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Changes in a temperate estuary during the filling of the biggestEuropean dam

Pedro Morais⁎, Maria Alexandra Chícharo, Luís ChícharoCIMAR/CCMAR – Centro de Ciências do Mar, Faculdade de Ciências do Mar e do Ambiente, Universidade do Algarve, 8005-139 Faro, Portugal

A R T I C L E D A T A

⁎ Corresponding author.E-mail address: pmorais@ualg.pt (P. Morai

0048-9697/$ – see front matter © 2008 Elsevidoi:10.1016/j.scitotenv.2008.11.037

A B S T R A C T

Article history:Received 24 April 2008Received in revised form11 November 2008Accepted 14 November 2008Available online 19 January 2009

This study aimed to determinewhether and how the disruption of river flow, during the fillingof the Alqueva dam, influenced the variability of abiotic and biotic factors in the Guadianaestuary, particularly the abundance and distribution of anchovy eggs. River inflow was foundto be the most important factor in determining abiotic and biotic variability in the Guadianaestuary. Seasonal patterns were obscured by long periods of low inflow (mid April to earlyDecember 2002), which caused marked changes in the estuary. The estuarine turbiditymaximum zone was displaced towards the upper estuary, to at least 38 km from the rivermouth, 8 to 16 km upstream from previous records. The dynamics of nutrient stoichiometrywas also affected. In the upper and middle estuary, P was more potential limiting than N andpotential Si limitation was only frequent on the coast, with direct and/or indirect influence inchanging phytoplankton dynamics and composition. Previously, the upper estuary alternatedbetween potential P limitation duringwinter, Si limitation during spring andmid summer andN limitation during mid summer and autumn. The flooding of vast areas in the catchment ofthe dam probably caused the increase of DSi concentrations, as well as maximal N and Ploadings. The abundance of larval stages of anchovy decreased, putatively because estuarineproductivity has also decreased. In April 2002 there was an uncontrolled discharge from theAlqueva dam, which reduced the abundance of anchovy eggs by 99.99%. It is suggested thatdammanagers shouldmimic, asmuch as possible, the natural river flow, in order tominimizethe impact on downstream ecosystems. Management efforts should not be restricted to theareas upstream of the dam, but should also encompass the estuary and adjacent coastal area.

© 2008 Elsevier B.V. All rights reserved.

Keywords:River inflowAbiotic and biotic parametersAnchovyDam impactAlqueva damGuadiana estuary

1. Introduction

Estuaries and adjacent coastal areas are highly dynamicecosystems, because of the tidal regime to which they aregenerally subjected (Gianesella et al., 2000). This is especially thecase in temperate regions which have marked biological (Coull,1999), physical (Bodineau et al., 1998), chemical (Cabeçadas etal., 1999) and geological seasonal cycles (Sherwood andCreager,1990). River inflow is another major structuring factor of abioticparameters and biota in estuaries and adjacent coastal areas(Cabeçadas et al., 1999; Snow et al., 2000), occasionally more sothan tides (Ande and Xisan, 1989). River inflow is crucial for

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setting nutrient concentration and stoichiometry (Grange et al.,2000; Nixon, 2003), which in turn affect primary and secondaryproduction (Canuel, 2001; Nixon, 2003. Ultimately, it also affectsestuarine dynamics and landings from coastal fisheries (Lone-ragan and Bunn, 1999; Whitfield and Harrison, 2003; Erzini,2005), with an inherent economic impact.

River inflow varies substantially within and among years intemperate estuaries under the influence of a Mediterraneanclimate, suchas theGuadianaestuary (SWIberia, Europe) (CEDEX,2006; INAG, 2006) (Fig. 1). River flow in the Guadiana basin iscontrolled by numerous dams (Euronatura and IIDMA 2003),whichshift thenatural flowregime (BrandãoandRodrigues, 2000)

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Fig. 1 –Geographical context of the Guadiana estuary in the Iberian Peninsula (Europe) (A) and location of sampling stations along the estuary and in the adjacent coastal area (B).Map of Iberian Peninsula modified from http://www.maps-for-free.com.

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and decrease natural variability. The Alqueva was the last largedam to be built; its floodgates were closed on February 8th 2002.Since then, river flow regulation increased from 75% to 81%(Rocha et al., 2002). Many changes in the Guadiana estuary wereexpected to happen after the construction of the dam (Brandãoand Rodrigues, 2000; Chícharo et al., 2001a,b; Rocha et al., 2002;Lopes, 2004; Domingues et al., 2005; Erzini, 2005). Prior to itsconstruction, the location of the estuarine turbidity maximum(ETM) was evaluated (Chícharo and Chícharo, 2000, 2001a) andmodelled (Lopes, 2004). Phytoplankton dynamics and the linkwith nutrient variability were established (Rocha et al. 2002;Domingues et al., 2005). Additionally, a positive relationship hasbeen found between river inflow and the abundance of larvalstages of anchovy (Pisces: Engraulidae – Engraulis encrasicolussensu lato), which is the most abundant planktivorous fish in theGuadiana estuary (Chícharo et al., 2001b, 2006b).

The purpose of this study was to determine whether andhow the disruption of river flow, caused by the filling of theAlqueva dam, influenced the variability of abiotic and bioticfactors in the Guadiana estuary, particularly the abundanceand distribution of anchovy eggs (Pisces: Engraulidae – En-graulis encrasicolus sensu lato).

Fig. 2 –A – Daily average Guadiana river inflow and Guadianahistorical river inflow monthly average (1947–1999) registeredat Pulo do Lobo hydrometric station. Vertical lines correspondto the days of sampling. B – Average river inflow (±SD) andcumulative rainfall registered between the different samplingsessions at Pulo do Lobo hydrometric station and MartimLongo meteorological station, respectively.

