An experimental study of plant habitat

choices by macroinvertebrates in brackish

soft-bottom bays

by

Hanna Axemar

Supervisors:

Joakim Hansen & Lena Kautsky

Plants & Ecology

Plant Ecology 2007 /9 Department of Botany Stockholm University

Plants & Ecology Plant Ecology Department of Botany Stockholm University S-106 91 Stockholm Sweden © Plant Ecology ISSN 1651-9248 Printed by Solna Printcenter Cover: Myriophyllum spicatum with epiphytes in a shallow soft-bottom bay. Photo by Joakim Hansen.

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Summary During the last decade there has been an increased research focus on shallow soft-bottom bays in the Baltic Sea. This study examined animal and plant interactions in this environment, which previously has been scantly studied. The habitat preference of four macroinvertebrates between three different aquatic plants was studied in experiments on the Askö laboratory in July 2006. The aim of the study was to examine if macroinvertebrates have any habitat preference between plant species and between single and multi-species habitats. The study was conducted with pair-wise comparisons of the habitats. A total of four different habitats were planted; three habitats of a single plant species and one habitat consisting of all three species. The hypotheses were that a single species habitat with a more complex plant structure will be preferred over a less complex by macroinvertebrates and that a heterogeneous multi-species habitat will be preferred by over a homogeneous single-species habitat. The most significant result showed that the amphipod Gammarus spp. clearly preferred a habitat consisting of Myriophyllum spicatum, with its delicate and complex structure, and the habitat with all three plants combined; M. spicatum, Potamogeton pectinatus and Chara baltica. The isopod Idotea spp. made a similar choice, however not significant. The habitat preference of the gastropod Theodoxus fluviatilis was not significant, but this species had a trend of preferring P. pectinatus. The gastropod Bithynia tentatculata did not make any active habitat choice.

Sammanfattning Det är först under det senaste decenniet som forskning om Östersjöns grunda mjukbottenvikar har kommit i fokus. Den här studien undersökte interaktioner mellan djur och växter i dessa miljöer, något som tidigare ringa studerats. Fyra makroevertebraters preferens av växthabitat bestående av tre olika vattenväxter studerades i experiment på Askölaboratoriet i juli 2006. Syftet var att ta reda på om makroevertebrater har någon habitatpreferens mellan olika växtarter, samt mellan enarts- eller flerartshabitat. Studien genomfördes med parvisa jämförelser mellan habitaten. Totalt planterades fyra olika habitat; tre enartshabitat och ett habitat bestående av alla tre växtarter. Hypoteserna var att ett enartshabitat med en mer komplex växtstruktur föredras framför ett mindre komplext habitat av makroevertebrater, samt att ett heterogent flerartshabitat föredras framför ett homogent enartshabitat. Resultaten visade att de olika djuren har olika habitat preferens. Det mest signifikanta resultatet visade att märlkräftan Gammarus spp. hade en klar preferens för både axslinga Myriophyllum spicatum, med sin fina komplexa struktur, samt habitatet bestående av alla tre växter; M. spicatum, borstnate Potamogeton pectinatus och grönsträfse Chara baltica. Tånggråsuggan Idotea spp. visade tendens på ett liknande val, dock utan signifikans. Östersjöbåtsnäckan Theodoxus fluviatilis gjorde inte något signifikant habitatval, men visade en tendens att fördra P. pectinatus. Bithyniasnäckan Bithynia tenatculata gjorde inte något aktivt habitatval.

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Introduction The Baltic Sea is a brackish and geologically young sea. The flora of the Baltic Sea consists

of both marine and freshwater species (Snoeijs 1999). In the northern Europe there is a

continuous isostatic land rise, which results in a succession of coastal bays that are slowly

isolated from the sea. These shallow, soft-bottom, wind and wave-protected environments

have a rich flora community consisting mainly of freshwater angiosperms and charophytes

(Munsterhjelm 2005).

Submersed plants can affect the associated aquatic fauna community in different ways. The

structure of the plant is important for the macrofauna by both offering the animals shelter to

avoid predators or to help the predator lurking on its prey. Plant species that have a

morphological structure with many branches and delicate leaves have been observed to house

a rich macroinvertebrate fauna community (McAbendroth et al. 2005; Humphries 1996; Xie

et al. 2006). Furthermore, a larger leaf area is likely to contain a higher density of epibionts,

and thereby offers more food. The aquatic plants could also be a direct food source for several

macrofauna species (e.g. Bodström & Mattilla 2005, Kornijow et al. 1995, Nicotri 1980,

Hartvig & Kraufvelin 2004, Kotta et al. 2004). Moreover, the whole composition of the flora

community can influence the fauna. A heterogenic community, containing several species of

plants, may theoretically house a more diverse fauna by offering an addition of niches in

comparison to a homogenic plant community (Statzner & Moss 2004).

Disturbance, like eutrophication, can alter interactions between organisms in an ecosystem.

Enhanced levels of nutrients in coastal bays have the consequence of favouring some plants

more than others, which results in changes in the plant community (Schramm & Nienhuis

1996). In eutrophied shallow sheltered coastal bays of the Baltic Sea charophyte populations

have declined and been replaced by angiosperm species more tolerant to turbid conditions,

e.g. Ceratophyllum demersum, Myriophyllum spicatum and Potamogeton pectinatus

(Munsterhjelm 2005). These changes can in turn affect the fauna community in the bays.

Other forms of disturbance, like introduced (non-native) species, pollution, boat traffic and

dredging may in a similar way affect the plant and animal community (Eriksson et al. 2004,

Schramm & Nienhuis 1996, Cheruvelil et al. 2002). For example Eriksson et al. (2004)

reported a decline of flora species in shallow areas of the Baltic Sea with excessive boat

traffic.

