

Aquatic invertebratesDAVIDE TAGLIAPIETRA · ALESSANDRO MINELLI

Mouth of the Idume near Lecce (Apulia)

Cerastoderma shells

energy is increased. Here, both inside and outside the inlet, the sediments,composed generally of sand or silty sand, fan out to form tidal flats.Channels then lead to a second zone, the central basin, which is generallyrather shallow. This is where the energy from both sea and river is lowest andabundant deposits of fine sediments are found. As the central basin formsthe most extensive part of the lagoon, it is not homogeneous and slopesgradually towards more internal shallows, with sandbanks along the edgesof the channels.The complex interplay of tidal energy, river energy and hydrodynamics ingeneral decides how close the sandbanks approach the sea, reducing areasof open water in the central basin.Marshland and sandbanks succeed one another towards the lagoon edge asfar as the third large zone, composed of the mouth of the river. Here we findsandbanks, first mesohaline and then oligohaline, dominated by reed-beds,which are progressively replaced by less salty environments, until the true riverenvironment is reached. It should be noted that drainage operations in almostall Italian lagoons have greatly reduced the zones of lagoon inlets andoligohaline marshes.In a delta dominated by tides, river energy is such that the central basindisappears and, with it, the capacity to trap fine sediments, which are thusdeposited further inland, past the line of the mouth.

repercussions on the biodiversity andstructure of the communities. It istherefore to be expected that, fromthe sea landwards, the progressivedivergence from marine environmentalconditions are tolerated by fewer andfewer species.The opposite situation occurs forspecies of river origin. Towards the rivermouth, there is a progressive departurefrom typical river environments and thenumber of species tolerating the newenvironmental conditions graduallyfalls. This transition from a continentalto a marine environment in deltas,estuaries and lagoons combines thesetwo-way changes in biodiversity fromthe sea towards rivers and vice versa.

■ Classification zones and their fauna

A salinity-based empirical method to classify coastal and transitional waterswas proposed in 1958 after the IUCN Venice Symposium, an event at which themajor international experts met to agree on a system of classification. Thefollowing zones were identified: hyperhaline, with salinity >40‰; euhaline 40-30‰; mixohaline 30-0.5‰; and limnetic 5-0.5‰.As most transitional environments fall within the wide mixohaline zone, thelatter was further subdivided into: mixo-euhaline with salinity >30‰ but lowerthan that of the adjacent sea; (mixo-)polyhaline, 30-18‰; (mixo-)mesohaline,18-5‰; and (mixo-)oligohaline, 5-0.5‰.However, zoning based on salinity is extremely variable over time and is onlybased on one of the factors of the transition gradient. Instead,physiographical zoning is based on the most obvious morphological andstructural characteristics of the environment, is less variable over time, andmay be integrated, where needed, with the “Venice System”. The broadphysiographical zoning of a typical transition environment such as a coastallagoon is as follows.Moving from the sea towards the river, a lagoon basin is divided into threelarge sections. The first is the zone of the actual inlet in which the sea’s

41

Detail of the shell of Acanthocardia spinosa,a bivalve

40

Lagoon of Venice, with the typical bricola, signalling deep-water channel for shipping

Marco SigoviniThe crab

The common crab or green crabCarcinus aestuarii (= C. mediterraneus) isa decapod crustacean characteristic ofthe transition and coastal environmentsof the Mediterranean and the Black Sea.However, its status as a species is underdiscussion - according to some authors,it is merely a subspecies of Carcinusmaenas, the affine and vicariant form(i.e., occupying the same ecologicalniche) which lives along the Atlanticshores of Europe.The species is well-adapted to the highspatial and temporal variability ofenvironmental conditions in lagoons,estuaries and deltas. It is able totolerate salinity values higher than thosetypical of the sea or ranging as low asan oligohaline state, and temperaturesof between approximately 0°C and30°C. The wide ecological range of thecrab is confirmed by its omnivorousfeeding habits, although predatory and

detrivorous behaviour prevails. Inconditions of high density, the commoncrab can regulate the abundances of itsprey species (bivalves, polychaetes,and other invertebrates), significantlyinfluencing the community structure.In turn, it constitutes an important foodresource for limicolous birds andestuarine and sea fish such as eels,gobies and sea bass.Like all arthropods, the crab must moultseveral times during its lifespan,replacing the outer shell or exoskeleton,to allow for growth (see drawing). Thisprocess is divided into four stages: pre-moult; true moult, which only lasts for afew hours; post-moult, and inter-moult,the stage of “resting” and preparing forthe next cycle. In general, adult males (m)moult in spring and autumn, whereasfemales (f) undergo only the first moult,later than the males. Mating takes placeduring this latter stage (a). The females lay their eggs between autumn and

winter, depending on location. In manyMediterranean estuaries and lagoons,female crabs migrate towards the seain winter, where the eggs then hatch.This is followed by a planktonic larvalstage, during which stages called theprotozoea, zoea (A) and megalops (B),follow one another. At the end of the laststage, the larvae, having returned to theestuarine or lagoon environment, settledown as benthic organisms, preferablyin protected habitats like seagrassmeadows, where they find shelter duringtheir juvenile stage (C). From thismoment onwards, they will moult severaltimes a year, with lessening frequency,until reaching sexual maturity (D).Exploitation of crab meat as a humanfood resource is only of importance inthe northern Adriatic, where a traditionaltype of fishing exists, principally at

Burano and Chioggia, of highethnological and economic interest.Its targets are the moulting crabs,called moleche, or moeche, which areimmediately removed from the water tostop re-calcification of the exoskeleton.Detailed knowledge of the crab’s life-cycle is extremely important in theprocess of selection, and the molecanti,as these fishermen are called, have awhole series of names for the crabsduring their various stages: gransio bon,an individual that still has to moult;gransio mato or duro, specimens whichwill not moult again during the fishingseason; spiàntano, close to moulting;capelùo, harvested at the moment whenthe shell detaches; and mastrùzzo orstruzzo, a few hours after the moult.Females with mature ovaries (masanéte)are also considered as delicacies andare gathered in autumn for the market.

42 43

Carcinus aestuarii

IXII

XI

X

IX

VIII

VII

VI

V

IV

III

II

AB

C

D

fm

a

The life-cycle of the crab. On the left, a diagram showing moult phases - dashed line: periods of themoult; continuous line: inter-moult (for abbreviations, see text)

marina, which is responsible for the mounds of sand with the appearance ofthick spaghetti that may be seen on sandy silty beds at low tide. This worm isextensively used as bait by fishermen and can tolerate sometimes considerablereductions in salinity. Another interesting polychaete is Sabellaria spinulosa,whose colonies have recently begun to cover hard substrates with a compactlayer of tubes constructed with cemented sand, giving the overall appearanceof a beehive. These polychaetes are important in that they create newsubstrates for other organisms.Among the crustaceans are some species of portunid crabs with moderateswimming ability belonging to the genus Liocarcinus, like the sand crab(Liocarcinus vernalis). Close to the shore lives the crab which might be definedas the king of the lagoon, Carcinus aestuarii (see box). Among the nooks andcrannies of breakwaters and outer sea walls lurks what is probably the mostrobust of the Mediterranean crabs, the yellow or warty crab (Eriphia spinifrons),accompanied in the intertidal zone by the very fast-moving Mediterraneanshore crab (Pachygrapsus marmoratus).The echinoderms include Echinocardium cordatum and Schizaster canaliferus,two heart-shaped sea urchins which live buried in sediment. Rocky substratesare home to Paracentrotus lividus, the classical sea urchin that causes somuch pain to careless bathers descending the cement blocks often used tocreate breakwaters.

■ Benthic fauna

In analogy to the succession ofdifferent types of bed, which from sandbecome progressively silty sand, siltand then clay, populations of benthicmacroinvertebrates evolve - from thecommunities classically attributed tothe biocoenoses of fine sand in tidaldeltas, to fine surface sand and muddysand in environments sheltered from

the more open areas of the central basin, until we reach the euryhaline andeurythermal lagoon biocoenoses of the more internal parts. We will thereforestart from an imaginary inlet in a tidal delta, first on the seaward and then on thelandward side.In the area facing the inlet, we find species typically connected with substrateswith a high sand content and more turbulent waters. Some species of bivalvesare well-known in fish markets, like the clam (Chamelea gallina) and the razorshell (Ensis minor), whose elongated shape allows it to bury itself rapidly in thecase of danger. Tellins (Tellina pulchella, T. nitida, T. tabula, Donax semistriatusand D. trunculus) are also extremely common.Frequenters of beaches are certainly familiar with Acanthocardia tuberculataand A. echinata, bivalves with a lovely heart-shaped shell, the surface of whichis characterised by radial longitudinal ribs with more or less pronouncedtubercles, marked by deep inter-rib furrows. These species are accompaniedby the miniscule Lentidium mediterraneum, millions of which often remainstranded at low tide, rapidly destined to enrich the fine shell detritus on theshore, the white Spisula subtruncata and the larger Mactra stultorum.It is also easy to identify some of the gastropods, such as the murex (Bolinusbrandaris), together with its cousin Hexaplex trunculus, an active predator ofbivalves whose spawn, like small white sponges, are often beached along theshore, and Nassarius mutabilis, a small ventricose snail which acts as ascavenger, feeding mostly on dead animals.Some families of molluscs arouse particular interest, in that species belonging tothem succeed one another in a characteristic manner along the transitiongradient. This is the case of venerid bivalves, mainly represented here byChamelea gallina, and nassariid gastropods, represented by Nassarius mutabilis.Polychaetes include Owenia fusifromis, a tubicolous filtering worm, which livesin the sediment in a tube it creates by cementing grains of sand, and Arenicola

