Suspended material availability and filtration–biodeposition processes performed by a native and invasive bivalve species in streams

Unionid mussels are among the most threatened group of freshwater organisms globally. They are known for their ability to filter food particles from flowing and standing waters. However, invasive bivalve species, such as the Asian clam (Corbicula fluminea) in North America, have the potential to overlap in feeding and potentially out-compete the native species. Yet, the feeding preferences of unionid mussels and C. fluminea are incompletely understood. We hypothesized that Elliptio crassidens (native) and C. fluminea (invasive) would select for specific organic components present within seston. We examined changes in seston (dry mass and ash-free dry mass) resulting from bivalve feeding activity for three size classes of material that were isolated using gravimetric filtration. The treatments were also sub-sampled for flow cytometry (FC) which separated the suspended materials in the stream water into five categories: detritus, heterotrophic bacteria, picoautotrophs, nanoautotrophs, and heterotrophic nanoeukaryotes. Our results indicated that both species of bivalve showed preferences for organic and living materials. E. crassidens preferentially filtered nanoeukaryotes, whose decreases were associated with an increase in bacteria. In contrast, C. fluminea preferred smaller materials through selective filtration of picoautotrophs. In addition, both species increased the concentration of large materials toward the end of the experiment because of the suspension of their pseudofeces biodeposits. To our knowledge, this study is the first to examine grazing by bivalve species on natural stream particulate matter using FC. Our results suggest that native and non-native mussels have different functional roles, which has important implications for organic matter processing and food webs in streams.     

Use of Bivalve Metrics to Quantify Influences of Coal-related Activities in the Clinch River Watershed, Virginia

Previously, we reported that survivorship and growth of field-caged Asian clams (Corbicula fluminea [Müller]) were significantly reduced downstream of the wastewater effluent discharge of the Clinch River Plant (CRP), a coal-fired electric power-generating facility in Russell County, VA (USA). These findings warranted the present study, which investigated whether reduced survivorship and growth of transplanted C. fluminea were predictive of adverse effects on resident bivalves – most notably the Clinch River’s (CR’s) federally protected freshwater mussel fauna (Unionoidea). Thus, surveys of unionoid assemblages and C. fluminea population densities and age distributions were conducted to assess ecotoxicological effects on bivalve assemblages naturally occurring in the CR. Results of these surveys suggested that field bioassays with transplanted C. fluminea were predictive of in-stream C. fluminea densities and the presence/absence of indigenous unionoid assemblages. Relative to C. fluminea, additional field bioassays with a juvenile unionoid species (Villosa iris) native to the CR were less predictive of adverse effects on resident bivalves. Concurrence of transplant studies and in-stream surveys was evident at most study sites, particularly within 0.6 km downstream of the CRP effluent where resident bivalves were virtually non-existent relative to other stream sites having similar habitat conditions. Our findings indicate that in situ field bioassays with transplanted C. fluminea are valuable tools for predicting in stream occurrence and distribution of resident freshwater bivalves, and that field studies integrating multiple bivalve metrics may be useful for prioritizing source-reduction efforts in similar watersheds. Discrepancies observed for transplanted C. fluminea and V. iris have important implications to selection of bivalve species used in field bioassays and warrant further study.     

Effects of depth, sediment and grazers on the degradation of drifting filamentous algae (Cladophora glomerata and Pilayella littoralis)

Eutrophication in the northern Baltic Sea promotes growth of annual filamentous algae. The algae detach, gather at the bottom and give rise to algal mats of varying size, density, composition and condition. Dense mats of filamentous algae induce anoxia, which in turn leads to faunal mortality. By a set of field experiments, we have studied the fate of the abundant Cladophora glomerata after detaching from the rocky substrate, and the effect of water depth and sediment on its decay. Further, we have studied the importance of common mesograzers (Gammarus and Idotea) on the rate of degradation of C. glomerata and Pilayella littoralis.Our results show that loose algae at shallow sites (8 m) decompose faster than algae in deeper (18 m) areas. Drifting C. glomerata on the sediment is more rapidly broken down and dissolved than algae floating in the water column, which depends on higher microbiological activity. Dominant amphipods (Gammarus spp) colonise near-shore drift algae quickly, and juvenile bivalves (Cerastoderma glaucum) utilise algae in the water column for settling. Moderate natural densities of grazers (Gammarus spp and Idotea baltica) in the drifting algae did not increase the degradation rates of C. glomerata and P. littoralis. C. glomerata was completely decomposed in 4 months.Our experiments demonstrate the effects of position (depth, water/sediment) and grazing on the degradation of drifting filamentous algae. Mass developments of opportunistic algae occur annually in the study area, and information on the destiny of detached drift algae may help us to predict their longevity and the damage they cause, and hence, to decide on long-term measures needed to improve environmental conditions.     

