Estuarine fish communities respond to climate variability over both river and ocean basins

Estuaries are dynamic environments at the land–sea interface that are strongly affected by interannual climate variability. Ocean–atmosphere processes propagate into estuaries from the sea, and atmospheric processes over land propagate into estuaries from watersheds. We examined the effects of these two separate climate‐driven processes on pelagic and demersal fish community structure along the salinity gradient in the San Francisco Estuary, California, USA. A 33‐year data set (1980–2012) on pelagic and demersal fishes spanning the freshwater to marine regions of the estuary suggested the existence of five estuarine salinity fish guilds: limnetic (salinity = 0–1), oligohaline (salinity = 1–12), mesohaline (salinity = 6–19), polyhaline (salinity = 19–28), and euhaline (salinity = 29–32). Climatic effects propagating from the adjacent Pacific Ocean, indexed by the North Pacific Gyre Oscillation (NPGO), affected demersal and pelagic fish community structure in the euhaline and polyhaline guilds. Climatic effects propagating over land, indexed as freshwater outflow from the watershed (OUT), affected demersal and pelagic fish community structure in the oligohaline, mesohaline, polyhaline, and euhaline guilds. The effects of OUT propagated further down the estuary salinity gradient than the effects of NPGO that propagated up the estuary salinity gradient, exemplifying the role of variable freshwater outflow as an important driver of biotic communities in river‐dominated estuaries. These results illustrate how unique sources of climate variability interact to drive biotic communities and, therefore, that climate change is likely to be an important driver in shaping the future trajectory of biotic communities in estuaries and other transitional habitats.     

Zonation of intertidal macrobenthos in the estuaries of Schelde and Ems

Based on data, collected in 1980–1990, the intertidal benthic macrofauna of the Schelde and Ems estuaries was compared. The spatial occurrence of the benthic macrofauna along the salinity gradient, including the freshwater tidal area was emphasized. Both estuaries appeared to have a very similar species composition, especially at genus level. The higher number of species observed in the Schelde estuary was probably due to a greater habitat diversity. In both estuaries species diversity decreased with distance upstream. The total density did not vary along the estuarine gradient, whereas biomass is highest in the polyhaline zone.In both estuaries distinct intertidal benthic communities were observed along the salinity gradient: a marine community in the polyhaline zone, a brackish community in the mesohaline zone, and a third community in the oligohaline and freshwater tidal zones of the estuary. These three communities were very similar between both estuaries. Their main characteristics were discussed together with the occurrence and distribution of the dominant species.For the Schelde estuary and to a lesser extent also for the Ems estuary, there was evidence that anthropogenic stress had a negative effect on the intertidal macrobenthic communities of the oligohaline/freshwater tidal zone. Only Oligochaeta were dominating, whereas the very euryhaline and/or true limnetic species were missing. In the mesohaline zone, the Schelde estuary was dominated by large numbers of short-living, opportunistic species, whereas in the Ems estuary relatively more stable macrobenthic communities were observed. A comparison with some other European estuaries showed in general similar trends as those observed for the Schelde and Ems estuaries.     

Defining winter trophic habitat of juvenile Gulf Sturgeon in the Suwannee and Apalachicola rivermouth estuaries, acoustic telemetry investigations