45

Nassarius mutabilis

44

Sand crab (Liocarcinus vernalis)

The isopod Tylos latreillei passesdaytime hours buried in dry sand,usually a long way from the foreshore;at sunset it comes out into the open onto the wet beach, where it feeds onvarious organic materials: beachedmarine animals, algae, etc. Beforedawn, it is once again resting in the drysand. Regarding this species, worthy ofmention here is the mass migrationobserved on Volano beach on theevening of 10 August 1968 by AntonioGiordani Soika, an attentive anduntiring scholar of the natural history ofthe Veneto lagoons and the Po Delta. He saw millions of individuals, composingan almost compact column a dozen metres wide, marching on the damp sandalong the shore, in a southerly direction.Many of these species enter the lagoon and settle in internal areas in tidal flats.Here the razor shell (Solen marginatus) replaces Ensis minor, while Chameleagallina is less frequent, being flanked by other venerids like Dosinia lupinus andPaphia aurea. The Philippine clam (Tapes philippinarum) starts to put in anappearance here, but is much more common in the next zone. This species,introduced into the northern Adriatic in the last twenty years, has the typicalcharacteristics of invasive species, such as a high reproduction rate and greattolerance to estuarine and lagoon environments. Unfortunately, very destructivesystems are used to harvest it. As well as the Philippine clam, other exoticspecies introduced by man have become established in these environments -in particular, the bivalve Scapharca inaequivalvis and the gastropod Rapanavenosa. The spread of Scapharca is easily explained by the fact that it isresistant to anoxia, being able to rely on haemoglobin as a respiratory pigment.The large gastropod Rapana venosa, of Indo-Pacific origin, causes problems,since it is a predator which grazes on mussel and oyster beds. Its flesh isedible, but not greatly appreciated.The extremely common mussels (Mytilus galloprovincialis) find their ideallagoon habitat in this zone. Indeed, this is where artificial mussel-beds are setup, prior to being moved some miles offshore, where the contribution ofbrackish water stimulates the growth of phytoplankton and whereenvironmental conditions during the critical summer and winter periods aremore stable, guaranteeing a longer growing period.

47

Mussels (Mytilus galloprovincialis)

46

Echinocardium cordatum

Nassarius mutabilis declines in abundance, being replaced by other nassariidssuch as Nassarius corniculus, N. nitidus and, to a lesser extent, Cyclopeneritea. Nassariids pass their time buried in the sediment, leaving only theirsiphons protruding, and with these they monitor the water, waiting to detect thesmell of the dead or injured animals on which they feed. Their sense of smell isparticularly acute, as they have a chemical sensor inside their siphons enablingthem to locate odours dozens of metres away with great precision.The zone adjacent to lagoon inlets often contains luxuriant seagrass meadows,with a prevalence of Cymodocea nodosa and Zostera marina. This is a species-rich environment, where organisms which benefit from organic matter in thesediment that accumulates at the base of the plants are associated withorganisms which choose the fronds as their preferred habitat. Among theformer, both oligochaetes and sedentary polychaetes are to be found inabundance, such as the capitellid Notomastus latericeus, which is tubicolousas a juvenile but lives in tunnels in sediment as an adult.The invertebrates populating the seagrass fronds include isopods like Idoteabaltica, which look like small fragments of leaves. They may sometimes be seen“navigating”, clinging to pieces of leaf that they use as boats, pushed by thebeating of their paddle-shaped pleopods. Tanaidacea like Apseudes latreilleiare also frequent, living in small tubes on the surface of the seagrasses, andeasily distinguished from isopods by the pincers on their first pair of limbs. A

great many amphipods live among the seagrass fronds, like the gammaridGammarus aequicauda, and graceful shrimps like Palaemon elegans, with theirslender legs ringed in yellow and blue, and P. adspersus, which nibble at anyparticles they are able to grasp. Browsers on the microalgal film that grows onthe leaf blades include various species of gastropods, among which Gibbulaare particularly widespread, for example, G. adriatica, and the small cerithiidgastropods Bittium reticulatum and B. scabrum.In this environment rich in submerged vegetation, Asterina gibbosa also maketheir appearance - delicate starfish a couple of centimetres in diameter, and thesmall and fascinating anthozoan Anemonia viridis, which anyone who hasventured into underwater meadows without adequate protection knows, totheir cost, because these anemones can sting with painful results.In the meadows closer to the inlets Pinna nobilis is also present. This is a prettypinnate-shaped bivalve of large size, whose fan may rise about thirtycentimetres from the bottom and whose byssus used to be spun and woven insome Italian localities (the island of Sant’Antioco off the south-western coast ofSardinia was once famous for this). P. nobilis is accompanied by the dark cockle(Chlamys varia) and the lucinid Loripes lacteus. The latter is an unusual bivalve,in that it plays host to bacterial symbionts in its gills. These fix carbon dioxidepresent in the water, using energy obtained from oxidation of sulphides, whichare abundant in the sediments at the base of seagrasses. Some of the organic

4948

Pinna nobilisPalaemon elegans

Marco SigoviniXilophagous organisms

Among the rigid substrates available tobenthic communities, wood has thedouble function of acting as a supportfor encrusting populations, and foodfor other organisms, mainly bacteria,fungi and xylophagous invertebrates(“wood-eaters”).As the activity of bacteria and fungi isvery slow, most of the macroscopicdegradation of wood is the work ofinvertebrates.These include bivalve molluscsbelonging to the family of the teredines,among which the most common areBankia carinata, Lyrodus pedicellatus,Nototeredo norvegica and Teredonavalis, which excavate large tunnelsas wide as a finger, and crustaceans -the isopods Limnoria lignorum andLimnoria tripunctata and the amphipodChelura terebrans, which are responsible

for a denser network of galleries.The evolutionary history of theseorganisms has mostly taken place withinenvironments like lagoons, estuaries anddeltas, where there are large quantitiesof wood in contact with the water.In heavily anthropised environments,the majority are breakwaters or otherwooden structures, placed there byman, and their degradation involveshigh replacement costs.The teredines have a wormlike bodywith a small shell at the front end.Evolutionary processes havetransformed this from a protection forthe animal into a digging implement,rendering it similar to the head of adrill bit, which these molluscs rotatewith movements of their robust foot.These animals colonise a site whenplanktonic larvae arrive and

subsequently metamorphose and growinside the wood, building tunnels alongthe entire submerged section.The dispersal dynamics of individualspecies are influenced by the durationof their larval stage, which lasts for twoor three weeks in T. navalis but only afew hours in L. pedicellatus. The tunnels,lined by a calcareous layer, havepractically imperceptible dimensions inthe first stretch, but increase markedlyin diameter and length as the organismgrows. The wood is pulverised andpartly used as food by means ofdigestion assisted by symbiont bacteria.Salinity and temperature are theprincipal environmental factorsdetermining spatial distribution:T. navalis, euryhaline (tolerating salinityof 10‰), is the species best adaptedto estuary environments; Lyrodus

pedicellatus finds its optimum in lessdesalinated conditions, and otherspecies require water more like that ofthe sea.Xylophagous crustaceans are alsowidely distributed in estuaries, deltasand lagoons. Although they lack aplanktonic larval stage, their diffusion isguaranteed by currents which transportindividuals to new sites.These organisms only attack woodwithin the mid-littoral belt. In synergywith the waves, the outer layersbecome detached, conferring thetypical “hour-glass” shape on highlydegraded wooden piles at the point ofmean sea level.Although the system of tunnels is oftenhighly unstable, it represents a newhabitat that can be colonised by manyother species of invertebrates.

50 51

The “hour-glass effect” caused by xylophagous organisms eating away the woodThe teredine Lyrodus pedicellatus

53compounds obtained from carbondioxide fixation are then transferred tothe bivalve.The central basin of a lagoon may bedivided into two large bands. The first,more external, facing towards the sea,has a succession of facies of classicalbiocoenoses of the muddy sands foundin sheltered environments; the second,more internal, towards the river and dryland, contains some facies of euryhalineand eurythermal lagoon biocoenoses. Inthe external band of the central basin,characteristic venerid bivalves are

Paphia aurea and the carpet shell clam (Tapes decussatus). As alreadymentioned, Tapes philippinarum is very common, if not completely dominant.Other species of bivalves characteristic of this zone are Corbula gibba, Gastranafragilis and Loripes lacteus.The rich phytoplankton and abundance of suspended organic matter explainthe frequency of other efficient filterers, such as Mytilaster minimus and theoyster Crassostrea gigas. The latter species is extremely common, both hereand in the more internal area. Oysters were introduced into Italian lagoons andestuaries around the middle of the last century and spread rapidly, creating trueminiature reefs in some sites which form a habitat of hard substrate -particularly important for the promotion of biodiversity in environmentsotherwise characterised by muddy substrates.The gastropods Nassarius reticulatus and Cyclope neritea abound on lagoonbeds, together with high numbers of Cerithium vulgatum and C. alucaster, withits elongated shell which may reach eight centimetres in length. Bittium andGibbula are also abundant on beds covered by macrophytes.Roving polychaetes include some predators of considerable size, sometimes afew dozen centimetres long, such as Nephthys hombergii, Marphysa andGlycera. In this band, the numbers of nereidid polychaetes like Alitta succinea(=Neanthes succinea) and the ragworms Perinereis cultrifera, P. rullieri andHediste diversicolor begin to be significant, although they reach their peak inthe next area. The mouth apparatuses of these polychaetes, even when dueaccount is taken of scale, are extraordinary and formidable: among thepredators, eunicids like Marphysa possess many pairs of mandibles of varyingshape, whereas Glycera has a characteristic evertible pharynx with four hooked