Linking terrestrial and benthic estuarine ecosystems: organic matter sources supporting the high secondary production of a non-indigenous bivalve

The Asian clam, Corbicula fluminea, is among the most pervasive invasive species in freshwater ecosystems worldwide. Our objective was to study C. fluminea’s functional response in terms of feeding behavior and food selectivity, using the natural variation in organic matter (OM) sources that occur in estuarine environments. Using C and N stable isotopes, we identified and quantified the contribution of different OM sources supporting the production of C. fluminea along the salinity gradient occupied in the Minho River estuary (NW-Iberian Peninsula, Europe), where this species presently dominates the benthic macrofauna biomass. We observed a pronounced shift in the quality of OM available for C. fluminea along the estuarine mixing zone. Stable isotope analysis, POM C/N, and phytoplankton contribution estimates based on C:Chl a revealed that POM was largely comprised of terrestrial-derived OM in tidal freshwater stations (TFW) and was increasingly comprised of phytoplankton, a more palatable food source, towards the polyhaline estuary. A similar shift in the isotopic composition along the estuarine mixing zone was observed in C. fluminea, suggesting a shift in food resources. Accordingly, based on a Bayesian stable isotope mixing model, there was an upstream–downstream counter gradient in the contribution to C. fluminea biomass from terrestrial-derived OM (41–64 % in TFW) and phytoplankton (29–55 % in the brackish estuary). Although the majority of the food sources identified were filtered from the water column (70–80 %), reliance on sediment OM and microphytobenthos provided evidence for deposit feeding by C. fluminea. We conclude that C. fluminea has the ability to adapt to environments with low food quality because it can consume terrestrial-derived OM. This can be a competitive adaptation in systems with perennial low food quality such as the Minho River estuary. Moreover, its ability to couple benthic and pelagic environments and terrestrial ecosystems demonstrates a strong potential to alter food web flows in aquatic ecosystems.     

Impact of Dreissena fouling on the physiological condition of native and invasive bivalves: interspecific and temporal variations

The impact of Dreissena fouling on unionids has hardly been studied in Europe, despite the fact that in some ecosystems (e.g. Lake Balaton, Hungary) infestations of several hundreds to a thousand individuals per unionid have been observed. At present, the zebra mussel Dreissena polymorpha is a dominant species in Lake Balaton and in the last decade three other invasive bivalves were introduced, potentially increasing the pressure on native unionid survival. We examined whether the fouling of dreissenids (zebra and quagga (D. rostriformis bugensis) mussels) has a negative impact on native (Anodonta anatina, Unio pictorum and U. tumidus) and invasive (Corbicula fluminea and Sinanodonta woodiana) bivalves and whether there are any interspecific and temporal variations in fouling intensity and physiological condition measured by standard condition index and glycogen content. A significant negative impact was detected on native unionids only in July and September (no impact was detected in May), when the fouling rate was high. For invasive species, a significant negative impact was detected on S. woodiana with a high level of dressenid infestation; whereas no significant impact was detected on C. fluminea. Overall, this study confirms that Dreissena may threaten unionid species including the invasive S. woodiana, although high interspecific and temporal variations were observed. This situation should be taken into account in future ecological and conservational assessments because species respond differently to Dreissena fouling and effects seem to be more pronounced in late summer/early autumn. In addition, this study provides the first evidence that the invasive C. fluminea appear to be less vulnerable to dressenid fouling.     