4

The research on shallow soft-bottom bays of the Baltic Sea was until the 1990´s limited, but

has during the last years received more focus. Surveys of the submersed vegetation and fish

community have been conducted in several areas (e.g. Dahlgren 2002, Andersson 2000,

Persson & Schreiber 2004, Persson & Johansson 2006, Edlund & Siljeholm 2003). Many of

these studies have been conducted within the framework of the EU habitat and water

directive. However, there are few studies on the macroinvertebrate fauna community in

vegetated shallow soft-bottom bays of the Baltic Sea.

The aim of this study was to see if there were any difference in the habitat preference among

macroinvertebrates between three submerged plants; Myriophyllum spicatum, Potamogeton

pectinatus and Chara baltica. The plants were chosen as they differ in tolerance to

eutrophication, differ in structure and possess different levels of allelochemistry. A further

aim was to test the effect of plant diversity on the habitat choice by the macroinvertebrates

comparing a single species habitat with a three species habitat. For the study four common

invertebrates in shallow soft-bottom bays of the Baltic Sea were used; i.e. Idotea spp,

Gammarus spp, Bithynia tentaculata and Theodoxus fluviatilis.

The following hypotheses were tested;

I.) A habitat with a more complex plant structure will be preferred by the individual animal

taxon compared to a less complex plant structure.

II.) A heterogenic habitat consisting of three plant species will be preferred by the individual

animal taxon compared to a homogenic one plant species habitat.

Material and method

The study was conducted in July 2006 at the Askö laboratory, situated in the Trosa

archipelago (N58º 49' E17º 38'). The habitat choice of the fauna species were studied in pair-

wise sets of flora species and for these plastic boxes where used (5.58 * 3.6 * 2.8 dm on the

inside). In each end of these, two different habitats were created consisting of one of the three

aquatic plant species, M. spicatum, P. pectinatus and C. baltica, or a mixture between the

5

three of them. A layer of 4-7 cm of sand covered the bottom of each box. The sand was

collected at the southeast part of the island (Fig.1) and filtered through a 5 mm sieve. During

the experiment the containers were filled with about 50 L of filtered seawater.

A.

C.B.

Figure 1. A map over Askö island in the Trosa archipelago. (A) Askö laboratory where the study was

conducted (N58º 49' E17º 38'). The plants and animals were collected from (B) Norra Flan and at

some occasions the animals also were taken from (C) Södra Flan.

The plants were collected from a shallow soft-bottom bay, Norra Flan (Fig. 1). All of the

plants grew at a depth of approximately 1.2 m. The site was chosen because all of the

concerned species were well represented. In the laboratory the plants were rinsed from

epiphytes, eggs, roe and fauna. For each habitat 22 g wet weight was planted (7.33 g * 3 in

the mix). The wet weight was obtained by spinning the plants in a salad spinner for about 1

minute and thereafter weighted (Delta range PB303; 2mg-310g, deviation=1mg). The plant

biomass was decided by estimating the abundance of the plants in the field. Plants were kept

for 3-5 days in sea-tempered, oxygenated water under ~80 PAR before being used in the

experiment.

In the experimental boxes plants were planted on an area of 5.25 dm2 (1.5 * 3.5 dm). The

pair-wise comparisons of the four habitats resulted in 6 combinations. Hence, one replicate of

all pair-wise combinations was constituted of 6 boxes. These were kept in a large tub (20.5 *

6

20.5* 6 dm) with circulating seawater, taken from a depth of 15 m, to cool the water in the

containers. Two replicates were run at the same time and a total of 9 replicates were done

over a 5 weeks period.

The animals were collected from two sites, Norra Flan and an adjacent shallow bay, Södra

Flan. The animals were collected from Fucus vesiculosus plants growing on the border

between hard and soft-bottom. The reason for this was that there were not enough animals to

be found in the soft-bottom vegetation further into the bay. The animal species were kept in

separate aquariums and fed F. vesiculosus with epiphytes. About 12 hours before being used

in the study, the food was removed from the animals.

In each box 10 individuals of Gammarus spp, T. fluviatilis and B. tentaculata were put

together with 6 individuals of Idotea spp. We mainly used Idotea viridis, but some Idotea

baltica were also put in the boxes because of the difficulties of finding enough specimens for

the replicates (I . baltica were evenly distributed between the different habitats). The number

of animals was decided depending on the abundance of animals found together with the

minimum of individuals needed to get a good statistic result. The animals were dropped in the

middle of each box and then allowed to swim around freely between the habitats for 24 hours

where after a plastic divider was placed in the middle. For each replicate the plants/habitats

were randomly planted, so that a specific location in the big tank would not affect the results.

The long side of each box was placed to the south and the experiments were terminated when

the sun was standing in the south. Thereby both habitats in each box had equal amount of

sun/shadow. Then the animals were counted to determine their distribution between the two

habitats. The plants and animals were then kept frozen at -18 to -20oC until further analyses.

Gammarus spp were later identified into 3 different species, G. oceanicus, G. locusta and G.

salinus.

During the 24-hour period oxygen and temperature was measured at two occasions, once in

the afternoon after starting the experiment and once in the morning the following day before

terminating the experiment. There was no significant difference in oxygen and temperature

between the pair-wise comparisons (p<0.4; ANOVA in R.2.4.0).

7

Study organisms

Plants

Myriophyllum spicatum, Potamogeton pectinatus and Chara baltica were chosen because

they are among the ten most common macrophytes in shallow soft-bottom bays from

Blekinge up to Söderarm (G. Johansson, Upplandsstiftelsen, pers. comm.). They also co-

occur in the same order of plant succession with the isolation of soft-bottom bays from the

sea, caused by the isostatic uplift of the landmass (Munsterhjelm 2005).