Three automated listening post‐telemetry studies were undertaken in the Suwannee and Apalachicola estuaries to gain knowledge of habitats use by juvenile Gulf Sturgeons (Acipenser oxyrinchus desotoi) on winter feeding grounds. A simple and reliable method for external attachment of small acoustic tags to the dorsal fin base was developed using shrink‐tubing. Suspending receivers on masts below anchored buoys improved reception and facilitated downloading; a detection range of 500–2500 m was realized. In the Apalachicola estuary, juvenile GS stayed in shallow water (< 2 m) within the estuarine transition zone all winter in the vicinity of the Apalachicola River mouth. Juvenile GS high‐use areas did not coincide with high density benthic macrofauna areas from the most recent (1999) benthos survey. In the Suwannee estuary, juveniles ranged widely and individually throughout oligohaline to mesohaline subareas of the estuary, preferentially using mesohaline subareas seaward of Suwannee Reef (52% of acoustic detections). The river mouth subarea was important only in early and late winter, during the times of adult Gulf Sturgeon migrations (41% of detections). Preferred winter feeding subareas coincided spatially with known areas of dense macrofaunal benthos concentrations. Following a dramatic drop in air and water temperatures, juvenile GS left the river mouth and estuary, subsequently being detected 8 km offshore in polyhaline open Gulf of Mexico waters, before returning to the estuary. Cold‐event offshore excursions demonstrate that they can tolerate full‐salinity polyhaline waters in the open Gulf of Mexico, for at least several days at a time. For juvenile sturgeons, the stress and metabolic cost of enduring high salinity ( Jarvis et al., 2001 ; McKenzie et al., 2001 ; Singer and Ballantyne, 2002 ) for short periods in deep offshore waters seems adaptively advantageous relative to the risk of cold‐event mortality in shallow inshore waters of lower salinity. Thus, while juveniles can tolerate high salinities for days to weeks to escape cold events, they appear to make only infrequent use of open polyhaline waters. Throughout the winter foraging period, juvenile GS stayed primarily within the core area of Suwannee River mouth influence, extending about 12 km north and south of the river mouth, and somewhat seaward of Suwannee Reef (< 5 km offshore). None were detected departing the core area past either of the northern or southern acoustic gates, located 66 and 52 km distant from the river mouth, respectively.     

The temporal variation of intertidal nematodes in the Westerschelde I. The importance of an estuarine gradient

Biannual meiobenthic sampling (Spring and Autumn) was carried out in 1983–1989 at two fine sandy intertidal stations in the Westerschelde estuary. Both stations are exposed daily for more than one hour and are situated in the polyhaline and the mesohaline zone of the estuary. Average density data of non-selective deposit-feeders > predators > epigrowth-feeders > selective deposit-feeders for both stations are presented in spite of different nematode species composition. No difference between Spring and Autumn nor trend over 7 years could be detected. Higher total nematode densities are found at the polyhaline station (average 3200 ind. 10 cm^?2) in comparison with the mesohaline station (average 2300 ind. 10 cm^?2), a difference mainly due to higher non-selective deposit-feeders and predators densities in the polyhaline station. Each year, heterogeneous variance is found for all feeding types at the mesohaline station, but only for epigrowth-feeders and predators at the polyhaline station. The higher nematode density at the polyhaline station is probably caused by the more stable nematode structure. An unstable nematode temporal pattern at the mesohaline station is suggested to be combined with the detritus food chain system in the mesohaline zone. The unstable estuarine habitats are mainly caused by their upstream effects: the River Schelde, which clearly influences the stability of the nematode communities.     

Assessing estuarine benthic quality conditions in Chesapeake Bay: A comparison of three indices

Legislation in US and Europe has been adopted to determine the ecological integrity of estuarine and coastal waters, including, as one of the most relevant elements, the benthic macroinvertebrate communities. It has been recommended that greater emphasis should be placed on evaluating the suitability of existing indices prior to developing new ones. This study compares two widely used measures of ecological integrity, the Benthic Index of Biotic Integrity (B-IBI) developed in USA and the European AZTI's Marine Biotic Index (AMBI) and its multivariate extension, the M-AMBI. Specific objectives were to identify the frequency, magnitude, and nature of differences in assessment of Chesapeake Bay sites as ‘degraded’ or ‘undegraded’ by the indices. A dataset of 275 subtidal samples taken in 2003 from Chesapeake Bay were used in this comparison. Linear regression of B-IBI and AMBI, accounted for 24% of the variability; however, when evaluated by salinity regimes, the explained variability increased in polyhaline (38%), high mesohaline (38%), and low mesohaline (35%) habitats, remained similar in the tidal freshwater (25%), and decreased in oligohaline areas (17%). Using the M-AMBI, the explained variability increased to 43% for linear regression, and 54% for logarithmic regression. By salinity regime, the highest explained variability was found in high mesohaline and low polyhaline areas (53–63%), while the lowest explained variability was in the oligohaline and tidal freshwater areas (6–17%). The total disagreement between methods, in terms of degraded-undegraded classifications, was 28%, with high spatial levels of agreement. Our study suggests that different methodologies in assessing benthic quality can provide similar results even though these methods have been developed within different geographical areas.     