52 mandibles. Although they cannot bedefined purely as predators, thenereidids possess two powerful,falciform, serrated mandibles, with aset of conical teeth. Among thesedentary polychaetes, some aretubicolous like Amage adspersa andthe maldanids, also known as bambooworms, because of the unusualsegmented structure of their body,which is similar to a bamboo cane.The cirratulids of the genus Cirriformiaare sedentary polychaetes of a reddish-orange colour, with long filaments liketentacles, which give them the appearance of jellyfish. They live buried justbeneath the surface of the sediment, from which their tentacular filamentsemerge to capture the organic matter on which they feed.The high eutrophication of lagoons, estuaries and deltas, especially in thecentral basin, also explains the abundance, among the polychaetes, of speciestypical of environments with high organic loads, such as Capitella capitata,Heteromastus filiformis and Polydora spp.Polychaetes of the genus Polydora are not only found in areas rich in organicmatter. When we eat an oyster or observe the inside of the valves, we maynote blackish blisters containing mud and covered by a thin layer of mother-of-pearl. In them live specimens of Polydora cornuta (=P. ligni) and P. ciliata.These polychaetes can perforate the shell of their host the oyster, aided byspecialised bristles and, probably, acid secretions, until they penetrate theinternal cavity. Sensing a foreign body, the oyster secretes a layer of mother-of-pearl around the hole made by the polychaete, forming a chamber withinwhich the worm then lives.Along the edge of channels, many holes about as wide as a finger may be seen:these are the ventilation shafts of the burrows of a crustacean prized as bait byfishermen: Upogebia pusilla. This thalassinid decapod, pale greenish-beige incolour, vaguely similar to scampi, reaches a length of around ten centimetresand lives in Y-shaped tunnels that reach down into the seabed to a depth ofabout fifty centimetres.The shells of gastropods, especially those of Nassarius nitidus, Cyclopeneritea and Cerithium, are often inhabited by the hermit crab Diogenespugilator, which is very abundant in this lagoon environment, especially

Hermit crab (Diogenes pugilator)Upogebia pusilla

along the edges of the largest tidal creeks between sandbanks and nearseagrass meadows.A characteristic anthozoan of this belt is Cereus pedunculatus, whichattaches itself by its basal disc to shells lying just below the surface of thesediment, from which its crown of tentacles protrudes.Some echinoderms are to be found In the most highly vivified areas.Examples are the holothurians Trachythyone tergestina and T. elongata, smallsea cucumbers a few centimetres in length, and the starfish Asterina gibbosain seagrass meadows.Many crustaceans can be found on hard substrates such as channel banksbuilt in bricks, stone or cement, the poles and brìcole (the typical triangulargroups of thick oak pites defining deep-water channels), including theamphipods Corophium acherusicum, C. acutum, C. sextonae, Stenothoetergestina and Jassa marmorata, and the cirripede Balanus amphitrite, easilyrecognised by its vertical purple stripes on a white background.We are now entering the innermost part of the central basin, an area of highsedimentation, distinguished by many sandbanks; the seabed is silty-clayeyand the water is often desalinated. All the inhabitants of this area generallytolerate both seawater dilution and the resulting decrease in salinity, andrelatively stagnant waters.The bivalves which characterise this zone are Cerastoderma glaucum, Abrasegmentum (=A. ovata) and, in the more brackish areas also containinggroundwater springs, Scrobicularia plana. Cerastoderma glaucum is veryabundant in this band. Its rounded, slightly heart-shaped form prevents itfrom sinking very deep into the substrate, from which it often protrudes,becoming easy prey for crabs.Scrobicularia plana is a bivalve reaching more than five centimetres in length,whitish in colour and flattened in shape, which facilitates its burial in muddysediments. Its long siphons brush the surface of the sediment, producingstar-shaped traces around the holes from which they protrude.The flattened shape and long siphons of Abra segmentum are very similar tothose of Scrobicularia, but are much smaller, being only about one centimetrelong. This bivalve also alternates a detrivorous diet with food, which it obtainsby filtering the water, and may be found in dense populations, sometimessubject to great fluctuations in numbers.It is in this belt that Nassarius nitidus progressively gives way to Cyclopeneritea. More than a general tolerance to environmental factors, it is likely thattheir methods of reproduction determine the distribution of these twonassariids. N. nitidus lays rows of pointed capsules containing around a

5554

Cereus pedunculatus

Two very common amphibious gastropods, Truncatella subcylindrica andMyosotella myosotis, are to be found on sandbanks.Lastly, this small world between water and land is home to the amphipodspreferring sandy shores belonging to the genera Talitrus, Talorchestia andOrchestia. These are extremely lively animals of marine origin, which hopabout swiftly on damp sand or amongst the beached detritus where they findtheir food.We are now nearing the estuary zone, where the entry of river water greatlyreduces the salinity of lagoon waters and enriches sediments with organicmatter.On deep bottoms, Cerastoderma makes way for Scrobicularia, while the tinygastropods of the genus Hydrobia accompany the amphipod crustaceansCorophium orientale and C. insidiosum in pools between sandbanks.Corophium looks like a sort of mechanical excavator in miniature - only 1 or 2centimetres in length, it has two very robust arm-like antennae about half thelength of its body, with which it displaces sediments, raking up the particulateon which it feeds. This is the zone where the roving polychaete Hedistediversicolor, which fisherman often use as bait, finds its most favourableenvironment. Despite being a highly resistant species, it does need a certainamount of oxygen, and so is often found in the intertidal zone. This polychaetereproduces only once during its lifetime, which lasts at most a few years.

57hundred eggs, preferably on seagrassleaves; the resulting larvae, beforesettling, pass slightly less than a monthas planktonic forms carried bycurrents. Conversely, Cyclope neriteadoes not go through a larval dispersalstage: it lays single eggs on hardsubstrates, such as shells, from whichhatch miniature adults that canimmediately cope with a less stableenvironment.A sedentary polychaete typical of thiszone is the spionid Streblospioshrubsolii. This small annelid, abouthalf a centimetre in length, lovesenvironments close to estuaries. It hasa very high growth rate and its coloniesare very large, so it represents a good

food source for the fry of many species of fish.Other polychaetes are associated with these species, such as Alitta succineaand the predator Nephthys hombergii. The behaviour of the nereidid A.succinea is particularly interesting, as it passes most of its existence on theseabed but, when it reaches sexual maturity, abandons it at night. The gametesof this worm are thus freed in the water close to the surface, which explainswhy, at certain times of the year, towards midnight, up to 4-5000 Alitta eggs perlitre of water may be counted.The most common amphipods are Microdeutopus gryllotalpa, which feeds onfragments of algae, the detrivore Melita palmata, and the filter-feederEricthonius punctatus. The hard substrates of this belt are encrusted withbarnacles (Balanus eburneus and B. improvisus), which also enter the nextzone, often in large numbers.Our curiosity will undoubtedly be aroused by mounds of mud that rise from thebottom of pools between sandbanks: these mark the exit holes of the complextunnels of another thalassinid crustacean, the ghost shrimp Callianassatyrrhena, which may extend for more than half a metre beneath the surface ofthe sediment. The name of this species comes from its white translucentappearance, which does make it appear as if it were the ghost of a prawn orscampi. Another characteristic of the species is the larger size of one of the twochelipeds (pincers), which is used for digging the tunnels.

56

Amphipod of the genus Talitrus

Nassarius sp.

■ Planktonic fauna

Mesozooplankton (planktonic animals0.2-2 mm long) play a very importantrole as food source for the larvae of fishand other filter-feeders. Their seasonalvariations in density follow those ofphytoplankton with a slight delay,peaking between the end of spring andsummer. The importance of plankton isrecognised for all filtering organisms ofthe benthos, but less expert naturaliststend to under-estimate the role ofplankton in the diet of vertebrates.Tangible proof of this comes from anexamination of the stomach contentsof the most common fish which live inthese basins. The planktophagousdietary regime of the young of five species of fish of commercial interest is well-documented, such as the sea bass (Dicentrarchus labrax), the giltheadseabream (Sparus auratus) and three species of mullet (Liza ramada, L. saliensand L. aurata). The young of all these species, until they reach a length of fivecentimetres, have a planktophagous diet, their preferred prey being calanids,polychaete larvae and cirripede nauplii. Whitebait (Atherina boyeri) are alsoactive consumers of plankton. Small individuals feed mainly on the moreminute fraction of zooplankton, composed of the earliest larval stages (nauplii)of copepod and cirripede crustaceans, as well as the larvae of polychaetes andmolluscs. As the fish grow, their attention turns more to the larger componentsof the plankton, such as mysidacean crustaceans and decapod larvae, as wellas benthic organisms, especially small polychaetes and amphipods.In proximity to the sea inlets, where fluctuations in salinity are morepronounced, the biocoenosis is rich in species, with cladocerans, cyclopoidcopepods and neritic harpacticoids (for example, Oithona spp., Oncaea spp.,Euterpina acutifrons) and tunicates (appendicularians).In lagoons, zooplankton is mainly marine (neritic: i.e., typical of the open sea)around the inlets and in the more external belt of the central basin. In internalparts it also includes a native component, composed of organisms that canpass their entire life-cycle in brackish waters, and also types of freshwaterorigin. An important component of meroplankton are the larvae of various

59Gametes are released more or less in synchrony in the entire population. Eggsand sperm leave the animal through an opening in the body wall, almost alwaysclose to the head. However, not all animals of the same age reproduce in thesame year; some late developers mature during the following year, thusconstituting a valuable reserve if any unforeseen events compromise thebreeding success of their siblings.Victorellid bryozoans are to be found on hard substrates. The hydrozoanCordylophora caspia and the tubicolous polychaete Ficopomatus enigmaticusare introduced species capable of creating reefs of great size.The amphipods Gammarus aequicauda, Leptocherirus pilosus and thetanaidacean Heterotanais oerstedi form large populations in the first stretchesof this sector, but they are also found in other areas of the central basin,because of their tolerance to environmental factors.Shallows particularly rich in organic matter, in which there is very little availableoxygen, are inhabited by capitellid polychaetes, including Capitella capitataand Heteromastus filiformis. Such is the affinity of Capitella for this type ofsediment that its larvae appear to be attracted by the hydrogen sulphide thatrises from it. The very abundant and tiny members of some species of tubifidoligochaetes are often to be found together with Capitella. Capitellids andoligochaetes are also abundant in environments rich in organic matter and notdesalinated, as in the deposits forming at the base of seagrasses.