The functional role of burrowing bivalves in freshwater ecosystems

1. Freshwater systems are losing biodiversity at a rapid rate, yet we know little about the functional role of most of this biodiversity. The ecosystem roles of freshwater burrowing bivalves have been particularly understudied. Here we summarize what is known about the functional role of burrowing bivalves in the orders Unionoida and Veneroida in lakes and streams globally. 2. Bivalves filter phytoplankton, bacteria and particulate organic matter from the water column. Corbicula and sphaeriids also remove organic matter from the sediment by deposit feeding, as may some unionids. Filtration rate varies with bivalve species and size, temperature, particle size and concentration, and flow regime. 3. Bivalves affect nutrient dynamics in freshwater systems, through excretion as well as biodeposition of faeces and pseudofaeces. Excretion rates are both size and species dependent, are influenced by reproductive stage, and vary greatly with temperature and food availability. 4. Bioturbation of sediments through bivalve movements increases sediment water and oxygen content and releases nutrients from the sediment to the water column. The physical presence of bivalve shells creates habitat for epiphytic and epizoic organisms, and stabilizes sediment and provides refugia for benthic fauna. Biodeposition of faeces and pseudofaeces can alter the composition of benthic communities. 5. There is conflicting evidence concerning the role of resource limitation in structuring bivalve communities. Control by bivalves of primary production is most likely when their biomass is large relative to the water volume and where hydrologic residence time is long. Future studies should consider exactly what bivalves feed upon, whether feeding varies seasonally and with habitat, and whether significant overlap in diet occurs. In particular, we need a clearer picture of the importance of suspension versus deposit feeding and the potential advantages and tradeoffs between these two feeding modes. 6. In North America, native burrowing bivalves (Unionidae) are declining at a catastrophic rate. This significant loss of benthic biomass, coupled with the invasion of an exotic burrowing bivalve (Corbicula), may result in large alterations of ecosystem processes and functions.     

Can suspension feeding by bivalves regulate phytoplankton biomass in Lake Waccamaw, North Carolina?

Suspension feeding by bivalves has been hypothesized to control phytoplankton biomass in shallow aquatic ecosystems. Lake Waccamaw, North Carolina, USA is a shallow lake with a diverse bivalve assemblage and low to moderate phytoplankton biomass levels. Filtration and ingestion rates of two relatively abundant species in the lake, the endemic unionid, Elliptio waccamawensis, and an introduced species, Corbicula fluminea, were measured in experiments using natural phytoplankton for durations of 1 to 6 days. Measured filtration and ingestion rates averaged 1.78 and 1.121 ind.^–1 d^–1, much too low to control phytoplankton at the observed phytoplankton biomass levels and growth rates. Measured ingestion rates averaged 4.80 and 1.50 µg chlorophyll a ind.^–1 d^–1, too low to support individuals of either species. The abundance of benthic microalgae in Lake Waccamaw reaches 200 mg chlorophyll a m^–2 in the littoral zone and averages almost an order of magnitude higher than depth-integrated phytoplankton chlorophyll a. Total microalgal biomass in the lake is therefore not controlled by suspension feeding by bivalves.     

Molecular detection of potentially toxic cyanobacteria and their associated bacteria in lake water column and sediment

We investigated the molecular diversity of cyanobacteria and bacteria during a water bloom in a lake with a long history of toxic cyanobacterial blooms (Lake Kastoria, Greece). We also tested the hypothesis whether bloom-forming cyanobacteria are preserved in the lake’s sediment 2 years after the bloom. The dominant cyanobacteria during the bloom included the potentially toxin-producing Microcystis aeruginosa and several other Chroococcales forms closely related to the genus Microcystis. This suggests that the use of cyanobacterial-specific primers seems to be very informative in describing the cyanobacteria during the water blooms. The bacterial community showed high diversity, consisting mostly of singleton and doubleton phylotypes. The majority of the phylotypes were typical lake bacteria including some potential pathogens and toxin metabolising bacteria, suggesting that the dominant toxic cyanobacteria did not have any significant effect on the bacterial community structure. In the sediment, 2 years after the water bloom, no bloom-forming cyanobacteria were retrieved, suggesting that they cannot be preserved in the sediment. Similar to the water column, sediment bacterial diversity was also high, consisting mostly of yet-uncultured bacteria that are related to environments where organic matter degradation takes place.     

Spatial distribution, seston removal, and potential competitive interactions of the bivalves Corbicula fluminea and Elliptio complanata, in a coastal plain stream

SUMMARY.