2. Materials and methods

2.1. Study site

The Guadiana estuary constitutes the southern border betweenPortugal and Spain (Fig. 1A). It is approximately 70 km long,encompassing a total area of 22 km2 and averaging 6.5 m indepth. It is a mesotidal estuary, with tidal amplitudes rangingfrom 1.3 to 3.5 m. The estuary is partially stratified when theaverage river flow (approx. 150 m3 s−1) and tidal prism (approx.3×107 m3) occur (Michel, 1980). The majority of the Guadianabasin is under the influence of a climate with Mediterraneancharacteristics, therefore, the river flow is strikingly variableamong and within years. The annual average air temperaturevaries from 14 to 18 °C. Rainfall is very irregular throughout theyear, around 80% of precipitation occurs during autumn andwinter months and summers are very dry. The annual averagerainfall fluctuates between 561 and 600 mm in the Portuguesebasin. Climate variability imposes a similar trend to riverflow; thus, the average river inflows are as follows: dryyears, 8–63 m3 s−1; average years, 170–190 m3 s−1; humid years,412–463 m3 s−1 (Bettencourt et al., 2003).

Intense regularization of river flow has occurred in theGuadiana basin since the mid 1950s (Brandão and Rodrigues,2000). The Alqueva dam was the most recent dam to be builtand is located 150 km from themouth of the estuary. The damreservoir has a maximum flooded area of 250 km2 and a totalcapacity of 4150 hm3, making it the largest artificial lake(in volume) in Europe (INAG, 2006).

2.2. Sampling strategy and methodology

Eulerian sampling was carried out during new moon springtides, fromMarch 2002 to February 2003. Nine sampling stationswere considered: 7 stations inside the estuary and 2 in thecoastal area. Station 1 was off Praia de Santo António, outside

the direct influence of the estuarine outflow, and station 2 waspositioned in theareawhere the river plume is formed. Station 9was the uppermost station, situated in the high estuary in frontof Alcoutim (Portugal) and Sanlucar de Guadiana (Spain), at38 km from the river mouth (Fig. 1B). Sampling was performedfrom a boat equipped with an 80 hp engine, except in February2003, when a boat equipped with a 30 hp engine was usedbecause of technical problems. Sampling was generally done atlow and high tides, except in January and February 2003, whenhigh tide sampling could not be done.

In each station, vertical profiles of temperature and salinityin the water column were recorded with a YSI 6600 probe.Subsurface zooplankton trawls were done with a 250 μm meshnet, equipped with a flowmeter (General Oceanics), and theorganisms collected were immediately preserved in bufferedformaldehyde (4% final concentration). Water samples werecollected at the same depth as the zooplankton trawls, for thedetermination of dissolved inorganic macronutrients, seston,suspended organic matter, chlorophyll a and phaeopigments.These sampleswere kept cool until processed in the laboratory.

2.3. Laboratory analyses

To determine dissolved inorganic macronutrients (ammonium,nitrate, nitrite, orthophosphateand silicate),water sampleswere

Fig. 3 –Spatial and temporal patterns of surface temperature, salinity and seston in the Guadiana estuary and adjacent coastalarea, at low and high tides, from March 2002 to February 2003.

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Fig. 4 –Temporal patterns of N and P loadings at Pulo do Lobo(A) and DIN, DRP, DSi and Chl a concentrations in stations 1,4, 6 and 9 during low tide (B–E), from March 2002 to February2003.

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filtered through 0.45 μm pore size cellulose acetate filters (MSI)and preserved by freezing (Kirkwood, 1996) until spectrophoto-metric analyses (Grasshoff et al, 1983). Water samples to beanalysed for seston and suspended organic matter were filteredthrough 0.7 μmpore size filters (WhatmanGF/F). The filtersweresubsequently washed with distilled water (three times thefiltered sample volume), dried at 60 °C and finally burned at450 °C (Greenberg et al., 1992). Water samples for analysis ofchlorophyll a and phaeopigments were filtered through 0.7 μmpore filters (Whatman GF/F). Care was taken not to exceed100mmHgof vacuumpressure during filtration. The filterswerethen frozen (−20 °C) until fluorimetric analyses (Knapet al., 1996).Anchovy eggswere sorted from the zooplankton samples, undera stereomicroscope, and their abundance was determined.

2.4. Data analysis

Dataon theGuadiana river inflow, rainfall, N andP loadingswasmade available by the Portuguese National Water Institute(INAG, 2006). River inflow and N and P loadings to the estuarywere measured in Pulo do Lobo hydrometric station (code: 27L/01) situated at 15 km above the tidal limit of the estuary.Approximately 90% of all freshwater water going into theestuarypasses through this station (Ribeiro et al., 1988).Monthlyaverage river inflowsandcumulative rainfallwere calculated forthe intervals between samplings. Nitrogen loading is thesummation of ammonium, nitrate and nitrite (dissolvedinorganic nitrogen; DIN), while P loading corresponds to totalphosphorus. Rainfall was determined at Martim Longometeor-ological station, the closest to Pulo do Lobo. Surface plots oftemperature, salinity and seston were done in Surfer 8, usingkriging (linear variogram model) as the gridding method.

One-way ANOSIM and non-parametric multidimensionalscaling (MDS) analyses were performed to evaluate spatial andtemporal patterns of nutrients, seston, organic matter, chlor-ophyll a and phaeopigments in the water. Eight factors wereconsidered in these analyses: Month, Season (spring – April,May and June 2002; summer – July, August and September 2002;autumn –October, November andDecember 2002;winter –March2002, January and February 2003), Station (from sampling station1 to 9), StudiedArea (coastal area – st. 1 and 2; low estuary – st. 3 and4; middle estuary – st. 5, 6 and 7; upper estuary – st. 8 and 9), Tide(low tide and high tide), Inflow (low – b 8 m3 s−1; moderate/low –19.0 m3 s−1, moderate – between 47.6 and 56.0 m3 s−1; high –105.3 m3 s−1), Rainfall (undetermined – March 2002; low – b8 mm;moderate – between 31.4 and 77.6 mm; high – N133.1 mm) andCoastal upwelling (present and persistent; absent; undetermined). Thesimilarity matrix constructed to perform these analyses wasdone after log (x+1) data transformation and setting Euclideandistance as the measure of similarity. The non-parametricANOSIM, which employs R statistics, was used to test forsignificant differences among the different levels within eachfactor. R values close to 0 indicate that there are smalldifferences in the evolution pattern of the analysed parameters,as opposed to R values near 1 (Clarke and Warwick, 2001). Bothanalyses were done using Primer 5 software (Primer-E Ltd.).