Shifts in the relative availability of phosphorus and nitrogen along estuarine salinity gradients

Phosphorus (P) availability in estuaries may increase with increasing salinity because sulfate from sea salt supports production of sulfide in sediments, which combines with iron (Fe) making it less available to sequester P. Increased P availability with increased salinity may promote the generally observed switch from P limitation of primary production in freshwater ecosystems to nitrogen (N) limitation in coastal marine waters. To investigate this hypothesis, we analyzed pore water from sediment cores collected along the salinity gradients of four Chesapeake Bay estuaries (the Patuxent, Potomac, Choptank, and Bush Rivers) with watersheds differing in land cover and physiography. At salinities of 1–4 in each estuary, abrupt decreases in pore water Fe²⁺ concentrations coincided with increases in sulfate depletion and PO₄ ^3? concentrations. Peaks in water column PO₄ ^3? concentrations also occur at about the same position along the salinity gradient of each estuary. Increases in pore water PO₄ ^3? concentration with increasing salinity led to distinct shifts in molar NH₄ ⁺:PO₄ ^3? ratios from >16 (the Redfield ratio characteristic of phytoplankton N:P) in the freshwater cores to <16 in the cores with salinities >1 to 4, suggesting that release of PO₄ ^3? from Fe where sediments are first deposited in sulfate-rich waters could promote the commonly observed switch from P limitation in freshwater to N limitation in mesohaline waters. Finding this pattern at similar salinities in four estuaries with such different watersheds suggests that it may be a fundamental characteristic of estuaries generally.     

Nanosilver inhibits nitrification and reduces ammonia‐oxidising bacterial but not archaeal amoA gene abundance in estuarine sediments

Silver nanoparticles (AgNPs) enter estuaries via wastewater treatment effluents, where they can inhibit microorganisms, because of their antimicrobial properties. Ammonia‐oxidising bacteria (AOB) and archaea (AOA) are involved in the first step of nitrification and are important to ecosystem function, especially where effluent discharge results in high nitrogen inputs. Here, we investigated the effect of a pulse addition of AgNPs on AOB and AOA ammonia monooxygenase (amoA) gene abundances and benthic nitrification potential rates (NPR) in low‐salinity and mesohaline estuarine sediments. Whilst exposure to 0.5 mg L^?1 AgNPs had no significant effect on amoA gene abundances or NPR, 50 mg L^?1 AgNPs significantly decreased AOB amoA gene abundance (up to 76% over 14 days), and significantly decreased NPR by 20‐fold in low‐salinity sediments and by twofold in mesohaline sediments, after one day. AgNP behaviour differed between sites, whereby greater aggregation occurred in mesohaline waters (possibly due to higher salinity), which may have reduced toxicity. In conclusion, AgNPs have the potential to reduce ammonia oxidation in estuarine sediments, particularly where AgNPs accumulate over time and reach high concentrations. This could lead to long‐term risks to nitrification, especially in polyhaline estuaries where ammonia‐oxidation is largely driven by AOB.     

Plant responses to increased inundation and salt exposure: interactive effects on tidal marsh productivity

Flooding and high salinity generally induce physiological stress in wetland vascular plants which may increase in intensity with sea-level rise (SLR). We tested the effects of these factors on seedling growth in a transplant experiment in a macrotidal estuary in the Pacific Northwest. Seven common wetland species were grown at mean higher high water (MHHW, a typical mid-marsh elevation), and at 25 and 50 cm below MHHW in oligohaline, mesohaline, and polyhaline marshes. Increased flooding reduced shoot and root growth in all species, including those typically found at middle or lower tidal elevations. It also generally disproportionately reduced root biomass. For more sensitive species, biomass declined by >50 % at only 25 cm below MHHW at the oligohaline site. Plant growth was also strongly reduced under polyhaline conditions relative to the less saline sites. By combining inundation and salinity time-series measurements we estimated a salt exposure index for each site by elevation treatment. Higher values of the index were associated with lower root and shoot biomass for all species and a relatively greater loss of below-ground than above-ground production in most species. Our results suggest that inundation and salinity stress individually and (often) interactively reduce productivity across a suite of common marsh species. As relative SLR increases the intensity of stress on coastal marsh plants, negative effects on biomass may occur across a range of species and especially on below-ground production.     