Myriophyllum spicatum (L.) is a submersed plant, which thrives in eutrophic waters (Preston

& Croft 1997; Stanley et al. 1986). It is the fourth most common aquatic plant found on

shallow soft-bottom bays in Sweden from Blekinge up to Söderarm (G. Johansson,

Upplandsstiftelsen, pers. com). There is no known documentation (to my knowledge) of M.

spicatum being grazed on by macroinvertebrates. This may be related to its production of

allelochemicals (chemical substances that may affect other organisms in its surroundings)

(Lindén & Lehtiniemi 2005). Myriophyllum spicatum has been observed to have an inhibiting

effect on phytoplankton (review in Gross 2003) and to be harmful or even deadly to

macroinvertebrates (Lindén & Lehtiniemi, 2005, Dhillon et al. 1982).

Potamogeton pectinatus (L.) is a submersed plant that can be found in nutrient waters on

sand, clay or mud bottoms (Preston & Croft 1997, Mossberg & Stenberg 2003). It is common

in eutrophic waters and can be found in both moving water and in water standing still. It

easily exploits newly made unconquered areas and often dominates with plants growing

closely together (Preston & Croft 1997). In shallow soft-bottom bays of the Baltic Sea it is

documented as the most commonly occurring aquatic plant (Blekinge up to Söderarm; G.

Johansson, Upplandsstiftelsen, pers. comm.). There are few documentations of grazing by

macroinvertebrates on P. pectinatus, but in the Baltic Sea I. baltica have been observed to

feed of the plant to the same degree as on epiphytic algae (Bodström & Mattila 2005).

Chara baltica (Bruzelius) is the tenth most common aquatic plant found on shallow soft-

bottom bays in Sweden from Blekinge up to Söderarm (G. Johansson, Upplandsstiftelsen,

pers. com). It separates itself from the other two angiosperms by belonging to the alga and the

class Charophyceae. They differ from other alga by not attaching directly onto hard

substrates. Instead they use their root-like parts to fasten themselves in soft substrates. Most

charophytes grows preferably in sheltered environments (Tolstoy & Österlund, 2003). Several

8

submersed angiosperms have most of their biomass closer to the surface, while the

charophytes have their closer to the bottom and may therefore be more sensitive to poorer

light conditions (Blindow 1992). In the Baltic the charophytes are threatened and have already

declined in several areas. The main reason for this is probably eutrophication (Schubert &

Blindow 2003). Chara baltica can be found from the Bothnian Bay along the coast down to

Öresund and Skagerak. Charophytes are known to produce allelochemical substances. This

also includes C. baltica, which contains allelopathic substances that has been shown to inhibit

epiphytic algae growth (Wium-Andersen 1982).

To measure the morphological structure of the three different plants 12 individuals of each

species were taken from 4 randomly chosen replicates of the non-mixed habitats.

Additionally, 5 individuals of each species were taken directly from their growing place (N.

Flan) (Fig. 1). The plants were pressed, dried and scanned (grey scale) using the software

program Arc Soft Photo Studio 5.5.0.70. Thereafter the area and perimeter were measured

using the program ImageJ 1.37v. Because of practical difficulties to measure the perimeter of

M. spicatum this was only conducted on 5 individuals. Furthermore, the number of branches

on each plant was counted. For M. spicatum and C. baltica, which are more delicately

branched than P. pectinatus, only the main branches were counted. Thereafter the total

number of branches was calculated by multiplying the number of main branches with the

number of divisions per branch and leaf taken from an average value from the literature

(Mossberg & Stenberg 2003; Schubert & Blindow 2003). These three different measures of

morphological structures; area, perimeter and number of branches; were divided separately

with the dry weight of each plant. An average for each plant species was then calculated. This

resulted in an index that helps to describe the differences of the morphological structures

between the plants in the study (Table 1). Myriophyllum spicatum had the highest average in

all the three categories. Both M. spicatum and C. baltica have more branches (though very

delicate ones), bigger perimeter and a larger respectively an almost as big area as P.

pectinatus.

9

Table 1. Features of morphological structures of the plants included in the study. The three measures

of morphological structures are; area per dry weight (A/D), perimeter per dry weight (A/D) and

branches per dry weight (A/D).

Plant species A/D (inch2/g) P/D (inch) B/D (no.) Chara baltica 13.8 651.7 849.2 Potamogeton pectinatus 15.8 543.5 440.2 Myriophyllum spicatum 21.9 3396.7 6353.2

Animals

In the Baltic Sea there are three different species of Idotea (Crustacea); Idotea granulosa

(Rathke), Idotea baltica (Pallas) and Idotea viridis (Slabber) (Salemaa 1979). Idotea viridis is

together with Ascellus aquaticus (L.) the most common isopod in shallow soft-bottom bays in

the Baltic Sea (southern Bothnian Bay to the northern Baltic Proper; J. Hansen, Stockholm

University, pers. comm.). It is more tolerant to lower salinity than many other species of

Idotea (Naylor 1955 a). In general Idotea seem to be omnivorous; however the different

species may have a different preference of food depending on what their habitat may offer

(Naylor 1955 b). Idotea feed for example on macrophytes, filamentous algae and sometimes

of each other (Boström & Mattila 2005, Goecker & Kåll 2003, Nicotri 1980, Naylor 1955 b).

However, the preference of food seem to differ depending on the size of the Idotea (Naylor

1955 a). According to Nicotri (1980) Idotea with their flat body shape are well suited for

broad algae with a thick thali (e.g. fucoids; Pavia 1999), which it can grab on to if there are

heavy water movements. It also provides them with an effective protection against predators

(Nicotri 1980). The reason for choice of habitat for Idotea is not quite clear. According to

Nicotri (1980), it is likely that Idotea chooses habitat because of the morphology and

availability of a plant rather than because of a feeding preference. According to other more

recent studies on I. baltica however, it seems that the choice of the animals is more related to

food quality (or availability) than shelter from predation (e.g. Boström and Mattila 1999;

Orav-Kotta & Kotta 2004). Though this depends on what kind of food that is offered and

whether it is under eutrophicated conditions or not. In a study by Salemaa (1978) on rocky

shores in the northern Baltic Sea, the breeding period for both I. baltica and I. viridis was

observed to start in late May. Before the end of July most of the juveniles had hatched after

been kept in a pouch by the female. The population of both species were largest in September.