The occurrence of coastal upwelling was inferred byanalysing NOAA 17 satellite images from the 7 days prior toeach sampling. Those images have a dimension of 760×1100pixels and a resolution of 8 bits pixel−1 (NERC and PML, 2004).

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Dissolved inorganicnitrogen (DIN)was calculatedas thesumof ammonium, nitrate and nitrite. Dissolved reactive phos-phorus (DRP) anddissolvedsilica (DSi) correspond to orthophos-phate and silicate concentrations. The concentrations of DIN,DRP and DSi during the studied period were analysed only forstations 1, 4, 6 and 9, which we considered as representative ofthe four areas studied.N:P and Si:Nmolar ratioswere calculatedand plotted on an XY logarithmic graph (Rocha et al., 2002).

To evaluate the effect of intra-annual changes of riverinflow on several water parameters, a comparison betweenthree distinct months was made. Selection criteria consideredthe one-way ANOSIM results, plus data on river inflow andanchovy eggs. Such evaluation relies on analyses of the shiftsof temperature and salinity profiles in the water column, aswell as on changes in nutrient stoichiometry along the estuaryin each of these 3 months.

3. Results

3.1. Global analysis

Daily average river inflow varied from 1.5±0.0 m3 s−1 (July 3rd2002) to 656.3±405.0 m3 s−1 (April 9th 2002), with an annualaverage river flow of 24.0±61.3 m3 s−1 (Fig. 2A). River inflowduring the studywasgenerally lower than theaveragehistoricalriver inflow. Drastic changes in river inflow were observed, themain one being 6 days prior to the April 2002 sampling, whenthere was a 44.8 fold increase in only 2 days (Fig. 2A). If those 6days are omitted, the annual average river inflow during thestudy drops by 20.4%. The highest average river inflow

Fig. 5 –Nutrient stoichiometry, during low tide, along the GuadianFebruary 2003.

determined between sampling events was 105.3±179.8 m3 s−1

(March28th toApril 15th).Moderate inflowswereobserved fromFebruary 27th toMarch 27th (47.6±55.8m3 s−1), December 4th toJanuary 5th (56.0±16.1 m3 s−1) and January 6th to February 4th(48.0±12.1 m3 s−1). The lowest inflows, under 7.7±0.6 m3 s−1,were registered between May 14th and December 4th (Fig. 2B).

The patterns of monthly average river inflow and cumu-lative rainfall did not always coincide, namely when themaximum cumulative rainfall was registered (206.2 mm fromSeptember 7th to October 7th). A similar pattern was observedfor these parameters from March 28th to April 15th; never-theless, the observed cumulative rainfall was inconsistentwith the high river inflow. Cumulative rainfall in October was4 times higher than in April (a similar period was considered),whereas monthly average river inflow remained low andconstant (4.1±1.8 m3 s−1) (Fig. 2B).

Surface temperature varied from 11.6 to 26.7 °C, showingthe usual seasonal evolution. Higher temperatures were gen-erally registered in the upper estuary and decreased towardsthe coast, yet this pattern became less clear (March and April2002) or inverted (December 2002 to February 2003) during thecolder months (Fig. 3A). Surface salinity decreased towardsupstream stations, varying from 0.09 to 36.5. Periods of higherriver inflow (Fig. 2) coincided with weaker intrusion of salinewater, namely during April 2002 and from December 2002 toFebruary 2003 (Fig. 3B).

Seston tended to decrease towards the coastal area, varyingfrom 3.0 mg L−1 (st. 1, September 2002) to 132.0 mg L−1 (st. 9,August 2002). Maximum seston concentration occurred in theuppermost stationduring low inflowperiods, but duringperiodsof more intense river inflow it was found further downstream,

a estuary and in the adjacent coastal area fromMarch 2002 to

Table 1 – Global R values for each factor obtained in thenon-parametric ANOSIM analysis

Factor Global R P(%)

Studied area 0.460 0.1Station 0.431 0.1Inflow 0.170 0.1Month 0.167 0.1Season 0.141 0.1Rainfall 0.114 0,1Tide 0.054 0.2Coastal upwelling −0.040 87.6

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between stations 6 (January 2003) and 8 (March 2002 andFebruary 2003) (Fig. 3C).

DIN (Fig. 4B), DRP (Fig. 4C) and DSi (Fig. 4D) concentrations,like seston, increased with distance from the mouth of theestuary and were generally highest during low tide (unpub-lished data). Maximum concentrations were concomitant withhigher river inflow (Fig. 2) and N and P loadings (Fig. 4A). Nloading reached its maximum (10,046 t) in January 2003, whilethe maximum for P loading occurred in February 2003, with asupply of 77 t to the estuary. Maximum DIN, DRP and DSiconcentrationswere 101.2 μM (Jan. 2003, st. 9), 3.9 μM (Feb. 2003,st. 9) and 179.0 μM (Feb. 2003, st. 9), respectively. Chlorophyll aconcentration peaked in June and July 2002 (12.3 μg L−1) (Fig. 4E),coinciding with minimumDIN and DRP concentrations. Subse-quently, chlorophyll a concentration decreased sharply, whileDIN and DRP concentrations increased, but showed no corre-spondence with N and P loadings. DSi concentration alsoincreased after reaching its minimum in August 2002, althoughthere was no corresponding increase in river inflow.

Seasonal evolution of nutrient stoichiometry reveals that theGuadiana estuary tends to be P limited. However, during sum-mer, N is potentially more limiting in the high and middleestuary and Si more limiting in the low estuary (Fig. 5A-C).Seasonally, the coastal area (st. 2 and st. 1) was more hetero-geneous, being potentially N limited during spring and autumn,Si limited during summer and P limited during winter (Fig. 5D).