Spatial distribution of subtidal Nematoda communities along the salinity gradient in southern European estuaries

This study investigated the spatial distribution of subtidal nematode communities along the salinity gradients of two Portuguese estuaries exposed to different degrees of anthropogenic stress: the Mira and the Mondego.The nematode communities were mainly composed of Sabatieria, Metachromadora, Daptonema, Anoplostoma, Sphaerolaimus and Terschellingia species, closely resembling the communities of Northern European estuaries. In both estuaries, nematode density and community composition followed the salinity gradient, naturally establishing three distinct estuarine sections: (i) freshwater and oligohaline – characterised by the presence of freshwater nematodes, low nematode density and diversity; (ii) mesohaline – dominated by Terschellingia, Sabatieria and Daptonema, with low total density and diversity; and (iii) polyhaline and euhaline – where nematodes reached the highest density and diversity, and Paracomesoma, Synonchiella, and Odontophora were dominant.Despite the similarities in community composition and total nematode density, the proportion of different nematode feeding types were remarkably different in the two estuaries. In Mira, selective deposit feeders were dominant in the oligohaline section, while non-selective deposit feeders were dominant in the other sections. On the contrary, in the Mondego estuary, epigrowth-feeders and omnivores/predators were dominant in the freshwater sections and in the euhaline sector of the southern arm.Differences observed along each estuarine gradient were much stronger than overall differences between the two estuaries. In the Mondego estuary, the influence of anthropogenic stressors seemed not to be relevant in determining the nematodes' spatial distribution patterns, therefore suggesting that mesoscale variability responded essentially to natural stressors, characteristic of estuarine gradients. Nevertheless, the proportion of the different feeding types was different between the two estuaries, indicating that the response of nematode feeding guilds is able to reflect anthropogenic-induced stress and can be useful in assessing biological quality in transitional waters ecosystems.     

Setting reference conditions for mesohaline and oligohaline macroinvertebrate communities sensu WFD: Helping to define achievable scenarios in basin management plans

The Water Framework Directive requires all Member States to achieve good ecological quality status for all waters (e.g., transitional waters). For that purpose, Member States must assess water bodies based on information on the Biological Quality Elements listed for each of them (e.g., benthic macroinvertebrates). However, the production of such a quality status classification (high, good, moderate, poor, bad) requires high reference conditions (associated with the absence of, or very low, human pressure) against which the deviation of the samples to be assessed can be measured. In transitional waters, upper stretches have seldom been included in monitoring activities, resulting in very little knowledge of mesohaline and oligohaline areas, which means further difficulty when defining the required reference conditions for these zones.Regarding the benthic macroinvertebrates, large datasets from the mesohaline and oligohaline stretches of the Mondego estuary (four seasons, five years, environmental parameters, density and biomass data) were used to estimate high reference condition values. In terms of environmental conditions, summer was identified as the most stable season and the most suitable for defining reference conditions for selected ecological indicators. For each indicator, the multivariate linear model expressing the best correlation with measured environmental parameters was selected. These models were used afterwards, by replacing the environmental parameters in those equations with their high reference values, to calculate the reference condition for each ecological indicator.Generally, macrobenthic communities within each stretch changed over the years, being mainly influenced by salinity and sediment organic matter. In both stretches, only a few taxa occurred and two species (the amphipod Corophium multisetosum and the bivalve Corbicula fluminea) were clearly dominant. Diversity values (for Margalef, Shannon and ES₅₀ – Hurlbert indices) were low in both stretches, although higher in the mesohaline, and for the most part the ecological condition was low (AMBI – AZTI Marine Biotic Index, MEDDOC – Mediterranean Occidental index, BENTIX biotic index, BO2A – Benthic Opportunistic Annelida Amphipod index). On the whole, the RC estimated for each index followed the same trend, being different for each stretch and below those found for lower sections of the estuary in other surveys.     