• 1 Within the past 10 years the introduced exotic bivalve, Corbicula fluminea, has spread from the Savannah River through a riverine swamp into a third-order blackwater tributary on the Savannah River Site (SRS). We examined the effects of this invasion on a native bivalve, Elliptio cornplanata, and on seston concentration in the stream.
• 2 Corbicula fluminea has not yet colonized the entire stream and its distribution limit is coincident with the occurrence of gravel beds.
• 3 Corbicula fluminea appears to cause localized reduction of seston concentration and may rapidly clear the sediment boundary layer of food. There was no evidence of a negative impact on the distribution of the native bivalve in spite of high measured rates of water clearance by C. fluminea.

     

Filtration activity of invasive mussel species under wave disturbance conditions

Self-purification is one of the most important ecosystem functions of rivers. Multiple human activities regularly impact this ecosystem service, consequently altering river morphology, hydrology, and the composition of biotic assemblages that contribute to self-purification. However, little quantitative information is available about the importance of such impacts. Hence, we tested how invasive mussel species contribute to self-purification under disturbed riverine conditions. In laboratory experiments, invasive mussel species equipped with magnetic sensors that recorded filtration activity were exposed to artificial waves of varying intensity that simulated the hydraulic effects of inland navigation. Our results suggest that invasive mussel species are more resistant to wave disturbance compared to native species, as estimated threshold values for initiating shell closure are very high (Dreissena rostriformis bugensis) or the duration (Corbicula fluminea) and degree of shell closing (D. rostriformis bugensis, C. fluminea) very low. Also we demonstrated that the invasive species D. rostriformis bugensis and C. fluminea continued filtering during wave impact, whereas Dreissena polymorpha did not behave significantly differently than previously studied native mussel species, based on the studied susceptibility parameters. Thus, D. rostriformis bugensis and C. fluminea appear to be pre-adapted to hydraulic or morphological disturbance, and may compensate against other losses regarding this important ecosystem function in rivers that are intensively used for inland navigation. However, as the dominance of invasive species in river systems may disrupt natural food webs, this compensation of filter-feeding activity may be accompanied by the loss of other ecosystem functions.     

Potential Impact of Filter-feeding Invaders on Temperate Inland Freshwater Environments

Since the 1990s, biological invasions have captured the attention of the scientific community as an important element of global change and a major threat to biodiversity. The inland waters of South America provide two examples of biological invasions. This review examines bivalve invasions in South America, summarizes the research results for two species, the Asian clam (Corbicula fluminea) and the golden mussel (Limnoperna fortunei), and suggests further studies. The rapid expansion of invasive bivalves into these environments involves significant changes. Until now, C. fluminea, the Asian clam, did not produce generalized macrofouling in the Neotropical region, as is common in the Holarctic region. However, the first specific cases of macrofouling by C. fluminea were recently detected in heat interchangers of power stations in Brazil. On the other hand, L. fortunei is provoking new economic impacts in South American freshwaters through macrofouling. Before the invasion by the golden mussel, macrofouling was recorded only in the marine and estuarine environments of the Neotropical region. The impact caused by invasive bivalves in this region is not only economic, however. Rapid changes in the benthic community, favoring the presence of Oligochaeta and Hirudinea, as well as the displacement of native species of mollusks, are among the problems related to the presence of the golden mussel. Another issue is the settlement of golden mussels on native bivalves. This bivalve is now a new element in the diet of some native fish species, being the main food item in some cases.     

Improved Method for Determining Bacterial Filtration Rates in Zooplankton

Filtration rates were determined for a natural population of zooplankton grazers (Bosmina longirostris [Müll.], Cyclops vicinus vicinus [Ulianine], Acanthodiaptomus denticornis [Wierz.], and Daphnia longispina [Müll.]) by using ³H-labeled bacteria as food for these organisms. There was a relationship between filtration rates of the major zooplankton grazers and the prevailing algal and bacterial composition in the lake water. Low filtration rates were obtained in the presence of colonial and filamentous cyanobacteria. The rapid process of bacterial adhesion to the external organs of grazers can result in an overestimation of filtration rates. By using the simple method presented here, filtration rates, with simultaneous correction for bacterial adhesion, can be quickly determined.     