Studied area and Stationwere the factors that best explainedthe differences of abiotic and biotic parameters, with global Rvalues of 0.460 (p=0.1) and 0.431 (p=0.1), respectively. Thesimilarity between studied areas and sampling stationsdecreased with increasing distance; minimum similaritieswere observed between stations 1 and 9 (R=1.000; p=0.1) and

Fig. 6 –MDS plots of the factors Studied Area

between coastal area and upper estuary (R=0.960; p=0.1). Inflow,Month, Season and Rainfall had global R values between 0.170(p=0.1) and 0.114 (p=0.1) (Table 1). The biggest differences forthe Inflow factor were observed between the periods of low andmoderate inflow (R=0.252; p=0.1) and high inflow (R=0.194;p=0.1). Seasonally, winter and summer were the most distinctperiods (R=0.426; p=0.1), in contrast with spring vs. autumn(R=0.009; p=20.7) and spring vs. winter (R=0.074; p=1.4). As aresult, August and February were the most distinct months(R=0.747; p=0.1). August showed the greatest difference fromthe remaining samplingmonths, the least difference was withSeptember (R=0.167; p=0.3). With regard to Rainfall, theperiods of low vs. high rainfall (R=0.142; p=0.1) and moderaterainfall (R=0.125; p=0.1) show the highest differences. Tide(R=0.054; p=0.2) and Coastal Upwelling (R=−0.040; p=87.6) hadthe lowest global R values (Fig. 6).

Anchovy eggs were distributed mainly through the lowerandmiddleestuaryandwerecollected fromMarch toNovember

(A), Season (B), Inflow (C) and Rainfall (D).

Fig. 7 –Spatial and temporal evolution of abundance ofanchovy eggs at low (A) and high (B) tides.

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2002. Maximum abundance was registered in June 2002, with2106 eggs 100m−3 (station 5; high tide), and abundance steadilydecreased until November 2002. From March to April 2002,maximum abundance decreased by 99.99%, down to 0.4 eggs100 m−3 (Fig. 7).

3.2. Comparison of three periods with distinct riverdischarges

Only the results of April 2002, August 2002 and February 2003were selected for the insight analysis. August 2002 andFebruary 2003 were the months with the lowest similarity(R=0.747; p=0.1). The highest river inflows were registered inApril 2002, with the maximum daily average river inflow(656.3±405.0 m3 s−1) occurring 6 days before sampling (Fig. 2).Moreover, in April 2002 the maximum abundance of anchovyeggs decreased by 99.99% (Fig. 7). In these 3 months, therewere striking differences in the vertical profiles for tempera-ture and salinity (data not shown), seston (see previoussection), nutrient concentration (see previous section) andstoichiometry (Fig. 8).

In August 2002 there was no vertical stratification oftemperature and salinity through the water column, but asharp gradient was observed from the coast to the upperestuary. Higher temperatures were registered in the upperestuary, decreasing towards the coast. The greatest differencebetween the highest and lowest temperaturewas 6.1 °C, whichwas also registered in August 2002. In April 2002 and February2003, maximum and minimum temperature differences werelower, 2.0 °C and 2.5 °C, respectively. Despite the reduceddifferences in water temperature in February 2003, warmertemperatures were registered at the coast, decreasing slightlytowards the upper estuary.

During low tide in April 2002 and February 2003, sharpstratification of salinity along the water column was observedfrom stations 2 to 4 and from stations 3 to 5, respectively. InApril 2002, only station 5 was stratified during high tide. Inthese months, the isohaline 0–2 reached station 5, whereas inAugust 2002 it remained close to station 9.

Nutrient concentrations were higher during periods ofhigher inflow (April 2002 and February 2003) (Fig. 4B–D),decreasing sharply during periods of low inflow (August 2002)(Fig. 4B–D). In April and August 2002, estuarine waters tendedto be potentially N limited (Fig. 8Ai), mainly in the middleand/or upper areas (Fig. 8A, B). In August 2002, nutrientstoichiometry showed a marked gradient along the studiedareas (Fig. 8B), similar to that observed for surface tempera-ture and salinity (Fig. 3A–B). In contrast, greater homogeneitywas observed in April 2002 (Fig. 8A), and even greater inFebruary 2003 (Fig. 8C), when it was preceded by a continuousperiod of high and moderate river inflow (Fig. 2). In thesemonths, all estuarine stations and station 2 had similarpotential for P limitation (Si:N=1.9±1.4; N:P=31.9±25.0), inclear contrast to station 1, which was silica-limited (Si:N=0.06; N:P=131.6) (Fig. 8Ci). Coastal waters were usuallysilica-limited, imposing this characteristic on downstreamstations during periods of low inflow (August 2002) andduring high tides (Fig. 8Bii). Intrusion of coastal water into theestuary led to the influence of tides in changing nutrientstoichiometry from stations 1 to 4 in April 2002 (Fig. 8A). In

Fig. 8 –Si:N:P ratios in the Guadiana estuary and adjacent coastal area, from sampling stations 1 to 9, at low and high tides,during April 2002, August 2002 and February 2003.

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August 2002, tidal influence markedly influenced nutrientstoichiometry, as far as the middle estuary (Fig. 8B).

4. Discussion

4.1. Factors controlling abiotic and biotic variability alongthe Guadiana estuary

The natural characteristics of estuaries (e.g. tides, salinitygradient), the particular features of the basins through whichthey flow (e.g. climate, geology, anthropogenic activities) and ofthe adjacent coastal areas (e.g. upwelling events, anthropogenicactivities) set a variety of factors that influence the physical,chemical, geological and biological dynamics of estuaries.

The distance to the river mouth (i.e. salinity gradient) is usu-ally associatedwith themostmarked gradients in estuarine eco-systems (Blaber et al., 1997; Huang et al., 2003; Scharler andBaird,

2003). In the Guadiana estuary, it was also the distance to theriver mouth that better explained the variability of abiotic andbiotic parameters. It is interesting to note, however, that duringthe filling of the Alqueva dam, river inflow was more importantthan seasonality in explaining the variability of the studiedparameters (Table 1). The importance of river inflow overlappedthat of seasonality because, from mid April to mid December2002, river inflowwas fairly constant, less than10m3s−1 in90%ofthe days (INAG, 2006).

There are few studies about the impact of changes in riverflow, set by dams, to the water temperature in estuaries(Gillanders and Kingsford, 2002). In the Guadiana estuary, theuncontrolled discharge of water from the Alqueva dam (April2002), caused an average reduction of 2 °C in the water tem-perature. This pulse event probably had a pulse response by themajority of the Guadiana estuarine communities (Bender et al.,1984); however, in other rivers affected by dams, the release ofcold waters is a press perturbation. For example, the Colorado

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river is now too cold for the successful reproduction of nativefish as far as 400 km below the Hoover dam (McCully, 1996).The impact of this pulse event on anchovy is discussed inSection 4.3.