Modeling growth and carbon allocation in two reed beds (Phragmites australis) in the Scheldt estuary

Common reed (Phragmites australis) is a prominent species in the upstream part of the eutrophic Scheldt estuary (Belgium, The Netherlands). From 1996 till 1998, seasonal growth dynamics of the species were studied in two monospecific stands subjected to different salinity regimes (seasonal means 1.6 and 13.3 PSU, respectively). We addressed the following questions: how are these reed vegetations affected by meteorological conditions and by the growth site, what are the important growth processes and what is the fate of the annually fixed carbon. A mathematical model was developed and calibrated using the data from the oligohaline site. Subsequent application of the model to the mesohaline stand required adaptation of parameters relating to the partitioning of resources and timing of growth initiation only. At their peak, the aboveground biomass was 587–1678 g DW m⁻² at the 13.3 PSU site and 1116–2179 g DW m⁻² (1.6 PSU); more than 60% of the biomass was located underground. In 1996, biomasses were 2–3 times lower than in the other 2 years, caused by a retarded growth initiation. Probably due to a lower temperature in early 1996, rhizome bud burst occurred more than 1 month later compared to the other years. In addition, growth initiation was several weeks later in the mesohaline site. This appeared mainly responsible for the large difference in maximal aboveground biomass between both stations. Architecture of the plants was also affected, with a higher shoot density (about 50% more shoots), better-developed root system (15% of total belowground biomass compared to 5%) and more, but smaller leaves at the higher salinity site. Notwithstanding large differences in aboveground biomass, annual growth was similar at both stations (154 and 132 mol C m⁻² per year at the oligo- and mesohaline station, respectively). Primary production accounted for about 80% of all growth processes, rhizome remobilization for almost 20%, translocation of mass before sloughing of leaves accounting about 3%. Within a year, some 44% (oligohaline) and 36% (mesohaline) of new assimilates produced by photosynthesis accumulated as dead litter. The other part was respired by the plant itself, either to provide the energy for growth (23%) or maintenance costs (33–41% at the oligo- and mesohaline station, respectively). Calculated annual turnover rates of aboveground biomass, rhizomes and roots were 100, 62 and 73%, respectively.     

Inorganic sulfur turnover in oligohaline estuarine sediments

Inorganic sulfur turnover was examined in oligohaline (salinity < 2 g kg^-1) Chesapeake Bay sediments during the summer. Cores incubated for < 3 hr exhibited higher sulfate reduction (SR) rates (13–58 mmol m^-2 d^-1) than those incubated for 3–8 hr (3–8 mmol m^-2 d^-1). SR rates (determined with³⁵SO ₄ ^2- ) increased with depth over the top few cm to a maximum at 5 cm, just beneath the boundary between brown and black sediment. SR rates decreased below 5 cm, probably due to sulfate limitation (sulfate < 25 μM). Kinetic experiments yielded an apparent half-saturating sulfate concentration (K_s) of 34 μM, ≈ 20-fold lower than that determined for sediments from the mesohaline region of the estuary. Sulfate loss from water overlying intact cores, predicted on the basis of measured SR rates, was not observed over a 28-hr incubation period. Reduction of³⁵SO ₄ ^2- during diffusion experiments with intact core segments from 0–4 and 5–9 cm horizons was less than predicted by non-steady state diagenetic models based on³⁵SO ₄ ^2- reduction in whole core injection experiments. The results indicate that net sulfate flux into sediments was an order of magnitude lower than the gross sulfur turnover rate. Solid phase reduced inorganic sulfur concentrations were only 2–3 times less than those in sediments from the mesohaline region of the Bay, despite the fact that oligohaline bottom water sulfate concentrations were 10-fold lower. Our results demonstrate the potential for rapid SR in low salinity estuarine sediments, which are inhabited by sulfate-reducing bacteria with a high affinity for sulfate, and in which sulfide oxidation processes replenish the pore water sulfate pool on a time scale of hours.     

D-Leu1]Microcystin-LR, from the cyanobacterium Microcystis RST 9501 and from a Microcystis bloom in the Patos Lagoon estuary, Brazil

[D-Leu1]Microcystin-LR was identified as the most abundant microcystin from a laboratory strain of the cyanobacterium Microcystis sp. isolated from a hepatotoxic Microcystis bloom from brackish waters in the Patos Lagoon estuary, southern Brazil. Toxicity of [D-Leu1]microcystin-LR, according to bioassay and protein phosphatase inhibition assay, was similar to that of the commonly-occurring microcystin-LR, which was not detectable in the Patos Lagoon laboratory isolate. This is the first report of a microcystin containing [D-Leu1] in the cyclic heptapeptide structure of these potent cyanobacterial toxins.     