58

The harpacticoid copepod Euterpina acutifrons

Tubicules of polychaetes Ficopomatus enigmaticus

Calanipeda aquaedulcis, Canuellaperplexa, Halicyclops sp., Harpacticussp., Microarthridium fallax, Acartiamargalefi and A. tonsa.In the Lagoon of Venice, the seasonalabundance of zooplankton is at itsminimum in January, with dominantspecies Acartia clausi, Paracalanusparvus, Centropages spp. and Oithonaspp. There is a slight increase in April,and then a period of growth dominatedby Acartia tonsa until the peak in July.This is followed by a rapid, progressivedecline in the following months.Absolute maxima and minima, in summer and winter respectively, are recordedin the most internal areas of the lagoon, mainly due to the thermophily (heat-loving characteristics) of the dominant species. The larvae of polychaetes,molluscs, crustaceans, decapods and fish peak during late spring. Again in theLagoon of Venice, in the last fifty years there have been marked changes in thespecific composition of the lagoon zooplankton, especially within the genusAcartia. The most typically marine species, A. clausi, is still abundant near theinlets, while the brackish-water species A. latisetosa has become increasinglyrare. A. margalefi typically occupies areas with intermediate characteristics, butsince the 1980s it has progressively declined in favour of A. tonsa which,recorded for the first time in the Lagoon of Venice in 1992, has now become thedominant species of the community.In the oligohaline part, where a mass of denser saltier seawater penetratesupriver near the bottom beneath the river waters, there is a mixing of brackishand river water components of the plankton. Freshwater plankton, composedmainly of rotifers, is joined by marine copepods and a few larvae of anothertypically marine group, the polychaetes.True plankton, naturally, exists in the sea and in still internal waters, especiallyin lakes. Identification of true plankton in a river environment may be debatable.In the flowing waters of a river, as well as swimming organisms (essentially fish),organisms typical of lake beds may also be found, snatched from their naturalenvironment and carried away downstream. However, it is difficult to imagine atrue population of small river organisms, incapable of any particularly activemovement, which can live perennially in the water without being inexorablycarried off down towards the sea.

61species of annelid polychaetes, gastropod and bivalve molluscs, and cirripedeand decapod crustaceans.Within the marine component of plankton, the important group of the copepodsis represented by many species: the calanids, typical of neritic plankton,include Acartia clausi and Paracalanus parvus; the cyclopoids include Oithonanana, O. similis, O. plumifera, the pecilostomatoids include Oncaea sp.,Corycaeus sp. and, lastly, the harpacticoids include Euterpina acutifrons andMicrostella norvegica. Among the cladocerans, which are more abundant in thesummer months, the most frequent species are Podon polyphemoides andPenilia avirostris. Other zoological groups, present at least seasonally, are thetunicates (appendicularians), hydromedusae, turbellarians, nematodes,ostracods, amphipods, mysids, chaetognaths (Sagitta sp.), and also the eggsand larvae of fish.A very curious although irregular presence is Noctiluca miliaris, a sphericalunicellular organism almost a millimetre in diameter - an exceptional size foran organism composed of a single cell. It is capable of emitting light ifstimulated by water movement - a phenomenon easily observed on theoccasions between spring and summer when Noctiluca reproduces in infinitenumbers in coastal waters. The components of the central basin, especially inthe internal part, include the larvae of cirripede crustaceans, some rotifers (twospecies of Synchaeta and Brachionus plicatilis) and many copepods:

60

The copepod Canuella perplexa

The copepod Calanipeda aquaedulcis

The fish fauna in the brackish waters oflagoons, deltas and estuaries includesspecies that are best classifiedaccording to the characteristics of theirbiological cycles. Some are constantlypresent throughout the year, likeanadromous species, the adults ofwhich migrate to fresh water to spawn,and juveniles which follow the same butinverse route to grow to maturity in thesea; conversely, the adults ofcatadromous species go to sea to breed, whereas juveniles ascend internalwaters. Then there are species present in brackish-water environments forvarious periods during the year, with transfers to the sea to avoid unfavourableenvironmental conditions and also for breeding - because they have floatingeggs which would not be able to develop in internal waters. Lastly, there areboth marine and freshwater species which are found in brackish waters onlyoccasionally.

■ Resident species

These are typically euryecious fishes, i.e., species able to tolerate wide andrapid variations in the chemical and physical parameters of the environment inwhich they live. Their capacities for adaptation allow them to pass their entirelife-cycle in lagoon environments. Lagoons are populated by a very diversifiedresident fish fauna, due both to the different environmental conditions inindividual areas, and to influences determined by the times and ways in whichlagoons came into being in their particular geographical locations.The populations of fish species in internal waters have been affected by severalevents since the Messinian geological epoch, during a salinity crisis in theMediterranean about 5 million years ago. In the northern Adriatic, fish were alsoinfluenced by preceding events in the middle Pliocene, when the Po Valley district

63FishesGILBERTO GANDOLFI

Black goby (Gobius niger jozo)

Whitebait (Atherina boyeri)

6564 to overwinter. They are gregarious species which feed on zooplankton. Thefemales release fertilised eggs into the incubating pouch of the males, whichfeed the embryos through blood vessel connections until their development iscomplete.The peacock blenny (Salaria pavo) is a species with individual territorialbehaviour, linked to the benthic environment and found on beds on which hardobjects abound (pebbles, stones, submerged pieces of wood, etc.) suitable asshelters. The females lay their eggs in the nests of the males, which protectthem until they hatch. This species has conspicuous sexual dimorphism.The black goby (Gobius niger jozo), a typical inhabitant of the Mediterraneanbasin, has similar biological characteristics to those of the peacock blenny:sexual dimorphism accentuated in the breeding season, parental care by themales, and a diet based on benthic invertebrates or small fish. Both the blackgoby and peacock blenny have poor adaptive capacity to highly desalinatedwaters.The three-spined stickleback (Gasterosteus aculeatus) may be found in lagoonareas scarcely influenced by salt waters and in channels adjacent to lagoons. InItaly, the species is absent in southern regions and in Sicily; elsewhere there arepopulations living in brackish waters and also freshwater ones, even far fromthe coast. In the breeding season, the male assumes a conspicuous livery andbuilds a nest using plant material. He then courts the female, inducing her to lay

was affected by links then existingbetween the Adriatic and para-Tethysseas, and by more recent events, duringthe Pleistocene (approximately 2 millionyears ago), caused by phenomena in theIce Ages. As regards the fish fauna ofinternal waters in general, the Po Valleydistrict is richer in species and endemicforms than peninsular Italy and theislands, for the reasons mentionedabove and also because of the greater

quantities of internal waters and the consequent greater abundance and varietyof transition environments between fresh and sea waters.The characteristic common to the resident fish species in lagoon waters is thatthey are usually smaller than the marine or freshwater species belonging to thesame genera or families. Living in a highly unstable environment, with frequentand rapid variations in temperature, availability of dissolved oxygen, salinity ofthe water and availability of food, these populations anticipate sexual maturityas much as possible, with the consequently reduced size of adults. In otherwords, they adopt a strategy of population growth, defined as type r, in whichthe adults produce a large number of descendants which quickly replace them.In their turn, the new generations exploit favourable environmental conditionsand rapidly reach sexual maturity, managing to survive critical periods,although in fewer numbers.In lagoons, environmental crises generally occur in summer, because shallowwaters heat up rapidly and there is a consequent reduction in oxygen, whichdissolves in an amount inversely proportional to temperature. Conversely,autumn spates in rivers carry large quantities of nutrients into the lagoonswhich, after low winter temperatures, trigger a phase of high production in thefood chain in spring. This section describes resident fish species which arecommon to all Italian lagoons.The whitebait (Atherina boyeri) is a gregarious species, feeding mainly onzooplankton and also adaptable to freshwater environments.The tiny South European toothcarp (Aphanius fasciatus) is also a gregariousspecies with a wide ecological range: it prefers lagoon shores rich in vegetation,but may also live in fresh water and pools with high salt concentrations.The pipefish (Syngnathus acus) and the black-striped pipefish (Syngnathusabaster), with their slim threadlike bodies, live in lagoons on sandy or muddybeds with abundant aquatic vegetation, sometimes migrating to coastal waters