Quantification of sediment reworking by the Asiatic clam Corbicula fluminea Müller, 1774

Active organisms modify the substratum in which they dwell. This process, called “bioturbation”, affects the way that biogeochemical fluxes are mediated at the substratum–water interface. In the frame of this work, the bioturbation potential of the Asiatic clam Corbicula fluminea was characterized and quantified. We measured the displacement of fluorescent particles by C. fluminea burying in a size-based experimental design in order to explore the effects of body-size on sediment reworking. Our results stress that C. fluminea belongs to the functional group of biodiffusors, and that C. fluminea can be considered as an intermediate sediment reworker. We suggest that bioturbation was mainly induced by the pedal-feeding activity of the clams. Results also showed that, though large clams induced displacement of particles deeper into the sediment, small clams showed the highest net sediment reworking activity. This result was in contrast to the initial hypothesis of biovolume as the main driver for particle displacement by bioturbating organisms. Life-history traits and specific features of pedal-feeding could explain the observed pattern.     

Ecosystem services transcend boundaries: estuaries provide resource subsidies and influence functional diversity in coastal benthic communities

Background

Estuaries are highly productive ecosystems that can export organic matter to coastal seas (the ‘outwelling hypothesis’). However the role of this food resource subsidy on coastal ecosystem functioning has not been examined.

Methodology/Principal Findings

We investigated the influence of estuarine primary production as a resource subsidy and the influence of estuaries on biodiversity and ecosystem functioning in coastal mollusk-dominated sediment communities. Stable isotope values (δ¹³C, δ¹⁵N) demonstrated that estuarine primary production was exported to the adjacent coast and contributed to secondary production up to 4 km from the estuary mouth. Further, isotope signatures of suspension feeding bivalves on the adjacent coast (Dosinia subrosea) closely mirrored the isotope values of the dominant bivalves inside the estuaries (Austrovenus stutchburyi), indicating utilization of similar organic matter sources. However, the food subsidies varied between estuaries; with estuarine suspended particulate organic matter (SPOM) dominant at Tairua estuary, while seagrass and fringing vegetation detritus was proportionately more important at Whangapoua estuary, with lesser contributions of estuarine SPOM. Distance from the estuary mouth and the size and density of large bivalves (Dosinia spp.) had a significant influence on the composition of biological traits in the coastal macrobenthic communities, signaling the potential influence of these spatial subsidies on ecosystem functioning.

Conclusions/Significance

Our study demonstrated that the locations where ecosystem services like productivity are generated are not necessarily where the services are utilized. Further, we identified indirect positive effects of the nutrient subsidies on biodiversity (the estuarine subsidies influenced the bivalves, which in turn affected the diversity and functional trait composition of the coastal sediment macrofaunal communities). These findings highlight the importance of integrative ecosystem-based management that maintains the connectivity of estuarine and coastal ecosystems.     

Alien molluscs affect the composition and diversity of native macroinvertebrates in a sandy flat of Lake Neuchâtel,Switzerland

The spread of alien molluscs is a serious threat to native biodiversity in fresh waters. Alien freshwater molluscs may deplete the resources of native species and alter the physical structure of the habitat through their shell mass. These changes might have both positive and negative effects on native community members. We investigated the native macroinvertebrate community in relation to the densities of four alien mollusc species (Corbicula fluminea, Dreissena polymorpha, Potamopyrgus antipodarum and Lithoglyphus naticoides) in a sandy flat of Lake Neuchatel, Switzerland. The habitat examined was dominated by these alien mollusc species. The abundance of the alien molluscs did not directly impact the native community assembly. However, C. fluminea and D. polymorpha influenced the composition and diversity of native macroinvertebrates by transforming the sandy substratum into a partly hard substratum habitat. Substantial differences in community composition between shallow (<3.5 m) and (≥5 m) deep sites were recorded. At shallow sites, the abundance of D. polymorpha was significantly reduced as a result of depth-selective feeding of ducks. A controlled shell decay study revealed that shells of alien molluscs (C. fluminea, D. polymorpha) persist for a longer period in the sediment than those of native molluscs. Consequently, shells of alien molluscs have a long-lasting impact by modifying the sandy habitat. This form of ecosystem engineering favours the occurrence of several native taxa, but is disadvantageous for other taxa with specific habitat requirements, and thus can be regarded as an indirect impact of competition.     