After mating the males die off and the females disappear as soon as the eggs are hatched. The

time for this behaviour can stagger geographically (Kroer 1989).

10

Gammarus spp. (Crustacea) is also considered selective omnivores (Kotta 2004) and eats

everything from macrophytes, detritus, periphyton to each other (Christie & Kraufvelin 2004;

Goecker & Kåll 2003; Kraufvelin et al. 2006). They are very important grazers in the northern

Baltic and are usually found under rocks, in crevices or in algae (Salovius 2004). In the

central Baltic Sea the different gammarids separates themselves from gammarids in other

areas as they are not separated because of their different preference of salinity. Though, they

still differ in the habitat preference concerning depth, substrate and exposure (Fenchel &

Kolding 1979). The reason for habitat choice of Gammarus is unclear. In a study on G.

Locusta by Kraufvelin et al. 2006, it seems that macroalgae are more important as a habitat

than as a food source. In my study three species of Gammarus were found; Gammarus

oceanicus (Segerstråle), Gammarus salinus (Spooner), and Gammarus locusta (L.). These

three species have also previously been found in shallow soft-bottom bays from the southern

Bothian Bay to the northern Baltic Proper (J. Hansen, Stockholm University, pers.comm.).

They are also found along the shallow coasts of northern Europe. Some of the Gammarus

species have different time periods for breeding, dominating and a difference in numbers of

generations that they produce per year (Fenchel & Kolding 1979; Kolding & Fenchel 1979).

Bithynia tentaculata (L.) (Gastropoda) is a common freshwater snail in shallow soft-bottom

bays in the Baltic Sea (southern Bothnian Bay to the northern Baltic proper; J. Hansen,

Stockholm University, pers.comm.). It can receive food through both scraping and suspension

feeding. The later seems to be the preferred way for it (Brendelberger 1993).

Theudoxus fluviatilis (L.) (Gastropoda) is a very common freshwater snail in the northern

Baltic and is usually found on hard bottoms (Skoog 1971), often attached to hard substrates

like rocks and F. vesiculosus (Haage & Jansson 1970). On soft-bottoms they keep to the

vegetation (Skoog 1971). It is a scraping grazer (Steinman 1996, cited in Liess 2006) who

mainly eats diatoms, cyanobacteria and green algae (Skoog 1978).

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Statistics

The habitat choices of the fauna were tested by binominal goodness-of fit tests in the software

program R.2.4.0. Fisher’s exact chi2– test was used to explore any difference in the preference

of habitat between the three Gammarus species and the two Isopod species.

RESULTS

Gammarus spp. showed the clearest habitat preference among the fauna species used in the

experiment. It had a strong preference for M. spicatum and the mixture consisting of all three

species (Figure 2). However there was no significance in the selection between M. spicatum

and the mixture. Gammarus spp. had the least preference for P. pectinatus. In none of the six

combinations was P. pectinatus preferred. Among the other animal groups some preference

can be seen, but the results are less clear. The habitat preference of Idotea spp. was only

significant for the combination of M. spicatum and C. baltica. However, the distribution of

individuals between the different combinations of habitats is similar to that of Gammarus (but

not significant). Theodoxus fluviatilis preferred P. pectinatus to M. spicatum and the mixture

over C. baltica. The only species who did not make any significant selection of habitat was B.

fluviatilis.

Table 2. The numbers of each species of Gammarus spp. and Idotea spp. that were used in the study.

Number of individuals %

Total Gammarus 507 100%G. oceanincus 464 92%G. locusta 16 3%G. salinus 20 4%Unidentified 7 1% Total Idotea 308 100%I. viridis 264 86%I. baltica 44 14%Unidentified 0 0%

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There was no significant difference in preference between G. oceanicus, G. locusta and G.

salinus or between I. baltica and I. viridis (p>0.1). Though in the combination of M. spicatum

and C. baltica, the significance for the choice made by the 3 species of gammarids was not far

from significant (p=0.055). In other words there was a tendency towards the 3 gammarids not

choosing the same when it comes to that specific combination of habitats. However, the

number of individuals of G. locusta and G. salinus was very low (Table 2). The numbers of I.

baltica was also low and therefore, it was decided not to separate the animals into different

species groups of Idotea spp. and Gammarus spp.

Idotea spp.

0.0

0.2

0.4

0.6

0.8

1.0

Potpect

Chabal

Potpect

Myrspic

Chabal

Myrspic

Potpect

Mix Chabal

Mix Myrspic

Mix

Dis

tribu

tion

Gammarus spp.

0.0

0.2

0.4

0.6

0.8

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Potpect

Chabal

Potpect

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Myrspic

Potpect

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Mix

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ribut

ion

Theodoxus

0.0

0.2

0.4

0.6

0.8

1.0

Potpect

Chabal

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Chabal

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ion

*

Bithynia

0.0

0.2

0.4

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Potpect

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Potpect

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Mix Myrspic

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ribut

ion

** *** *** *** *** ns

ns ns ns ns ns ns ns * ns ns * ns

ns ns *** ns ns ns

Figure 2. Habitat preference of Gammarus spp., Idotea spp., Bithynia tentauclata and Theodoxus fluviatilis

between three species of macrophytes and a mix of the three macrophyte species; Potamogeton pectinatus

(Pot.pect), Chara baltica (Cha.bal) and Myriophyllum spicatum (Myr.spi). Bars show mean distribution of

individuals (±95% CI) between the macrophyte habitats. Significance according to; *** p<0.001, ** p<0.01,

* p<0.05, ns p>0.1).