The horizontal and vertical salinity profiles clearly re-flected the influence of river inflow variability, allowing anevaluation of the relationship between rainfall and waterrelease/retention from/in dams. In April 2002, a fall in salinitycoincidedwith the sudden increase of river inflow. In contrast,the decrease in salinity in October 2002 did not coincide withan increase in river inflow and it is more likely the result oflocal rainfall events (Figs. 2, 3). The low river inflow registeredfrom June to December 2002 indicates that water wasmassively retained behind the Alqueva dam.

The retention of water in the Alqueva dam, with the inher-ent river flow reduction, also influenced nutrients concentra-tions and stoichiometry, the position of the ETM and theanchovy population.

Thus, during the twomost disparate samplings, August 2002andFebruary 2003, nutrient stoichiometry revealed the impact ofdistinct river inflow. The constant high and moderate inflowsthat preceded the February 2003 sampling forced an intenseestuarine homogenization, as far as 12 km (st. 5) from the rivermouth. Thus, all estuarine stations and the one under directinfluence of estuarine outflow showed similar stoichiometry,with potential P limitation. The effect of river inflow was sostriking that the only station outside the direct and indirectinfluence of estuarine outflow, even with a river inflow of1000m3 s−1 (Cunha et al., 2000), exhibited strong silica limitation.Conversely, in August 2002, the prolonged reduced river inflowcaused a marked stoichiometry gradient. In this month, themajority of the estuary was potentially Si-limited, reflecting thedecreasing terrestrial input and stronger coastal influence(Turner et al., 2003b). In fact, between August 2002 and February2003, after some periods of high and moderate rainfall, DSiconcentration increased13.8 times, fromanoverall averageof 8.4

Table 2 – Comparison of minimum and maximum values of Nconcentrations determined in Alcoutim (St. 9) by Rocha et al. (2

ALCOUTIM (St. 9) Rocha et al. (2002)

Sampling period Oct. 1996–Mar. 1998Loadings N (t) min. 3 (Sep. 97)

max.7220 (Jan. 98)Loadings P (t) min.–

max. approximately 270 (Feb 97)DIN (μM) min. 2.4 (Jul. 97)

max. 86.4 (Mar. 98)DRP (μM) min. 0.02 (Mar. 98)

max. 8.7 (May 97)DSi (μM) min. 0.2 (Jun. 97)

max. 176 (Dec. 97)Chl a (μg L−1) min. 2.1 (Dec. 97)

max. 35.6 (Mar. 98)Seston (mg L–1) no data

706.5±1409.5 (Oct. 96–Set. 97)a

Avg. inflow (m3 s−1) 1498.3±1941 .3 (Oct. 97–Mar. 98)b

To allow more accurate comparisons, only high tide data were assessed.February (month preceding the first sampling) to December 2002 samplinNov., 28 Feb.–2 Mar.; 7–17 Mar.; 12–19 May; 1 Jun.–14 Jul.; 23 Jul.–18 Aug.;

to115.2μM. Itwasalsoexpected that theoutstandingpeak inriverinflow prior to theApril samplingwould result in a stoichiometrypattern similar to the one determined in February 2003, or evenmorehomogenised. Actually, similar ratioswere observedduringthe low tide, but not as pronounced as in February 2003, probablybecause constant high or moderate inflows might be moreimportant in terms of stoichiometry homogenization than asingle river flow pulse few days before sampling.

It was foreseen that the concentration of DSi woulddecrease in the Guadiana estuary after the construction ofthe Alqueva dam, due to increased retention in this newreservoir (Koszelnik and Tomaszek, 2008), however the lowestconcentration of DSi registered in this study (21.8 μM)corresponds to a 70.5 times increase, compared to the lowestDSi concentration previously registered in the Guadianaestuary (Table 2; Rocha et al., 2002). Furthermore, the max-imum DSi concentration (143.0 μM) was only 19% lower thanthe one registered in a year of higher river flow (Table 2; Rochaet al., 2002). These findings appear to be contrary to thoseobserved in other aquatic ecosystems undergoing the impactof dams, where DSi concentration decreased substantiallyafter the construction of dams (e.g. Africa – Aswan High Dam(Nixon, 2003); Asia – Three Gorges Dam (Gong et al., 2006),Europe – Danube River Dam (Humborg et al., 1997)). However,the increase of minimum and maximum DSi concentrationsin the Guadiana estuary, during the filling of the Alqueva dam,is most likely due to dissolution from the newly inundatedsoils (Humborg et al., 2006).

It is known that N and P are positively related withwatershed development (Turner et al., 2003b). However, theincrease of maximum N and P loadings in the Guadianaestuary, between 2001 and 2002, 79.4% for N (up to 10046 t) and302.1% for P (up to 77.2 t), were also possibly related to leachingfrom the inundated soils in the catchments of the Alquevadam and not with fertilizer use and urban runoffs, ascommonly observed (Howarth et al., 1996; Caraco and Cole,

and P loadings, DIN, DRP, DSi, chlorophyll a and seston002), Domingues et al. (2005) and Morais et al. (this study)

Dontingues et al. (2005) This study (high tide data)

Apr. 2001–Oct. 2001 Mar. 2002–Dec. 2002min. 9 (Jun. 01) min. 15 (Aug. 02)max. 5599 (Oct. 01) max. 10046 (Jan. 03)min. 0.2 (Aug. 01) min. 0.3 (Sep. 02)max. 19.2 (Oct. 01) max. 77.2 (Feb. 03)min. 4.6 (Jul. 01) min. 16.5 (Sep. 02)max. 99.9 (May 01) max. 52.9 (Mar.02)min. 1.3 (May 01) min. 0.6 (Nov. 02)max. 4.3 (Oct. 01) max. 3.2 (Oct. 02)min. 1.3 (May 01) min. 21.8 (Aug. 02)max. 87 (Apr. 01) max. 143.0 (Mar. 02)min. 9.6 (Set. 01) min. 1.2 (Apr. 02)max. 216.1 (Apr. 01) max. 8.0 (Jul. 02)min. 12.0 (Jun. 01) min. 27.3 (Nov. 02)max. 49.0 (May 01) max. 130.0 (Oct 02)15.6±33.2c 17.8±55.9 (Mar. to Dec. 02)

This study of average river inflow corresponds to the period from lateg. Legend – No daily average river flow for the following periods: a 3024 Aug.–30 Sep.; b 3–7 Nov. 97 and 31 Mar. 98; c 1–25 Apr.