Pipefish (Syngnathus acus)

Three-spined stickleback (Gasterosteus aculeatus)

■ Catadromous species

After feeding in internal waters, theadults of these species move intobrackish waters prior to migrating tothe sea in order to breed, while thejuveniles make the reverse trip. Theirpresence in brackish waters is thuslimited to specific periods of the year.Only one catadromous species, the eel(Anguilla anguilla), passes through theestuaries and deltas of Italian rivers in transit. It is known that the adults, havingmigrated from European waters for thousands of kilometres to reach theSargasso Sea in the Atlantic Ocean, spawn there and die after releasing theirgametes. The larvae (leptocephali) are transparent and have a characteristicallyflat shape. They are carried by the Gulf Stream towards the European coastlineover a period varying from one to more than two years and, once they reach thecontinental shelf of Europe or enter the Mediterranean, by which time they areabout 7 cm long, they metamorphose and assume a cylindrical shape. At thisstage, they are called elvers, and begin to ascend rivers, especially at nightwhen there is a rising tide. In Italian waters, this happens from October in thewest until February in the Adriatic. The elvers distribute themselves over a widerange of environments: coastal brackish waters, canals, ponds, rivers andtorrents, and lakes. They prefer soft bottoms and waters rich in vegetation,where they act as predators, feeding on invertebrates and small fish,prevalently at twilight and during the night. Their stay in fresh or brackish waterslasts for a few years - three to four for those which stay in brackish waters or onthe plains, and up to seven or eight years for those living in colder waters. Theformer appear to be destined to become males, the latter females. Thisdifferentiation takes place after migration to the sea, as the adults which begintheir breeding migration in autumn have not yet developed gonads.With maturity, their morphological characteristics modify, the colour on theirbacks varying from brownish-yellow to brownish-black, that of the belly fromyellowish to silvery; the eyes become larger, the skin thickens, the pectoral finsbecome more pointed and the intestine begins to degenerate because thebreeders do not feed during their long migration. They transform into what areknown as silver eels, with lengths and weights varying from less than 45 cmand less than 200 g for those destined to differentiate as males, up to morethan 1 m and 2 kg for the females.

her eggs in the nest and, after fertilisingthem, protects them by continuallyfanning them with his pectoral fins, toprovide them with oxygen and keepthem clean.In the lagoons of the northern Adriatic,as well as the black goby, there are alsoother species of the gobiid family.The grass goby (Zosterisessorophiocephalus) is the largest of thegobiids which has adapted to lagoon

environments, sometimes reaching more than 20 centimetres in length. It liveson muddy beds, in which the individuals of both sexes dig individual densamong the seagrass roots. In the breeding season, which begins at the end ofFebruary and lasts until May, the dens become more complex and are occupiedby one adult male, a few immature males, and two to four females.Canestrini’s goby (Pomatoschistus canestrinii) is a species endemic to thenorthern Adriatic, and frequents sandy beds. In the breeding season, the maleoccupies a territory around some small submerged object (a mollusc shell,submerged wood, seagrass rhizomes, etc.), beneath which the female lays hereggs. The male looks after the eggs, which hatch within a few days. Thepresence of Canestrini’s goby in a small lake at the mouth of the river Sinni inthe Gulf of Taranto is most probably due to accidental introduction into a fish-farm together with grey mullet fry.The lagoon goby (Knipowitschia panizzae) is a very small species that rarelyexceeds four centimetres in length. It is practically endemic to Italian waters,extending only as far as the lagoons of Dalmatia. Records relating to theLagoon of Lesina and lagoon environments of the Tyrrhenian coast veryprobably regard accidentally introduced populations that have recentlybecome acclimatised. The lagoon goby lives on muddy beds, and theterritorial behaviour of the male during the breeding season is similar to that ofthe previous species, but the hiding-place used as a nest is almost always ashell of some small bivalve of the genus Cerastoderma. Beneath this, aftercovering the half-shell with mud to conceal it, the male takes refuge to lookafter the eggs.Lastly, another representative of the gobiid family, Pomatoschistus tortonesei,is found in the Stagnone di Marsala in Sicily. This is another tiny species, evensmaller than the previous one, which is typical of the lagoons on the Africancoasts of the central Mediterranean; its biology is practically unknown.

6766

Fish-farm ponds in the Lagoon of Venice Eel (Anguilla anguilla)

69■ Anadromous species

The adults of these species, afterfeeding at sea, move seasonally intodeltas, estuaries and lagoons, toascend rivers until they reach suitableareas for spawning.After a stay in internal waters, theyoung individuals then take the inverseroute, to pass their growing stages inmarine waters until they reach sexualmaturity.The fish species which make breedingmigrations in internal Italian watersinclude the twaite shad and threedifferent species of sturgeon, plus twospecies of lamprey – the latterbelonging to a separate zoologicalclass.The breeding specimens of twaite shad (Alosa fallax) swim up Italian rivers inspring, to spawn on gravel beds, sometimes very far from the sea. The young,after an initial growth stage in the river, descend to the sea in autumn, wherethey remain until they reach sexual maturity. Recently, differences in thenumber of branchiospines have shown that the populations of the Tyrrhenianare best considered as a separate sub-species (rhodanensis) from those ofthe Adriatic (nilotica).Of the three species of sturgeon that in the past ascended the major Italianrivers in spring, the largest, the beluga (Huso huso), which is typical of theCaspian Sea, Black Sea and the Adriatic, has only sporadically been recordedin the Po for some decades, and appears to be close to extinction.The common sturgeon (Acipenser sturio), which in the past had a widerdistribution - ascending to spawn in all the major rivers of the northern Adriaticand also the Tiber and other rivers flowing into the Tyrrhenian Sea - seems to befollowing the same destiny.The third species, the Adriatic sturgeon (Acipenser naccarii), endemic to theAdriatic Sea and smaller than the others (rarely reaching more than 150 cmand 30 kg) appears to be able to pass its entire life-cycle in freshwater, asdemonstrated by the persistence of a small population in the middlestretches of the Po and lower stretches of the Ticino, upstream from barriers

68

Beluga (Huso huso)

Adriatic sturgeon (Acipenser naccarii)

71■ Seasonal species

A few marine species feed temporarilyin lagoons and estuaries, sometimesswimming for long stretches upriver,but they return to the sea both tospawn and to overwinter.Five species belonging to the mulletfamily are frequent in all the brackishwaters of the Italian peninsula: ● flathead mullet (Mugil cephalus): fryenter internal waters from August toDecember, and adults migrate to thesea from summer to early autumn;● thinlip grey mullet (Liza ramada): fryenter internal waters in autumn or atthe end of winter, and adults migrate tothe sea during the last months of theyear;● golden grey mullet (Liza aurata): fry enter internal waters in autumn or at theend of winter, and adults migrate to the sea from September to November;● leaping grey mullet (Liza saliens): fry enter internal waters in summer, whileadults migrate to the sea;● thicklip grey mullet (Chelon labrosus): fry enter internal waters from April toJune and adults migrate to the sea from February to April.These are all gregarious fishes with a diet based on plankton during the juvenilestages. Adults feed on algae and detritus rich in organic matter.These species are typical of lagoon and estuary environments with muddy andsandy beds; some swim upriver for varying stretches, being able to toleratevariations in salinity. In particular, the thinlip grey mullet has lengthened itsascent of the Po in the last few decades, now reaching distances of more than200 km from the delta in summer, probably as a consequence of recentchanges in the riverbed, which is much richer in organic matter than in the past.Other species which utilise brackish water for growing-on of juvenile stages arethe sea bass (Dicentrarchus labrax) and the gilthead seabream (Sparus aurata).In late winter and in spring, the fry of these two species enter lagoons to feedon zooplankton.Being less tolerant of desalinated waters and cold temperatures than themullets, they return to the sea during the winter months. Sea bass adults mostly

70 that would be insurmountable during migration. In the last year, operationsto repopulate the Adriatic sturgeon have been under way, and specimensfrom fish-farms have been introduced into the Po and the rivers of theVeneto region.Among the four species of lamprey present in Italian waters, two makespawning migrations in rivers after passing their trophic stage in the sea asectoparasites on fish or marine mammals.The sea lamprey (Petromyzon marinus) ascends rivers in spring or earlysummer, sometimes attached to the flanks of migrating twaite shad orsturgeon. It is found in all Italian rivers, whereas a second species, the riverlamprey (Lampetra fluviatilis), only goes up the estuaries of rivers flowing intothe Ligurian Sea and northern and central Tyrrhenian.Overall, the populations of species which make obligatory breeding migrationsfrom the sea to internal waters, or vice versa, are those which have been mostbadly affected by human acts of environmental degradation. As well as thenegative effects of water pollution on the development of eggs and initialgrowing stages of the species that make potamodromous (river) migrations,dams or other barriers built across rivers prevent adult individuals fromreaching the beds suitable for reproduction. Similarly, it is now oftenimpossible for eels to reach the stretches of rivers they inhabited in the pastduring their growing stage.