13

Discussion

The taxon that showed the clearest result in their choice of habitat was Gammarus. They

preferred M. spicatum and the three species mixture to the other plant habitats. The hypothesis

was that the macroinvertebrates would choose the most structurally complex habitat

according to the measured parameters on morphological structure (Table 1). Myriophyllum

spicatum had the largest area, perimeter and the most branches of the three plants and was

therefore the one with the most complex structure. The habitat choice of Gammarus is in

accordance with other studies where complex structured macrophytes contains a larger

biomass of invertebrates than those with a less complex structures (e.g. Jeffries 1993,

McAbendroth et al. 2005)

According to the second hypothesis a more diverse habitat would be preferred over a less

diverse habitat by macroinvertebrates. It is possible that Gammarus therefore chose the mixed

habitat because of the combination of all three plants. However it is also likely that they chose

the mixture because M. spicatum was represented in it. In the combination of M. spicatum and

the mixed species habitat there was no significance in the preference between them. Hence,

Gammarus seem to prefer a diverse habitat or the structurally complex M. spicatum.

In the study Idotea spp. showed no clear habitat preference, besides the combination C.

baltica and M. spicatum, where they preferred the latter species. However the pattern of

preference was similar to that of Gammarus, although it was not significant. Hence, the two

tested crustacean taxa in this study appear to favour the delicate and highly branched M.

spicatum over C. baltica and P. pectinatus.

Myriophyllum spicatum is known to be allellopathically inhibit e.g. phytoplancton (review in

Gross 2003). It can also have a negative effect on animals. In two different studies Lindén &

Lehtiniemi (2005) and Dhillon et al. (1982) found M. spicatum to be very harmful and even

deadly to mysids and mosquitoes. In another experiment on Daphnia the repellent effect of

Myriophyllum on the zooplankton was suggested to not only be chemical but also structural

(Lauridsen and Lodge 1996). The different response to the plant by Gammarus and Daphnia

is not contradictory considering the difference in size and feeding choice between the two

crustaceans.

14

There are different theories of explaining the habitat choice of Idotea (e.g. Boström and

Mattila 1999; Orav-Kotta & Kotta 2004; Nicotri 1980). For example some of them claim that

it is the feeding that decides, while others say that it is the morphology of the plant. Idotea

probably chooses their habitat for a similar reason as Gammarus. The less significant results

for Idotea compared to Gammarus may be a result of that a fewer specimens of Idotea were

used in the experiment.

The plants used in the study were fresh, only 1-3 days old. This can have affected the results.

Macroinvertebrates, like amphipods and isopods, have been observed to prefer decomposed

angiosperms and algae (e.g. charophytes) to fresh ones (Kornijóv 1995; Kotta et al. 2004;

Salemaa 1987). The reason for this may be that on decayed plants the microorganisms have

made the cell walls less resistant and the nutrition more available to herbivores (Birch et al.

1983; Mann 1988). Furthermore, in fresh Chara there is a lot of calcium (Schubert &

Blindow 2003), which may be hard to digest for the macroherbivores. Perhaps the results

would have looked different if the experiment had taken place in the autumn instead of

summer, when the plants would have begun to break down. If in fact the feeding is the main

reason for these animals choosing a habitat. Furthermore, it is possible that if the plants would

have had macroepiphytes, the result may have been different. Orav-Kotta & Kotta (2004)

showed that I. baltica under eutrophical conditions chooses the habitat with the epiphytes (for

feeding), while under conditions with little epiphytes it is the morphology (shelter) of the

plants that decides the choice.

Theodoxus fluviatilis chose P. pectinatus over M. spicatum and the mixture of all three plant

species over C. baltica. A possible reason explaining this pattern is that both M. spicatum and

C. baltica produce allelopathic substances. Chara species are known to have an inhibiting

effect on epiphytes (Wium-Andersen 1982) and M. spicatum have been shown to inhibit

planktonic microalgae growth (Körner & Necklish 2002). Therefore the two plants might not

have as much epibionts growing on them as P. pectinatus. Although the plants were cleaned

of macroepibionts before the experiment, a difference in microepibiont density on the plants

can have affected the choice of T. fluviatilis, and P. pectinatus was preferred. The tendency of

preference of the mixture may presumably depend on P. pectinatus being one of the plants

there.

15

In the case of B. tentaculata there was no significant preference in any combination of

habitats. Most of B. tentaculata individuals were found in the sand under the plants rather

than on them. As mentioned before B. tentaculata has two ways of feeding; grazing and

suspension feeding. Since suspension feeding seems to be the preferred way of feeding

according to Brendelberger & Jürgens (1993), perhaps this species simply took shelter in the

sand under or in a randomly chosen plant habitat and fed through suspension-feeding.

Another possible explanation for the borrowing in the sand could be that the sand contained

small food particles like e.g. bacteria.

Considering that this study only had 4 different animal species and just a few individuals of

each, it is hard to see weather or not they would prefer a mixed habitat with more niches for

the different species to inhabit. Perhaps the competition was not strong enough to show this as

might be the case out in the field. Another thing one should consider is that perhaps the result

would have looked different if the experiment would have taken place out in the field instead

and not in an artificial environment. Adding to this the removal of predators and food in

forms of epiphytes, could also have affected the outcome. The optimal habitat ought to

provide both food and shelter for the animals.