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1999; Turner et al., 2003b). Since the loadings of N and P aremore relatedwith anthropogenic factors thanwith the naturalbackground variability (e.g. climate, river discharge, soil,geomorphology), it is expected that with further developmentof the Guadiana watershed (Morais, 2008), the increment ofnutrients loadings might negatively influence the growth andcomposition of freshwater and marine food webs (Turner,2002; Turner et al., 2003b) (see Section 4.3).

Regarding DRP, there is no consistent trend relating theconcentrations of this nutrient with dams, implying thatmechanisms, such as biogenic uptake of P in reservoirs orsorption onto settling suspended sediments, do not signifi-cantly alter the downstream transport of DRP (Wu and Huh,2007). The pattern of DRP along the Guadiana estuary wasconsistent with the majority of estuarine ecosystems, withsignificantly lower DRP concentrations being registered in theupper reaches of estuaries (Caroco et al., 1990; Wu and Huh,2007; Jordan et al., 2008). The higher DRP concentrationsregistered in the lower Guadiana estuary might be related tosewage outfalls from two cities (Ayamonte – Spain: 17,500inhabitants; Vila Real de Santo António – Portugal: 13,880inhabitants) (Wu and Huh, 2007; Morais, 2008) and to phos-phorus biogeochemistry along estuarine salinity gradients(Jordan et al., 2008). Under conditions of low salinity, themajority of phosphate is bound and sorbed to iron-oxides,either in suspended or deposited sediments, being releasedfrom terrigenous sediments when they are deposited in salinesections of estuaries (Pant and Reddy, 2001; Jordan et al., 2008).During summer, the increase of salinity in the upper Guadianaestuary (Fig. 3), due to significant water retention in theAlqueva dam, probably enhanced desorbtion reactions ofphosphates from sediments, increasing DRP concentration inthe upper estuary after the sampling of July 2002 (Pant andReddy, 2001; Jordan et al., 2008). The release of phosphateunder saline conditions explains why phosphorus and nitro-gen are usually more limiting to phytoplankton growth infreshwater and seawater environments, respectively (Jordanet al., 2008). Indeed, the high and middle estuary was onlypotentially nitrogen limited during summer (Fig. 5), whensalinity increased (Fig. 3).

Thecontrasting inflowsobserved inApril (105.3±179.8m3s−1)and August 2002 (7.7±0.6 m3 s−1) increased the tidal influenceover estuarine stoichiometry along 18 km. This is also supportedby the marked change in the positions of the salt wedge andETM, commonly used to detect changes in river flow (Kurupet al., 1998; Nagy et al., 2002). Lopes (2004) modelled sedimenttransport, determining that, for a river flowof 20m3 s−1, the ETMshould lie between 22 and 30 km from the river mouth. Theavailable field data support themodel predictions (Chícharo andChícharo, 2000). However, the extended periods of low inflowthat occurred during the filling of the Alqueva dam, forced therelocation of the ETM to upstream areas, at least to station 9,38 km away from the river mouth. Chícharo et al. (2001a)considered that the stratification of the water column and thelocation and strength of the ETMmay significantly influence theretention of zooplankton in the Guadiana estuary. Effects ofchanges in river inflow on chlorophyll a concentration were notperceptible in this study. However, a simultaneous study withshorter sampling periodicity revealed that the exceptional highriver inflow registered in earlyApril 2002 caused a 58% reduction

of chlorophyll a content in the area between Mértola (70 kmaway fromrivermouthand30 kmupstreamof station 9) and Fozde Odeleite (20 km away from river mouth, station 7) (Sobrinoet al., 2004). Strong hydraulic pulses are one of the decisive fac-tors in the dynamics of planktonic ecosystems, having thepotential to enhance biodiversity, control eutrophication andprevent harmful algal blooms (Roelke et al., 2003). However, themagnitude and periodicity of flushing must be controlled andthe potential impacts on adjacent ecosystems ought to beevaluatedpreviously (Morais, 2007), particularly in sharedbasins(Gillanders and Kingsford, 2002).

As mentioned, river inflow was the most important factorin explaining the variability of the studied parameters(Table 1). The other factors analysed were relatively unim-portant, because their action was spatially and temporallyrestricted. Regarding coastal upwelling, it is known that it caninfluence estuarine ecosystems for quite long periods (Pérezet al., 2000; Hickey and Banas, 2003), while in others, it only hasa significant effect during periods of reduced river inflows(Taylor, 1992). In the Guadiana estuary, only one upwellingevent was registered during the filling of the Alqueva dam,namely from late August to early September 2002. This eventstarted in the south-west coast of Portugal, near Cape SãoVicente, extending eastwards along the southern coast, as faras the Guadiana estuary. The main consequences of thisupwelling event were the decrease of water temperature inthe low estuary (Fig. 3A) and the increase of DIN and DRP(Fig. 4B,C and unpublished data), mainly during high tides, asusual in areas affect by coastal upwelling (Van Bennekomet al., 1978; Hickey and Banas, 2003).