Sea lamprey (Petromyzon marinus)

Sea bass (Dicentrarchus labrax)

■ Occasional species

In certain conditions - for instance, the autumn spates of rivers, or the summerpenetration of the so-called “salt wedge”, which happens when waters are low -some typically freshwater or marine species may occasionally be found inbrackish-water environments, thus demonstrating their capacity to live withvery different degrees of salinity to that to which they are normally exposed.It is not rare, for example, to observe some species of cyprinids in estuarywaters, or the occasional pike (Esox lucius) in the most desalinated areas ofthe lagoons. Sporadically, in the hottest summers, the trout (Salmo trutta), afish typical of mountain and hill waters, may even reach the sea in search ofcooler waters.This sporadic behaviour mimics the regular anadromous migrations of the troutwhich populate the tributary rivers of the Black Sea, northern Atlantic Oceanand North Sea. The waters of the Mediterranean are too warm and salty for theneeds of this species.Recordings of coastal marine species are more frequent - for example, thesardine (Sardina pilchardus), sprat (Sprattus sprattus), corb (Umbrina cirrosa),red mullet (Mullus barbatus) and sole (Solea solea). These are very probablyonly episodic visits of species which are not resistant to significant variationsin salinity.

73prey on crustaceans and fish, and gilthead seabream mainly on bivalvemolluscs.The five species of mullet, together with sea bass and gilthead seabream, aretraditionally farmed in sometimes artificially enclosed brackish ponds. As thesespecies cannot reproduce in brackish water, the fry are caught as theyinstinctively try to move upstream to internal waters, or else, they are bred byartificial techniques in which the males are induced to mature by treatment withgonadotrophic hormones.The plaice (Platichthys flesus luscus) lives in the lagoons and terminal stretchesof the rivers of the northern Adriatic. This sub-species spawns in the sea in lateautumn and winter.The larvae, which still have bilateral symmetry, enter the internal waters wherethey undergo metamorphosis when they are slightly more than 10 mm long: theleft eye migrates to the right flank next to the right eye; the small fish acquiresbenthic habits, and its left flank gradually loses its pigmentation. The plaice,now flat, feeds on small invertebrates.Lastly, the sand goby (Pomatoschistus minutus elongatus) is found in lagoonand estuary waters along Italian coasts. It is slightly larger than its congenerswhich live permanently in lagoons. It spawns at sea and uses internal watersfor its trophic stage, returning to the sea in winter or when the rivers are in fullspate in early spring.

72

Sole (Solea solea).Fishing with large drop nets at the mouth of the Mignone (Latium)

75

The Po Delta

Estuaries, deltas and lagoons - thosecomplex transition areas between freshand salt waters, between land and sea -are environments where the incessantinterplay between the eroding action ofwinds and tides and the arrival of newmaterial from rivers continually remodelthe lines of the landscape and,following delicate equilibria, create aset of complex ecosystems in continualevolution.The deposition of sediments createsdunes, beaches and tongues of sandin the shape of sandbars which, overtime, will be demolished by the waves, the impetus of high river spates, orsubsidence.Vegetation plays a key role in stabilising and consolidating submerged andemerging sediments, which would otherwise be condemned to a state ofperennial movement. Marked geomorphological variability corresponds to avariety of habitats, and consequently gives rise to plant communities whichdiffer in their response to the salt gradient or particle size and texture of thesubstrate. Lagoon environments are differentiated into mudflats, sandbanks,tidal creeks, saltpans, marshes and inlets, which are all dominated byhalophilous (salt-loving) vegetation, and the coasts and islands which hostpsammophilous (sand-loving) vegetation.

■ Reed-beds

Reed-beds are one of the most representative and widespread types ofvegetation at river mouths and in coastal lagoons. In some situations - forexample, at the terminal stretches of the major Italian rivers (Po, Adige,Tagliamento), favoured by the width of the riverbed and slow current - reed-

Terrestrial vegetationFRANCESCO BRACCO · MARIACRISTINA VILLANI

Vegetation on coastal dunes

76 77beds are so extensive that they confera characteristic physiognomy on thedelta landscape. Reed-beds are easilyidentified by their typical appearance -a dense and impenetrable grassland ofgigantic herbaceous plants that maygrow more than three metres above thelevel of the water.The undoubted main actor in this plantcommunity is the reed (Phragmitesaustralis), a tall grass which thrives in avariety of environments: from coastalareas to an altitude of 2000 metres,from fresh to brackish waters, fromtranquil lakesides and ditches toriparian strips along streams, to lagoonsandbanks and marshes. Nor does thereed disdain environments without the

constant or frequent availability of water, like flood plains or riverbanks, even inhighly disturbed situations. Reeds have thin hollow stems, which may lengthento more than three metres, and grow straight and very close together, formingdense stands of vegetation. They bear two rows of greyish linear-lanceolateleaves with sharp edges, accompanied by fine tufts of hairs where they join thestem. The long culms of reeds become more conspicuous in summer, whengreyish-purple inflorescences appear at the top. Each panicle, 20-30centimetres in length, is formed of a large number of feather-like spikelets, eachbearing tiny flowers accompanied by soft and silky hairs. When the fruit haveripened, the entire spikelet breaks off the panicle, leaving only the basalelements (glumes) adhering. The hairs now become efficient organs of flight, toaid dissemination of the spikelet by air currents.The hypogeal (below-ground) apparatus of reeds is less obvious, but equallyunusual. Robust creeping rhizomes, buried in muddy beds, form a densesubterranean tangle, a sort of net that constitutes the foundations of the reed-bed. Each rhizome, covered in coriaceous scales, grows from year to year andmay reach several dozen metres in length, producing both anchoring roots andnew shoots at every node.Reeds are widely known for their traditional uses: at one time, the stems werecut during winter and used to insulate ceilings and roofs, to produce mats, stuffthe seats of chairs or make brushes. In the Polesine (part of the Po Delta), the

“arèle”, as reed mats are known locally, were placed vertically around the edgesof vegetable gardens as protection against the wind. Over time, these uses andpractices have progressively waned, but reeds are still exploited nowadays.They are successfully used in operations of natural engineering, thanks to thevigour of their vegetative reproduction. Portions of rhizomes or green cuttingsare collected in areas where reeds grow abundantly and transplanted towetland areas in need of environmental upgrading.Reeds are also one species very often used in the creation of phyto-purificationsystems, in which waste waters are channelled through a specially createdroute to prolong contact times with the plants. This permits efficient uptake ofnitrogen, which is thus removed from the water and stored in plant tissues. Thecapacity of reeds to survive conditions of prolonged submersion make themideal natural water purifiers.Owing to their wide ecological range, reed-beds may be found in differentterritorial ambits. In reality, the name reed-bed is used for different plantcommunities, which share physiognomic uniformity but which have substantialdifferences in composition, as the floral retinue changes according toecological conditions. The common denominator to all communities is the clearquantitative dominance of reeds, a highly competitive species that creates adense canopy, but which is not very conducive to the growth of other plants. Asa consequence, few accompanying species survive.

Reed-bed in the Po flood plain

Reed (Phragmites australis)

They are mostly heliophytes, i.e., with a subterranean portion that roots them tothe bed of the water-body, while the epigeal (above-ground) part emerges fromthe surface and lengthens in the subaerial environment. These are highlyspecialised plants. One of the main problems that plants in a reed-bed have tosolve is lack of oxygen, which is poorly soluble and diffuses slowly in water. Theroots, permanently buried in the asphyxial mud, with little oxygen, areparticularly affected. A mechanism that counterbalances this lack is thetranslocation of oxygen from tissues that have plenty of it to others that do nothave enough. The key to this process is the existence of a particularparenchymal tissue in the rhizome, called aerenchyma, which has many largeintercellular spaces that are interconnected forming a system of conduitsfacilitating the movement of oxygen. In this way, oxygen can easily reach theroot apparatus from the green epigeal parts, where it is produced byphotosynthesis or taken from the atmosphere.Reed-bed species also have to tolerate another negative characteristic of theanoxic mud in which their roots are buried. The oxygen-poor environmentstimulates the blooming of an anaerobic bacterial flora whose metabolismgenerates toxic products for the plants - although, by means of sophisticatedphysiological strategies, the latter manage not to suffer the consequences.Reed-beds are typical of wetlands and freshwater marshes, such as the smalllakes behind the dunes, but those which flourish best are found in the riparian

79Plants and coastlines

Humans have exploited deltasand lagoons throughout history.Since the earliest times, man and thesea, just like man and rivers, havecontended for space in repeatedcompetition, first conquered by one,then by the other, but more often withno lasting results, because nature hasoften re-appropriated surface areasthat man had previously succeededin wresting from it.A significant example of this ongoingbattle is the Lagoon of Venice, whichhas modified its geographyinnumerable times over the courseof millennia. When, for variousreasons, the coastline has receded,man has been able to exploit largeareas of grassland for crops or aspasture for livestock, sometimesalso building roads.An emblematic case of how plantscan help us to understand the past

structure of the territory is that of theancient road, the Via Annia which,traversing the plain between therivers Sile and Piave (NE of Venice),linked the two large Romansettlements of Altino and ConcordiaSagittaria.Vegetation gives clues as to why itsroute was moved further inland inRoman times (1st century AD): findsof fossilised plants, particularly thepollen of halophilous species,demonstrate the advance of thelagoon, which at that time invadedthe area corresponding to the currentvillage of Ca’ Tron (near Roncade,in the province of Treviso).In practice, the sea had won backwhat it had previously cededflooding the original route of theroad and making it necessary tobuild an alternative one, furtherfrom the coastline.