Conclusions

This study shows that the delicately branched M. spicatum is a preferred habitat over less

complex macrophytes C. baltica and P. pectinatus for the amphipods Gammarus spp. A

habitat of all three species was not preferred over a single species habitat with M. spicatum,

but over single species habitats containing the two other macrophytes. The results were

similar for the isopod Idotea spp., however only the combination of M. spicatum and C.

baltica, where M. spicatum was the preferred one, was significant. The gastropod T. fluviatilis

preferred P. pectinatus over M. spicatum and the mixture of all three plant species to C.

baltica while B. tentaculata showed no preferences. Hence, the habitat preference can vary

between different macroinvertebrate taxa.

16

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20

Serien Plants & Ecology (ISSN 1651-9248) har tidigare haft namnen "Meddelanden från Växteologiska avdelningen, Botaniska institutionen, Stockholms Universitet" nummer 1978:1 – 1993:1 samt "Växtekologi". (ISSN 1400-9501) nummer 1994:1 – 2003:3. Följande publikationer ingår i utgivningen: 1978:1 Liljelund, Lars-Erik: Kompendium i matematik för ekologer. 1978:2 Carlsson, Lars: Vegetationen på Littejåkkadeltat vid Sitasjaure, Lule Lappmark. 1978:3 Tapper, Per-Göran: Den maritima lövskogen i Stockholms skärgård. 1978:4: Forsse, Erik: Vegetationskartans användbarhet vid detaljplanering av

fritidsbebyggelse. 1978:5 Bråvander, Lars-Gunnar och Engelmark, Thorbjörn: Botaniska studier vid

Porjusselets och St. Lulevattens stränder i samband med regleringen 1974. 1979:1 Engström, Peter: Tillväxt, sulfatupptag och omsättning av cellmaterial hos

pelagiska saltvattensbakterier. 1979:2 Eriksson, Sonja: Vegetationsutvecklingen i Husby-Långhundra de senaste

tvåhundra åren. 1979:3 Bråvander, Lars-Gunnar: Vegetation och flora i övre Teusadalen och vid Auta-

och Sitjasjaure; Norra Lule Lappmark. En översiktlig inventering med anledning av områdets exploatering för vattenkraftsändamål i Ritsemprojektet.

1979:4 Liljelund, Lars-Erik, Emanuelsson, Urban, Florgård, C. och Hofman-Bang, Vilhelm: Kunskapsöversikt och forskningsbehov rörande mekanisk påverkan på mark och vegetation.

1979:5 Reinhard, Ylva: Avloppsinfiltration - ett försök till konsekvensbeskrivning. 1980:1 Telenius, Anders och Torstensson, Peter: Populationsstudie på Spergularia marina

och Spergularia media. I Frödimorfism och reproduktion. 1980:2 Hilding, Tuija: Populationsstudier på Spergularia marina och Spergularia media.

II Resursallokering och mortalitet. 1980:3 Eriksson, Ove: Reproduktion och vegetativ spridning hos Potentilla anserina L. 1981:1 Eriksson, Torsten: Aspekter på färgvariation hos Dactylorhiza sambucina. 1983:1 Blom, Göran: Undersökningar av lertäkter i Färentuna, Ekerö kommun. 1984:1 Jerling, Ingemar: Kalkning som motåtgärd till försurningen och dess effekter på

blåbär, Vaccinium myrtillus. 1986:1 Svanberg, Kerstin: En studie av grusbräckans (Saxifraga tridactylites) demografi. 1986:2 Nyberg, Hans: Förändringar i träd- och buskskiktets sammansättning i

ädellövskogen på Tullgarnsnäset 1960-1983. 1987:1 Edenholm, Krister: Undersökningar av vegetationspåverkan av vildsvinsbök i

Tullgarnsområdet. 1987:2 Nilsson, Thomas: Variation i fröstorlek och tillväxthastighet inom släktet Veronica. 1988:1 Ehrlén, Johan: Fröproduktion hos vårärt (Lathyrus vernus L.). - Begränsningar och

reglering. 1988:2 Dinnétz, Patrik: Local variation in degree of gynodioecy and protogyny in Plantago

maritima. 1988:3 Blom, Göran och Wincent, Helena: Effekter of kalkning på ängsvegetation.

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1989:1 Eriksson, Pia: Täthetsreglering i Littoralvegetation. 1989:2 Kalvas, Arja: Jämförande studier av Fucus-populationer från Östersjön och

västkusten. 1990:1 Kiviniemi, Katariina: Groddplantsetablering och spridning hos smultron, Fragaria

vesca. 1990:2 Idestam-Almquist, Jerker: Transplantationsförsök med Borstnate. 1992:1 Malm, Torleif: Allokemisk påverkan från mucus hos åtta bruna makroalger på

epifytiska alger. 1992:2 Pontis, Cristina: Om groddknoppar och tandrötter. Funderingar kring en klonal

växt: Dentaria bulbifera. 1992:3 Agartz, Susanne: Optimal utkorsning hos Primula farinosa. 1992:4 Berglund, Anita: Ekologiska effekter av en parasitsvamp - Uromyces lineolatus på

Glaux maritima (Strandkrypa). 1992:5 Ehn, Maria: Distribution and tetrasporophytes in populations of Chondrus crispus

Stackhouse (Gigartinaceae, Rhodophyta) on the west coast of Sweden. 1992:6 Peterson, Torbjörn: Mollusc herbivory. 1993:1 Klásterská-Hedenberg, Martina: The influence of pH, N:P ratio and zooplankton

on the phytoplanctic composition in hypertrophic ponds in the Trebon-region, Czech Republic.