4.2. Responses of the phytoplankton/cyanobacterialcomponents to changes in river flow

Rocha et al. (2002) and Domingues et al. (2005) predicted anincreased dominance of cyanobacteria in the upper Guadianaestuary for the period after the construction of the Alqueva dam.The concern regarding the occurrence of cyanobacterial bloomsis prompted by the presence of several potential toxic species inthis estuary (e.g.Microcystis aeruginosa,Aphanizomenon flos-aquae,Oscillatoria spp.,Anabaena spp.) (Caetanoet al., 2001). Thenutrientstoichiometry in the upper Guadiana estuary evolved frompotential P limitation during winter, to Si limitation duringspring to mid summer and to N limitation frommid summer toautumn, corresponding to a transition from a diatom springbloom, to a flagellate bloom and to cyanobacteria dominanceduring summer (Rocha et al., 2002; Domingues et al., 2005).However, changes in nutrient concentrations and stoichiometryand phytoplankton dynamics are known to occur as a conse-quence of river flow changes after the construction of dams(Humborg et al., 1997; Li et al., 2006; Jiao et al., 2007). Indeed,during the first year of the filling of the Alqueva reservoir (March2002 to February 2003), the above-mentioned seasonal pattern inthe evolution of the nutrient stoichiometry (this study) andphytoplankton dynamics (Domingues et al., 2007) was notobserved. Throughout this period, the high and middle estuarytended to be more potentially P-limited and potential Silimitation was only frequent on the coast (Fig. 5). The commonspring diatom bloom did not occur and cyanobacteria bloomedfrom summer to winter (Domingues et al., 2007).

2256 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 4 0 7 ( 2 0 0 9 ) 2 2 4 5 – 2 2 5 9

Rocha et al. (2002) stated that low Si-loading and low N:Pratios, combined with highwater temperatures, constitute themain triggering features for the occurrence of cyanobacterialblooms in the upper estuary. During the filling of the Alquevadam, cyanobacteria bloomed with low concentrations of DINand N:P ratios, precisely during summer when the upperestuarywasmore nitrogen limited, but also bloomedwith highSi concentrations and high Si:N and N:P ratios (this study).Although there were high Si concentrations in the estuary, thepredominant potential P limitation conditions might havelimited the growth of diatoms, as revealed by Ferris and Leh-man (2007) in experimental bioassays.Moreover, the transitionto a cyanobacteria-dominated community has to rely on otherfactors apart from nutrient concentrations and ratios, such asturbidity and light intensity in the water column (Cloern, 1987,1999) and grazing by planktonic metazoans (Chícharo et al.,2006a) and by the abundant Asian clam, Corbicula fluminea(Bivalvia) (J. Teodósio, unpublished data). This bivalve does notexhibit differential grazing on phytoplankton groups and sizeclasses (Boltovskoy et al., 1995), the net effects of C. fluminea onphytoplanktonic assemblages differs between taxamainly dueto the growth rates of each phytoplanktonic group (Hwang etal., 2004). Thus, fast growing taxa, as cyanobacteria, wouldeventually dominate the phytoplanktonic assemblages, indetriment of slow growing taxa, such as diatoms (Harris, 1986).

It seems that the filling of the Alqueva reservoir alterednutrient concentration and ratios, changing phytoplanktondynamics in the upper estuary and replacing the typical foodweb diatom-zooplankton-fish (DZF) by a non-DZF food web(Turner, 2002), with cyanobacteria as the main item in thebottom of this food-web during most of the year (Domingueset al., 2007). It is likely that the estuarine productivitydecreased. The maximum chlorophyll a concentrationspreviously registered in the upper Guadiana estuary were35.6 μg L−1 (Rocha et al., 2002) and 216.1 μg L−1 (Domingueset al. (2005) (Table 2), which was coincident with springblooms of diatoms. However, in this study, the maximumconcentration of chlorophyll a measured during high tideswas 8.0 μg Chl a L−1 (the same tide as in the mentionedstudies; 12.3 μg Chl a L−1 at low tide), registered during acyanobacteria bloom (July 2002). The maximum concentra-tion of microcystins increased 5.8 times, up to 1010 ng L−1

(study period from March to May 2002 – Sobrino et al., 2004),when compared with 1999, a year with similar average riverinflow (25.2±79.6 m3 s−1) (study period fromMarch to October1999 – Caetano et al., 2001).

A recurrence of non-DZF food webs might be more frequentafter the development of the Alqueva irrigation plan, whichwillaffect 110,000 ha. This is a real threat to aquatic ecosystems andshould not be neglected (Morais, 2008). There is a worldwidetrend in aquatic ecosystems for increased P and Si limitationand a higher incidence of noxious planktonic blooms, generallycaused by increased eutrophication due to higher nitrateloading (Turner et al., 2003a). The ingredients to promoteincreased degradation of water quality in the Guadiana basinmay be a reality in the near future. Thus, “How to preventwaterquality degradation in the Guadiana basin?” is a question thatshould concern all, from local populations, to scientists andpolicy makers. A broad ecohydrological approach to manage-ment might be a potential solution to minimize the threats to

the Guadiana basin (Morais, 2008), which is one of the betterpreserved Portuguese river basins (Vasconcelos et al., 2007).

4.3. Indirect and direct impacts of the Alqueva dam on theecology of anchovy

The ecology of fishes in an estuary depends on the naturalchanges of the ecosystem and on the anthropogenic stressorsaffecting estuaries and their communities (e.g. fishing, pollu-tion, dams) (Gillanders and Kingsford, 2002). In the Guadianabasin, the impact of water abstraction and dams on fishpopulations can be synthesised in four topics, namely:a) decrease of fish conservation status; b) reduction of habitatfor freshwater fishes; c) reduced use of the estuary by marinefishes; d) shifts on the estuarine and on coastal communities(Morais, 2008 and references therein).

Alterations in the amount of river flow into the estuary andadjacent coastal areas affect the concentrations of nutrients,with consequences for the primary productivity and asso-ciated trophic chains (Loneragan and Bunn, 1999; Gillandersand Kingsford, 2002; Wolanski et al., 2006). There are severalstudies that positively relate the intensity of river inflow withthe abundance of estuarine and coastal fish and crustaceans(Loneragan and Bunn, 1999; Quiñones and Montes, 2001;Gillanders and Kingsford, 2002; Nixon, 2003), some statingthat, if in appropriate quantities and at suitable times,stronger river inflow may be favourable to fisheries, eitherby enhanced production mechanisms (Bergeron, 2004) or bypromoting concentration and catchability (Loneragan andBunn, 1999).