Seasonal and environmental influences on recruitment patterns and habitat usage among resident and transient fishes in a World Heritage Site subtropical estuary

This study investigated whether the fish communities inhabiting shallow non‐vegetated habitats in two divergent bays in a subtropical World Heritage Site estuarine system differed according to wet (spring–summer) and dry (autumn–winter) seasons or polyhaline and mesohaline zones, within the broader objective of facilitating spatio‐temporal management. Species richness (total of 74 taxa; total length, L_T = 11–552 mm) and abundance (51 109 individuals) were mostly greater in the wet than dry season and in polyhaline than mesohaline areas. There was a major effect of rainfall on recruitment, particularly among transient fishes, which could be the result of enhanced survival of young via greater productivity (food resources) and protection from predators (via turbidity reducing visual cues). Salinity had strong interactive effects with rainfall and temperature in one bay, with greater species richness and overall abundances as well as large abundances of four key species [Anchoa januaria and Atherinella brasiliensis (pelagic residents), Cetengraulis edentulus (pelagic transient) and Diapterus rhombeus (demersal transient)] during the wet season in polyhaline areas; possibly reflecting a biodiversity hotspot that might be affected by distance to the estuary mouth and convergence hydrology. Regionally, the results support enforcing spatio‐temporal restrictions to minimize anthropogenic activities within statutory (but not always enforced) protected areas. Globally, the data reiterate the need to identify and understand biotic and abiotic effects on estuarine ichthyofaunal distributions and abundances as a precursor to their management.     

The occurrence, distribution and toxicity of cyanobacteria in the estuary of Lagoa dos Patos, Rio Grande do Sul

Several blooms of Microcystis aeruginosa have been observed in the Patos Lagoon estuary during the last fifteen years without a proper investigation of their ecological importance or possible toxicity. The present study has identified and quantified the presence of cyanobacteria in the Patos Lagoon estuary, particularly of M. aeruginosa. During this survey, identification and quantification of the main phytoplankton groups were done in relation to geographical distribution in the estuary. The presence of M. aeruginosa colonies in the estuarine region confirmed their superficial distribution throughout the estuarine waters during twelve months with a maximum of 1, 3.10(6) cells. L-1 in December, 1994 and a minimum of 1, 5.10(5) cells. L-1 in August, 1995 and also confirmed that M. aeruginosa originated from waters in the north of the estuary. The period of the highest cell and colonies densities was coincident with high chlorophyll-a levels in surface waters. Toxicity of M. aeruginosa bloom material was determined by bioassay and concentrations of hepatotoxins microcystins were identified by HPLC-DAD. M. aeruginosa blooms were considered highly toxic, presenting a 24 h-LD50 lower than 100 mg.Kg-1 b.w. and a toxin content higher than 1 microgram.mg-1 d.w. Several microcystin variants were found in the extracts with microcystin-LR predominating.     

Environmental Variables and Planktonic Communities in Two Ponds of El Hondo Wetland (SE Spain)

The annual cycle of physical and chemical variables and plankton dynamics was studied in two shallow ponds (East and West Ponds) of the El Hondo wetland, an ecosystem of international importance. Water conductivity increased up to 31–49 mS cm^–1 as water level decreased due to high evaporation and minimal water inputs. Initially considered mesohaline, the waters were reclassified as polyhaline during the hot season. EP was subject to successive desiccation‐flooding cycles, and flooding of the dried sediment caused the release of high concentrations of nitrogen and phosphorus compounds, which quickly depleted. The algal species composition was typical of eutrophic ecosystems, and the chlorophyll content indicated that EP was eutrophic and WP mesotrophic. Phytoplanktonic species richness and diversity were low in both ponds. Algal assemblages, in terms of biovolume, were mainly dominated by Dinophyceae in EP and by Cryptophyta in WP. The zooplankton community was dominated by Rotifers (Brachionus and Hexarthra), although Copepods and Ciliates were also important. Different water inputs to the ponds, partial drying in EP during the warm season with the subsequent higher increment of salinity, and the presence of dense populations of submerged macrophytes in WP, explain the differences in plankton communities found between the two ponds.     