1994:1 Fröborg, Heléne: Pollination and seed set in Vaccinium and Andromeda. 1994:2 Eriksson, Åsa: Makrofossilanalys av förekomst och populationsdynamik hos Najas

flexilis i Sörmland. 1994:3 Klee, Irene: Effekter av kvävetillförsel på 6 vanliga arter i gran- och tallskog. 1995:1 Holm, Martin: Beståndshistorik - vad 492 träd på Fagerön i Uppland kan berätta. 1995:2 Löfgren, Anders: Distribution patterns and population structure of an economically

important Amazon palm, Jessenia bataua (Mart.) Burret ssp. bataua in Bolivia. 1995:3 Norberg, Ylva: Morphological variation in the reduced, free floating Fucus

vesiculosus, in the Baltic Proper. 1995:4 Hylander, Kristoffer & Hylander, Eva: Mount Zuquala - an upland forest of

Ethiopia. Floristic inventory and analysis of the state of conservation. 1996:1 Eriksson, Åsa: Plant species composition and diversity in semi-natural grasslands -

with special emphasis on effects of mycorrhiza. 1996:2 Kalvas, Arja: Morphological variation and reproduction in Fucus vesiculosus L.

populations. 1996:3 Andersson, Regina: Fågelspridda frukter kemiska och morfologiska egenskaper i

relation till fåglarnas val av frukter. 1996:4 Lindgren, Åsa: Restpopulationer, nykolonisation och diversitet hos växter i

naturbetesmarker i sörmländsk skogsbygd. 1996:5 Kiviniemi, Katariina: The ecological and evolutionary significance of the early life

cycle stages in plants, with special emphasis on seed dispersal. 1996:7 Franzén, Daniel: Fältskiktsförändringar i ädellövskog på Fagerön, Uppland,

beroende på igenväxning av gran och skogsavverkning. 1997:1 Wicksell, Maria: Flowering synchronization in the Ericaceae and the Empetraceae. 1997:2 Bolmgren, Kjell: A study of asynchrony in phenology - with a little help from

Frangula alnus.

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1997:3 Kiviniemi, Katariina: A study of seed dispersal and recruitment of plants in a fragmented habitat.

1997:4 Jakobsson, Anna: Fecundity and abundance - a comparative study of grassland species.

1997:5 Löfgren, Per: Population dynamics and the influence of disturbance in the Carline Thistle, Carlina vulgaris.

1998:1 Mattsson, Birgitta: The stress concept, exemplified by low salinity and other stress factors in aquatic systems.

1998:2 Forsslund, Annika & Koffman, Anna: Species diversity of lichens on decaying wood - A comparison between old-growth and managed forest.

1998:3 Eriksson, Åsa: Recruitment processes, site history and abundance patterns of plants in semi-natural grasslands.

1998:4 Fröborg, Heléne: Biotic interactions in the recruitment phase of forest field layer plants.

1998:5 Löfgren, Anders: Spatial and temporal structure of genetic variation in plants. 1998:6 Holmén Bränn, Kristina: Limitations of recruitment in Trifolium repens. 1999:1 Mattsson, Birgitta: Salinity effects on different life cycle stages in Baltic and North

Sea Fucus vesiculosus L. 1999:2 Johannessen, Åse: Factors influencing vascular epiphyte composition in a lower

montane rain forest in Ecuador. An inventory with aspects of altitudinal distribution, moisture, dispersal and pollination.

1999:3 Fröborg, Heléne: Seedling recruitment in forest field layer plants: seed production, herbivory and local species dynamics.

1999:4 Franzén, Daniel: Processes determining plant species richness at different scales - examplified by grassland studies.

1999:5 Malm, Torleif: Factors regulating distribution patterns of fucoid seaweeds. A comparison between marine tidal and brackish atidal environments.

1999:6 Iversen, Therese: Flowering dynamics of the tropical tree Jacquinia nervosa. 1999:7 Isæus, Martin: Structuring factors for Fucus vesiculosus L. in Stockholm south

archipelago - a GIS application. 1999:8 Lannek, Joakim: Förändringar i vegetation och flora på öar i Norrtälje skärgård. 2000:1 Jakobsson, Anna: Explaining differences in geographic range size, with focus on

dispersal and speciation. 2000:2 Jakobsson, Anna: Comparative studies of colonisation ability and abundance in

semi-natural grassland and deciduous forest. 2000:3 Franzén, Daniel: Aspects of pattern, process and function of species richness in

Swedish seminatural grasslands. 2000:4 Öster, Mathias: The effects of habitat fragmentation on reproduction and population

structure in Ranunculus bulbosus. 2001:1 Lindborg, Regina: Projecting extinction risks in plants in a conservation context. 2001:2 Lindgren, Åsa: Herbivory effects at different levels of plant organisation; the

individual and the community. 2001:3 Lindborg, Regina: Forecasting the fate of plant species exposed to land use change. 2001:4 Bertilsson, Maria: Effects of habitat fragmentation on fitness components. 2001:5 Ryberg, Britta: Sustainability aspects on Oleoresin extraction from Dipterocarpus

alatus.

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2001:6 Dahlgren, Stefan: Undersökning av fem havsvikar i Bergkvara skärgård, östra egentliga Östersjön.

2001:7 Moen, Jon; Angerbjörn, Anders; Dinnetz, Patrik & Eriksson Ove: Biodiversitet i fjällen ovan trädgränsen: Bakgrund och kunskapsläge.

2001:8 Vanhoenacker, Didrik: To be short or long. Floral and inflorescence traits of Bird`s eye primrose Primula farinose, and interactions with pollinators and a seed predator.