In the Guadiana estuary, anchovy was designated as apotential key species in detecting the impact of changes in riverflow on the estuary (Chícharo et al., 2001b, 2003), mostly due toits central position in the trophicwebof theGuadianaestuary (R.Santos, unpublished data). Thus, the reduction of river inflowand estuarine productivity should be reflected on the anchovypopulation of the Guadiana estuary (Chícharo et al., 2001b,2006b). Furthermore, during the filling of the Alqueva dam, themaximum concentration of chlorophyll a was 27.0 times lowerthan themaximumregistered in theGuadiana estuary (Table 2),in a year of similar river inflow, prior to the construction of thedam (Domingues et al., 2005). The decrease of estuarineproductivity might indirectly affect the anchovy population,both in termsof condition status and abundance of larval stages(Bergeron, 2004). Indeed, the maximum abundance of anchovyeggs and larvae registered in 2002 decreased 4.7 and 14.5 timesin comparison with the maximum registered in 1988, a year ofmoderate inflow andprobablywithhigher estuarine productionthan 2002 (Chícharo and Teodósio, 1991 – February–July 1988).

Gillanders and Kingsford (2002) pointed out that the natureof fish eggs (benthonic or pelagic) and larvae may reflectchanges in river flow and productivity. In the Guadianaestuary, an inter-annual comparison between river inflowand ichthyoplankton abundance suggested that the decreasein the abundance of larval anchovy, in comparison to larvalgobies (Pomatochistus spp.), is evidence of the response of thesespecies, with different life cycle strategies, to the reduction ofGuadiana productivity during the filling of the Alqueva dam(Bergeron, 2004; Faria et al.; 2006). Gobies have benthonic eggsand larvae are more highly developed and less dependent on

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the available food upon hatching, unlike anchovy larvae thatemerge from pelagic eggs and are planktitrophic, thereforemore dependent on the productivity status of the estuary.

As in other fishes, the ecology of anchovy is stronglydependent on temperature, one of themost important triggersthat dictate fish spawning periods (Holmes and Henderson,1990; Palomera, 1992). Anchovy is a planktivorous fish withpelagic eggs and a broad reproductive period (Millán, 1999;Plounevez and Champalbert, 1999) and no larval stages havebeen captured in waters with temperatures below 13 °C(Demir, 1965; Ré, 1984; Chícharo and Teodósio, 1991; Ribeiro,1991; Motos et al., 1996). This is likely to relate to theirpresence in the Guadiana estuary only in late March 2002,when the average temperature was 18 °C. Since anchovy hadalready started its broad and continuous spawning by lateMarch 2002, it was surprising to observe that in mid April 2002the maximum abundance of anchovy eggs had decreased by99.99%. Biological and hydrological hypotheses, or both, mightexplain this finding. Biologically, if populations of the prey ofadult anchovies had collapsed, then it might have causedovarian atresia (Abaunza et al., 2003). With regard to hydro-logical causes, the sudden increase of river inflow during the6 days prior to sampling might have led to the advection ofanchovy eggs out of the estuary. However, hydrodynamicsimulations strongly support the second hypothesis and dataon the gonadossomatic index of anchovy adults completelyreject the first hypothesis (Morais, 2007).

The intense river pulsemight had decreased the successfuldevelopment of anchovy larval stages, as a result of eitherincreased osmotic stress or increased probability of predationin the coastal area, which can cause a reduction of recruits insubsequent years (Whitfield, 1994). However, due to the innatecharacteristics of anchovy reproduction, this short pulseperturbation had a pulse response, while for other fish speciesitmight have had a press response, i.e. resulting in a long-termreduction in abundance (Bender et al., 1984), mainly for thosethat have restricted reproductive periods (Whitfield, 1994;Loneragan and Bunn, 1999).

The dynamics of anchovy eggs was not linked with anyother aspect of variability in river inflow. Its abundancepeaked in June, in accordance with previous observationsthat recorded maximum spawning in June and July in theGuadiana estuary (Chícharo and Teodósio, 1991). The abun-dance of anchovy eggs decreased from July 2002 onwards, dueto the decrease of anchovy spawning (Morais, 2007) andprobably also due to intense predation by the enormouspopulation of jellyfish (Morais, unpublished data), which areknown to be important regulators of zooplanktonic commu-nities (Schneider and Behrends, 1994).

5. Conclusions

River inflow was the most important factor in determiningabiotic and biotic variability in the Guadiana estuary and inthe adjacent coastal area, during the unique period of thefilling of the Alqueva dam. The seasonal patterns wereoverlaid by long periods of low inflow (mid April to earlyDecember 2002). The filling of the dam caused markedchanges in the estuary. The ETM zone was displaced towards

the upper estuary, to at least 38 km from the river mouth. Thedynamics of nutrient stoichiometry also changed in the highand middle estuary, which was more P limited than N limitedduring the whole year. Si limitation was only frequent on thecoast, with direct and/or indirect influence in changingphytoplankton dynamics and composition. The flooding ofvast areas in the dam catchments was probably responsiblefor increasing DSi concentrations in the estuary. The abun-dance of anchovy larval stages decreased, putatively due tothe decrease of estuarine productivity and to an uncontrolledriver discharge from the Alqueva dam, in April 2002.

Management of river inflow can be a useful tool to resolveecological constraints downstream, but bad managementpractices may increase such constraints. The assessment ofanchovy eggs seems to be an excellent indicator of the effectsof river inflow management. During and after the filling of adam, it is advisable that dam managers mimic the naturalriver flow as much as possible, in order to minimize theimpact on downstream ecosystems, since the generalisedperception that water flowing from a dam into downstreamareas is waste, is completely wrong. Moreover, dammanagersshould not restrict/limit their efforts to the areas upstream ofthe dam wall, but their management should begin in thedistant estuary and coastal area.

Acknowledgements

The authors acknowledge Antero Fernandes and Luís Cristóvãofor their invaluable help and support during field work, AndréNeves for his contribution in the laboratory analyses; Ana Fariaand Isabel Marques during ichthyoplankton sorting, Ester Diasand Rita Domingues for the discussions on the ecology of theGuadiana estuary and Pedro Range for the English revision. Thiswork was funded by “Fundação para a Ciência e a Tecnologia”through the PhD scholarship granted to PedroMorais (SFRH/BD/5187/2001) and the project “Effect of river flow changes on theichthyofauna communities in Douro, Tejo and Guadianaestuaries and in its adjacent coastal areas. Ecological andsocioeconomical predictions” (FCT/P/MAR/15263/1999).

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