Reproductive cycle and spatiotemporal variation in abundance of the one-sided livebearer Jenynsia multidentata, in Patos Lagoon, Brazil

Jenynsia multidentata is an important component of the fish assemblage of the Patos Lagoon estuary in southern South Brazil. In order to investigate its reproductive cycle and abundance patterns, standardized sampling was conducted over large spatial (marine, estuary and lagoon) and temporal (1996–2003) scales. Both females and males were significantly more abundant during summer (December–March) than winter (June–August). Total abundance was significantly positively correlated with water temperature (R=0.91), but not with salinity and Secchi depth. Females achieved higher average (49.1 mm L _T) and maximum size (91 mm) than males (37.7 mm; 66 mm), and average sex ratio was female-biased (3.2:1) across all months. An annual reproductive cycle composed of two cohorts was proposed: individuals born from December to March started reproducing during late winter and spring and individuals born from September to November started reproducing during late summer and fall. A 12-month survey conducted throughout the longitudinal gradient of the lagoon indicated that the species was only present in the estuary, and was absent from marine and upper lagoon areas. The abiotic factors analyzed could not explain this spatial distribution. Inter-annual variation in abundance was great, with higher abundance during drier years. A `dilution effect' was proposed to explain the low abundance of the species in the estuary during high-rainfall trigged by El Niño episodes.     

Potamopyrgus antipodarum(Mollusca:Hydrobiidae) in continental aquatic gastropod communities: impact of salinity and trematode parasitism

The structure of gastropod communities was examined from January to June 1999 in four sites of the streams of Mont Saint-Michel Bay along a gradient of salinity, and the occurrence of larval trematodes infecting snails was studied. Abundance and species richness of gastropods increased from polyhaline (95 snails, 1 species) to oligohaline waters (6672 snails, 6 species). Whatever the salinity, the most abundant species was Potamopyrgus antipodarum, an invasive non-indigenous species that represented 80% of the gastropods. Only one male was found in P. antipodarum populations suggesting a predominantly parthenogenetic mode of reproduction. Among 7218 gastropods collected, 1.2% were infected by larval trematodes: 5 species in Lymnaea peregra (4.4%), 4 species in Planorbis planorbis (12.0%), one echinostome in Physa acuta (0.2%), and a new species of Sanguinicola in P. antipodarum (0.5%). This is the first record of infected P. antipodarum in Europe. No parasites were found in polyhaline waters. The prevalence per host population varied from 0 to 100% depending on time of collection, salinity and host species. In the lowest-salinity site, abundance of gastropods and prevalence of trematodes were negatively correlated. The dominance of P. antipodarum in the gastropod communities is discussed in relation with euryhalinity, parthenogenesis and weak rate of parasitism.     

The food web in the lower part of the Seine estuary: a synthesis of existing knowledge

The Seine estuary illustrates the alterations to estuaries due to human activities heavy releases of pollutants of various origins and significant morphological changes beginning in the middle of the 19th century. The intertidal mudflat surface has been seriously reduced (< 30km²) since the channels of the Seine River came under management. While the role of the Seine estuary in the dynamics of the eastern English Channel ecosystem is recognized as important, the biological characteristics of the estuary remained relatively unknown until the 1990s. Biological diversity was progressively impoverished from the polyhaline zone to the oligohaline zone. In spite of a heavily contaminated environment, the macrobenthic and planktonic fauna of the Seine estuary remains similar to those of other northeastern Atlantic estuaries. The fauna exhibit clear contrasts between areas with very high abundance and others with very low abundance. The pelagic fauna, especially the copepod Eurytemora affinisand the shrimp Palaemon longirostris, are more abundant in the Seine estuary than in other estuaries. Diversified and abundant, Abra alba-Pectinaria koreni and Macoma balthica benthic communities occur, respectively, in the outer and inner parts of the estuary. In subtidal flats, benthic fauna is especially poor in terms of specific richness, abundance and biomass. Paradoxically, considering the high abundance of prey, fish are particularly scarce. Two food webs have been identified. In the oligohaline zone, where turbidity is maximum, the food web is exclusively planktonic, due to dredging that prevented benthic fauna from settling. In the polyhaline zone, fish populations that feed particularly on benthic fauna benefit from low turbidity and high oxygen concentrations. So, in spite of heavy organic and metallic contamination and human activities, the Seine estuary remains a highly productive ecosystem, which provides a nursery for marine fish and feeding grounds for migratory birds. A global management plan appears to be necessary in order to guarantee that the Seine estuary continues to function as it currently does.