2001:9 Wikström, Sofia: Plant invasions: are they possible to predict? 2001:10 von Zeipel, Hugo: Metapopulations and plant fitness in a titrophic system – seed

predation and population structure in Actaea spicata L. vary with population size. 2001:11 Forsén, Britt: Survival of Hordelymus europaéus and Bromus benekenii in a

deciduous forest under influence of forest management. 2001:12 Hedin, Elisabeth: Bedömningsgrunder för restaurering av lövängsrester i Norrtälje

kommun. 2002:1 Dahlgren, Stefan & Kautsky, Lena: Distribution and recent changes in benthic

macrovegetation in the Baltic Sea basins. – A literature review. 2002:2 Wikström, Sofia: Invasion history of Fucus evanescens C. Ag. in the Baltic Sea

region and effects on the native biota. 2002:3 Janson, Emma: The effect of fragment size and isolation on the abundance of Viola

tricolor in semi-natural grasslands. 2002:4 Bertilsson, Maria: Population persistance and individual fitness in Vicia pisiformis:

the effects of habitat quality, population size and isolation. 2002:5 Hedman, Irja: Hävdhistorik och artrikedom av kärlväxter i ängs- och hagmarker på

Singö, Fogdö och norra Väddö. 2002:6 Karlsson, Ann: Analys av florans förändring under de senaste hundra åren, ett

successionsförlopp i Norrtälje kommuns skärgård. 2002:7 Isæus, Martin: Factors affecting the large and small scale distribution of fucoids in

the Baltic Sea. 2003:1 Anagrius, Malin: Plant distribution patterns in an urban environment, Södermalm,

Stockholm. 2003:2 Persson, Christin: Artantal och abundans av lavar på askstammar – jämförelse

mellan betade och igenvuxna lövängsrester. 2003:3 Isæus, Martin: Wave impact on macroalgal communities. 2003:4 Jansson-Ask, Kristina: Betydelsen av pollen, resurser och ljustillgång för

reproduktiv framgång hos Storrams, Polygonatum multiflorum. 2003:5 Sundblad, Göran: Using GIS to simulate and examine effects of wave exposure on

submerged macrophyte vegetation. 2004:1 Strindell, Magnus: Abundansförändringar hos kärlväxter i ädellövskog – en

jämförelse av skötselåtgärder. 2004:2 Dahlgren, Johan P: Are metapopulation dynamics important for aquatic plants? 2004:3 Wahlstrand, Anna: Predicting the occurrence of Zostera marina in bays in the

Stockholm archipelago,northern Baltic proper. 2004:4 Råberg, Sonja: Competition from filamentous algae on Fucus vesiculosus –

negative effects and the implications on biodiversity of associated flora and fauna. 2004:5 Smaaland, John: Effects of phosphorous load by water run-off on submersed plant

communities in shallow bays in the Stockholm archipelago.

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2004:6 Ramula Satu: Covariation among life history traits: implications for plant population dynamics.

2004:7 Ramula, Satu: Population viability analysis for plants: Optimizing work effort and the precision of estimates.

2004:8 Niklasson, Camilla: Effects of nutrient content and polybrominated phenols on the reproduction of Idotea baltica and Gammarus spp.

2004:9 Lönnberg, Karin: Flowering phenology and distribution in fleshy fruited plants. 2004:10 Almlöf, Anette: Miljöfaktorers inverkan på bladmossor i Fagersjöskogen, Farsta,

Stockholm. 2005:1 Hult, Anna: Factors affecting plant species composition on shores - A study made in

the Stockholm archipelago, Sweden. 2005:2 Vanhoenacker, Didrik: The evolutionary pollination ecology of Primula farinosa. 2005:3 von Zeipel, Hugo: The plant-animal interactions of Actea spicata in relation to

spatial context. 2005:4 Arvanitis, Leena T.: Butterfly seed predation. 2005:5 Öster, Mathias: Landscape effects on plant species diversity – a case study of

Antennaria dioica 2005:6 Boalt, Elin: Ecosystem effects of large grazing herbivores: the role of nitrogen. 2005:7 Ohlson, Helena: The influence of landscape history, connectivity and area on

species diversity in semi-natural grasslands. 2005:8 Schmalholz, Martin: Patterns of variation in abundance and fecundity in the

endangered grassland annual Euphrasia rostkovia ssp. Fennica. 2005:9 Knutsson, Linda: Do ants select for larger seeds in Melampyrum nemorosum? 2006:1 Forslund, Helena: A comparison of resistance to herbivory between one exotic and

one native population of the brown alga Fucus evanescens 2006:2 Nordqvist, Johanna: Effects of Ceratophyllum demersum L. on lake phytoplankton

composition. 2006:3 Lönnberg, Karin: Recruitment patterns, community assembly, and the evolution of

seed size.2006:4 Mellbrand, Kajsa: Food webs across the waterline - Effects of marine subsidies on

coastal predators and ecosystems.2006:5 Enskog, Maria: Effects of eutrophication and marine subsidies on terrestrial

invertebrates and plants.2006:6 Dahlgren, Johan: Responses of forest herbs to the environment 2006:7 Aggemyr, Elsa: The influence of landscape, field size and shape on plant species

diversity in grazed former arable fields.2006:8 Hedlund, Kristina: Flodkräftor (Astacus astacus) i Bornsjön, en omnivors påverkan

på växter och snäckor.2007:1 Eriksson, Ove: Naturbetesmarkernas växter- ekologi, artrikedom och

bevarandebiologi.2007:2 Schmalholz, Martin: The occurrence and ecological role of refugia at different

spatial scales in a dynamic world.2007:3 Vikström, Lina: Effects of local and regional variables on the flora in the former

semi-natural grasslands on Wäsby Golf club’s course. 2007:4 Hansen, Joakim: The role of submersed angiosperms and charophytes for aquatic

fauna communities.

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2007:5 Johansson, Lena: Population dynamics of Gentianella campestris, effects of grassland management, soil conditions and the history of the landscape.

2007:6 von Euler, Tove: Sex related colour polymorphism in Antennaria dioica2007:7 Mellbrand, Kajsa: Bechcombers, landlubbers and able seemen: Effects of marine subsidies on the roles of arthropod predators in coastal food webs.2007:8 Hansen, Joakim: Distribution patterns of macroinvertebrates in vegetated, shallow, soft-bottom bays of the Baltic Sea.

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