Mariacristina Villani · Antonella Miola78

0 1 2 km

Meolo

Cà Tron

Marteggia

Sile

PortegrandiVia Annia

Via Annia

Maximum limit

of lagoon expansion

San Donà

Caposile

Piave

LAGOON OF VENICE

The Roman road Via Annia (south of Treviso) in relation to the maximum expansion of the Lagoon

Marshy area at the mouth of the Agri (Basilicata)

belt along the terminal stretches ofrivers, in conditions of perennial orprolonged submersion. Here, othertall grasses are associated with reeds,such as reed canary grass (Phalarisarundinacea) and reed sweetgrass(Glyceria maxima), and sedges like thegreat pond sedge (Carex riparia) which,due to their height, manage to becompetitive in capturing light.Also frequently found are other speciesof interest because they are rare andappear in the Red Lists of species atrisk of extinction, such as the marshspurge (Euphorbia palustris) or the fen ragwort (Senecio paludosus), with itsyellow flower clusters that stand out among the reeds during the summer.In situations with shallow water and recurring, prolonged periods of drought,species typical of water meadows are associated with reeds, such as wood club-rush (Scirpus sylvaticus), with its characteristic triangular stem and bracts likelarge leaves that enfold the inflorescences; marsh woundwort (Stachys palustris)with its purplish-red corolla, purple loosestrife (Lythrum salicaria) and yellowloosestrife (Lysmachia vulgaris), with its bright yellow petals. Hedge bindweed(Calystegia sepium), with white funnel-shaped flowers, is almost always present,and succeeds in reaching the light by climbing up the reed stems. Anotherrecurrent, but less frequent climbing species is bittersweet (Solanum dulcamara),which has clusters of dainty purple flowers with orange anthers.

Halophilic reed-beds. Salinity does not represent a grave problem for reed-beds: they grow both in moderately brackish waters and in more saltyconditions and, in response to the saline gradient, are accompanied by speciesmore or less tolerant of salinity. In littoral zones, at river mouths and in creeks,where abundant freshwater mixed with seawater reduces the saltconcentration, indicator species typical of areas with a moderate level ofsalinity appear next to reeds, such as the sea club-rush (Bolboschoenusmaritimus) with its inflorescence composed of reddish spikelets; soft-stembulrush (Schoenoplectus tabernaemontani), with its greyish-blue stems; spear-leaved orache (Atriplex prostrata), the leaves of which are spear-like bladescovered by a mealy bloom; and sea aster (Tripolium pannonicum ssp.tripolium), with its capitulum with tube-shaped flowers forming a central yellow

81Heterotopy

The proximity of the Eastern Alps tothe coast and the existence of efficienttransport systems towards the sea byrivers has led to the presence of whatare more properly Alpine speciesof plants in coastal areas and, inparticular, close to river mouths.The survival of their plant populationsis probably closely linked to rivercurrents regularly carrying seedsfrom higher land. This is an extremecase of heterotopy, a biogeographicalterm that describes the existence ofpopulations of species dislocatedoutside their typical distributionarea, due to transport of dispersedseeds. For example, the presence atthe northern Adriatic river mouths of

some species of shrubs which areusually components of holm-oakwoodland is interpreted in this way:rock buckthorn (Rhamnus saxatilisssp. saxatilis) and purple broom(Cytisus purpureus) are examples.One large tree also shares thispeculiar phyto-geographicalcondition: the Austrian pine (Pinusnigra) which, although it growsspontaneously on the dunesat the mouth of the Tagliamento,is generally typical of the north-eastern calcareous uplands.The presence of spring heath (Ericacarnea) in this context has also beeninterpreted in this way, but it mayalso be a relict of the last Ice Age.

Francesco Bracco · Mariacristina Villani

Marsh spurge (Euphorbia palustris)

Austrian pine (Pinus nigra) at the mouth of the Tagliamento (Friuli Venezia Giulia)

80

because they are covered with cells that secrete the excess salts absorbed bythe plant - a sophisticated physiological specialisation that allows them tosurvive high concentrations of toxic chlorides.The composition of halo-hygrophilic reed-beds also includes the annualseablite (Suaeda maritima), a plant species of the Chenopodiaceae that iscalled roscan in the hinterland of the Veneto lagoons. This term is alsocommonly used to designate other species belonging to the same familywhich, like the annual seablite, are well-known because they are edible.In halophilic reed-beds, another annual species plays a similar role. This is aglasswort (Salicornia veneta), an annual succulent species with fleshy stemsand branches that appear to be leafless. The flowers, rudimentary and joined intwo groups of three, are also only just observable with the naked eye. As theLatin name suggests, glasswort is a common and endemic plant of the Venetocoast, where it forms the most external strips of vegetation of the sandbankson the lagoon edges, which are periodically submerged by seawater.Another species composing halophilic reed-beds is shrubby glasswort(Sarcocornia fruticosa), a perennial plant with woody stems at the base andfleshy above; more robust than the previous Chenopodiaceae, it mayexceptionally reach one metre in height. With a similar morphology to that ofSalicornia veneta, with the exception of its size, shrubby glasswort used to beeaten - although nowadays pollution in the lagoons makes this unadvisable.

83disc surrounded by purple ligulate petals. In these situations, the saltmarshaster (Symphyotrichum squamatum) is also frequent: an alien species presentin disturbed environments and capable of living in soils with a modest saltcontent. It is a plant with flowering branches equipped with very thin linearleaflets bearing a large number of small heads of minute flowers which, atdissemination, develop a delicate pink pappus.Where the effects of seawater are more marked, salinity becomes a stronglyconditioning factor that has two effects: on one hand, it limits the vigour ofreeds, which grow shorter; on the other, it reduces floral richness, because onlya few species, with particular adaptation strategies, can tolerate highconcentrations of salt. In perennially haline environments, like sandbanks andsalt-marshes, some species are able to join the floral retinue of reed-beds,although, in conditions of more accentuated salinity, they would form plantcommunities and become dominant. The most eye-catching of these includethe sea-lavender (Limonium narbonense), belonging to the family of thePlumbaginaceae. This plant is insignificant in the vegetative stage, when it hasonly a basal rosette of spatula-shaped leaves, but becomes striking in latesummer and autumn, when the floral stem lengthens, bearing a corymbedpanicle dense with small pink-purplish flowers. These maintain their colour forsuch a long time, even when the corollas dry, that they are often gathered andsold in decorative bunches. The leaves and stem are sticky to the touch

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Flowering of pink sea-lavender (Limonium narbonense) Salicornia veneta

In areas where the influence of brackishwater is higher, halophilic reed-bedsgive way to Puccinellio palustris-Scirpetum compacti, we find thetypical formations of sea club-rush(Bolboschoenus maritimus).This robust member of the Cyperaceaepresents many affinities with reeds: arobust hypogeal apparatus, densestems, and a wide ecological rangethat allows it to live in both freshwaterand oligohaline environments.Another species that can tolerate lowrates of salinity is round-headed club-rush (Scirpoides holoschoenus), whichforms communities in which it isdominant, often in damp areas behinddunes.The bed of rushes is unmistakable when the inflorescences appear at the top ofthe stems: they have the appearance of small balls subtended by a bract thatlengthens upwards almost as though it were an elongation of the stem.If salinity increases, beds of maritime rushes appear, with sea rush (Juncusmaritimus), a species that is prickly to the touch, with its stems and rigidpointed leaves borne by robust horizontal rhizomes. Vegetation with sea rushflourishes best on sandbanks, where it usually grows in the more internal andhigher areas, where the effect of tides is felt less.One of the interesting types of vegetation in wetter areas with weak salinedistribution is Mariscetum serratae, the term used for the communitydominated by sawgrass (Cladium mariscus), a vigorous species belonging tothe family of the Cyperaceae. The stem, cylindrical at the base and triangular atthe top, a few centimetres across, bears leaves with serrated, sharp marginsfolded along the midrib. The inflorescence, which develops during the summer,consists of an anthela composed of more overlapping and often interruptedbrownish anthelas. This species is more common in freshwater environments,but its capacity to tolerate low levels of salinity means that it frequently appearsin delta areas. Although sawgrass is considered a sub-cosmopolitan species,i.e., diffuse over most continents, the drainage operations that have taken placein coastal areas in the past have drastically reduced the presence of thesecommunities throughout Italy.

85■ Other marsh vegetation

The tall herbaceous plants thataccompany reeds in reed-beds maytake on a dominant role in favourableconditions and characterise thephysiognomy of the vegetation, so thatreeds are relegated to a subordinaterole. An example are the tall grasslandsdominated by bulrushes (Typha latifoliaor T. angustifolia). Less extensive incomparison to reed-beds, these plantcommunities do not pass unobserved,especially during their flowering andfruiting seasons.The species of the Typha genus havean unusual inflorescence, in which themale and female flowers form two

separate clusters along the terminal axis. The male flowers, tiny and withoutpetals, constitute the upper portion of the inflorescence. The female flowers,which are also tiny and without petals, form a kind of velvety sausage,composed of many thousands of microscopic flowers set very close together.Each female flower is composed of an ovary and supported by a peduncle,surrounded by a whorl of long bracts. Not only the position, but also the timethe inflorescences remain on the axis are different: after the anthers haveopened and the pollen is freed, the male flowers have finished their biologicaltask, so they fall and leave a section of bare flowering axis above the sausage-shaped spike. Instead, the female part turns brown after flowering, andconserves the different parts of which it is formed. The ovary, transformed intoa small fruit, remains at the top of the peduncle. On this the bracts, which laterplay an important role, are also preserved. During the winter, the inflorescencedries and dissemination takes place: in response to hygroscopic stimuli, thebracts spread out and cause the floral peduncles to break; the sausage-shapedspike thus disintegrates and the seeds are transported by the wind in the formof cottony wisps.These communities are generally richer floristically than reed-beds, probablybecause bulrushes form less dense populations than reeds and create lesssevere shading conditions - and also because the long linear leavesperpendicular to the ground do not develop dense canopies.

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Typha latifolia Sea club-rush (Bolboschoenus maritimus)

■ Consolidated dunes and damp hollows: a complex vegetational landscape

As well as the more halophilous vegetation, the combined morphogeneticactivity of the sea and rivers creates a complex landscape, which is also richin communities far removed from psammophilous, marshy, salty or brackishones.A complex environmental picture is created that flanks forestry aspects, xeric(heat-loving) vegetation, internal hollows containing freshwater, and coastalwater-bodies of brackish or salt water. This is possible because of the detailedgeomorphology of the landscape in both raised areas and low-lying ones. Thehigher areas generally correspond to dunes stabilised by vegetation, and maylie at some distance from the present-day coastline. Hollows are stronglyinfluenced by the presence of salty or brackish water if they are in contact withthe sea: conversely, if they are affected by the water table or surface water ofland origin, they will have typically freshwater characteristics.In places where plants find it difficult to survive, as in the northern Adriaticlittoral arc, the complex environmental situation also provides a little space forspecies of plants with different geographical distributions and very differentecological requirements. In this area, and as far south as the Po Delta, on thetops and flanks of consolidated dunes where the evolutionary process of thesoil is more advanced, the landscape at river mouths hosts woodlands of holm

87Low salinity is also tolerated by cordgrass (Spartina versicolor), a robust grassthat forms dense carpets on dunes in formations where it tends to substitutemarram-grass, although it is also found on the edges of lagoons. Lesshalophilous than its congener small cordgrass (Spartina maritima), S. versicolormainly reproduces vegetatively, through subterranean rhizomes that give rise totussocks of leaves: because of this, its grasslands are very compact andspecies-poor. When they do appear, the inflorescences are composed of threeor four sessile ears up to five centimetres in length.Lastly, one peculiar aspect of the vegetational landscape of the deltas shouldbe mentioned - the belts of halo-nitrophilous vegetation. These communitiesare dominated by annual species with the distinctive feature that they cantolerate moderate salt contents and the physiological aridity that follows, butalso high nitrate contents. Although often covering small areas, they play animportant role as pioneer plants which anticipate the other more strictlyhalophilous species. The best-represented families are the Chenopodiaceae,with the oraches (Atriplex tartarica, A. prostrata) and the Polygonaceae, withgolden dock (Rumex maritimus) and sea knotgrass (Polygonum maritimum).Their presence is often associated with disturbed environments. Natural plantdebris is often accompanied by quantities of waste transported and depositedby water - a tacit but manifest accusation of modern man’s uncivilisedbehaviour towards nature.

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Sea knotgrass (Polygonum maritimum) Mouth of the Reno (Emilia Romagna)

88 oak (Quercus ilex), the evergreen oak more typical of the Italian Mediterraneanlandscape. This does not appear to be an exceptional case, as this woodlandformation grows on the dune areas at the mouths of the Tagliamento and Adigeand in the Po Delta. The low impact of this woodland in today’s landscape isdue to the relatively small extent of the dune and palaeo-dune systems, butalso largely to the progressive elimination of this type of vegetation by man’sexploitation of coastal areas.In contrast to the image of instability and strong dynamism of the coastallandscape, in these situations the soil - although essentially composed of seasand, and thus dry and well-drained, appears to be well-structured and has asignificant accumulation of organic matter in its shallower levels.The holm oak can form a closed, continuous tree canopy that may reach aheight of 20 metres. Together with the holm oak, the most common treespecies is the flowering ash (Fraxinus ornus), which is always co-present, butplaying a decidedly subordinate role. Two generally Mediterranean tree speciesbut with different phyto-geographical characteristics are therefore associated:the holm oak, typically distributed in the circum-Mediterranean areas withmarked summer aridity (steno-Mediterranean), and the flowering ash, with adistribution area that covers not only the northern coasts of the Mediterranean,but also gravitates around the Black Sea (euro-Mediterranean-northern Pontic).The shrubs beneath the trees develop into a rather scattered cover, with amixture of species with differing phyto-geographical characteristics.The tree vegetation of the dunes is associated in the landscape with shrubcommunities, which represent structurally simpler evolutionary stages of thevegetation cover, gradually preparing for colonisation by forest species.The first stage of colonisation by woody vegetation is that of low-growingshrubs, less than one metre high, defined as the precursor of the holm oakwoodland, characterised by climbing shrubs with a small herbaceouscontingent. The dominant species are wild asparagus, osyris and the heathErica carnea, accompanied, especially in the most northerly part of the Adriatic,by purple broom (Cytisus purpureus) and the buckthorn Rhamnus saxatilis ssp.saxatilis, a small spiny shrub with reddish bark and small brown flowers withfour petals. Very frequent, although not abundant, is the wall germander(Teucrium chamaedrys), a prostrate dwarf shrub, with small coriacious lobateleaves. The herbaceous species mainly derive from the neighbouring aridgrasslands and, among these, tor-grass (Brachypodium rupestre) and cypressspurge (Euphorbia cyparissias) are frequent. It is within these ambits that theholm oak makes a timid appearance together with the flowering ash, althoughonly at the seedling stage.

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Mosaic of vegetation at the mouth of the Reno (Emilia Romagna)

be found only rarely on the peaty soils ofthe high plain, montane and subalpinebelts. The delicate environment in whichit lives means that it is endangeredthroughout Europe, and its presence incoastal vegetation has also beendrastically reduced, so that it now onlysurvives in a few sites, such as thoseclose to the mouth of the Tagliamento.More generally, where freshwaterconditions converge towards the generalmesotrophic or eutrophic state moretypical of surface waters on the plains,the marsh and aquatic vegetation foundin ditches and flooded hollows tends to coincide with that present in the water-bodies of the plains. In the slow-moving waters of the Po Delta, for example,aquatic communities have been recorded with fennel pondweed (Potamogetonpectinatus), a species which may also penetrate into brackish water. In thebodies of freshwater, the typical communities are dominated by severalhydrophytes, all with large leaves growing on the surface of the water: the whitewater-lily (Nymphaea alba), yellow water-lily (Nuphar lutea), fringed water-lily(Nymphoides peltata) and water chestnut (Trapa natans). Only extremely raregroupings of marestail (Hippuris vulgaris) have been recorded.The hollows may also contain hygrophile or mesophile woodland vegetation,dominated by species of deciduous broadleafs that are in stark contrast to theevergreen woodlands on higher ground. This type of vegetation is even rarerthan that of the holm oak woodland. A prime example is the Bosco dellaMesola, which has been growing for the last thousand years on the complex ofdunes and depressions created by the sedimentation of the Po di Volano andPo di Goro, within the Po Delta. The forest community that settles on the moreancient dune systems with flatter morphology is, on one hand, reminiscent ofholm oak woodland and, on the other, is related to the mesophile forestvegetation typical of the plains. The soil is moderately damp and onlysporadically water-logged, so a multilayered forest develops, with tall trees,many shrubs, and sparse herbaceous undergrowth.Lastly, the survival of marsh woodlands with alder (Alnus glutinosa) is veryinfrequent on plains, but becomes exceedingly rare at river mouths. For example,there are small areas of alder woods on soft black soil at the mouth of the Adige,at times with marsh marigold (Caltha palustris) in the herbaceous undergrowth.

91Where the sea is close enough toallow the salty air to reach the cordonsof stabilised dunes, the vegetation isunable to form holm oak woodland,but reaches its climax as compactshrubland dominated by juniper(Juniperus communis), identifiable byits spiny-tipped leaves in whorls ofthree and its bluish-black berries.To this is associated, generallysubordinately, the sea buckthorn(Hippophae fluviatilis), a denselybranched shrub with silvery bark, linearleaves that are white beneath, and

characteristic bright orange berries. This community is completed by many ofthe shrubs already mentioned for holm oak woodland and scatteredherbaceous species including bladder campion (Silene vulgaris ssp. tenoreana)and the sedge Carex liparocarpos.River mouth environments sometimes contain systems of depressions that areoften affected by salinity, but others - for instance, along the coast of the Venetoand Friuli plains - are influenced by the presence of fresh surface and groundwater,with low nutrient loads and high concentrations of calcium carbonate. In thesecases, the depressions may contain accumulations of peaty sediments on whichunusual plant communities settle and which, at sea level and at a short distancefrom the coast, sometimes include species that require cool environments.Typical examples are the peaty grasslands of purple moor-grass (Molinia caeruleassp. caerulea) that are reminiscent of those in spring areas on the north-easternplain, of which they reproduce a good part of the floral retinue. This includes blackbogrush (Schoenus nigricans), narrowleaf plantain (Plantago altissima), fragrantleek (Allium suaveolens) and marsh gentian (Gentiana pneumonanthe). In slow-moving or still surface waters within these grasslands, rare hydrophytecommunities of waters with low nutrient contents may also find refuge. Anexample is the aquatic vegetation dominated by fen pondweed (Potamogetoncoloratus forma heterophyllus) with both floating and completely submergedelliptical leaves, both types being translucent, veined, and often reddish.The rosemary-leaved willow (Salix rosmarinifolia) also appears here as a woodycolonising species. This shrub, rarely exceeding a height of two metres, has linearor linear-lanceolate leaves that are hairy and silvery beneath, and dark green andshiny above. It is a plant with Euro-Asiatic distribution, which in northern Italy is to

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Rosemary-leaved willow (Salix rosmarinifolia) Marsh marigold (Caltha palustris)