Effects of Upper-Limit Water Temperatures on the Dispersal of the Asian Clam Corbicula fluminea

Temperature is a determinant environmental variable in metabolic rates of organisms ultimately influencing important physiological and behavioural features. Stressful conditions such as increasing temperature, particularly within high ranges occurring in the summer, have been suggested to induce flotation behaviour in Corbicula fluminea which may be important in dispersal of this invasive species. However, there has been no experimental evidence supporting this hypothesis. It was already proven that C. fluminea drift is supported by a mucilaginous drogue line produced by mucocytes present in the ctenidia. Detailed microscopic examination of changes in these cells and quantification of clam flotation following one, two and three weeks of exposure to 22, 25 and 30°C was carried out so that the effects of increasing water temperatures in dispersal patterns could be discussed. Results show that changes in temperature triggered an acceleration of the mucocytes production and stimulated flotation behaviour, especially following one week of exposure. Dilution of these effects occurred following longer exposure periods. It is possible that these bivalves perceive changing temperature as a stress and respond accordingly in the short-term, and then acclimate to the new environmental conditions. The response patterns suggest that increasing water temperatures could stimulate C. fluminea population expansion.     

Can bivalve veligers escape feeding currents of adult bivalves?

While the stock of introduced Pacific oysters (Crassostrea gigas) increased in the Oosterschelde estuary (SW Netherlands), so did the filtration pressure of all bivalve species together. In the same period, stocks of native bivalves declined slightly. The expansion of Pacific oysters in Dutch estuaries might be partially due to better abilities of their larvae to avoid or escape filtration, compared to larvae of native bivalves. In this context, escape and swimming abilities of Pacific oyster larvae and the larvae of the native blue mussel (Mytilus edulis) were compared.Swimming behaviour of C. gigas larvae and larvae of M. edulis was recorded in still water and in a suction current mimicking a bivalve feeding current, in a horizontal and in a vertical plane. Larval swimming behaviour in a suction flow field was reconstructed by subtracting local water movement vectors from the total movement of larvae, yielding movement paths due to larval swimming alone.Swimming speeds and the rate of displacement in vertical direction of C. gigas and M. edulis larvae were related to larval shell length, and to the pitch of up- or downward swimming.Larvae of both species did not show escape reactions in a suction flow field. With increasing shell length, larval swimming speeds of both species increased significantly. Swimming speeds of C. gigas larvae were significantly higher than swimming speeds of M. edulis larvae, resulting in a faster vertical displacement. The ability to migrate to more favourable water layers faster may offer C. gigas an advantage over native bivalves with slower swimming larvae.     

The use of growth and ingestion rates of Capitella sp. I as the bioassay approaches to determine the sediment quality of coastal wetlands of Taiwan

Food is a limiting factor for the deposit feeders. The availability of sediment nutrients thus has a tight relationship with the growth, survival and development of the animal. There are two purposes of this study: (1) to determine if the ingestion and growth rates can be used as a bioassay approach to assess the sediment nutrients; and (2) use the combination of bioassay approaches and chemical analyses to determine which chemical parameter is the better predicator of the sediment nutrients to the animals. In the preliminary study, the optimal growth length and average ingestion rate of Capitella sp. I were obtained from the laboratory. The standardized relationships of the growth and ingestion rates in response to different nutrients were prepared. Then, the sediments collected from different coastal wetlands in Western Taiwan were used in the feeding, growth experiments and chemical analyses. The comparisons were made between the field and laboratory experiments to determine the sediment nutrients in the wetland of Taiwan. In the growth rate standardized relationship, Capitella sp. I increased its growth rate with the total organic nitrogen (TON) concentration between 0 to 2.8 mgN·g sediment⁻¹, total organic carbon (TOC) concentration between 0 and 22.4 mgC·g sediment⁻¹, and enzymatically hydrolyzable amino acid (EHAA) concentration between 0 and 4.48 mg protein·g sediment⁻¹. After the nutrient concentrations exceed these values, the growth rates decreased gradually. In the ingestion rate standardized relationship, the animal increased its ingestion rate with the total organic nitrogen (TON) concentration between 0 and 2 mgN·g sediment⁻¹, total organic carbon (TOC) concentration between 0 and 14.1 mgC·g sediment⁻¹, and EHAA concentration between 0 and 3.2 mg protein·g sediment⁻¹. After the nutrient concentrations exceed these values, the ingestion rates also decreased. To determine which nutrient parameter is the best predictor for the sediment nutrient in the field, we first analyzed whether the data obtained from the laboratory fell within 99% confidence interval of the regression obtained from the field data. Then, to determine which parameter had the shortest perpendicular distance between the field and the laboratory regression curves. Both the growth and ingestion rates comparisons showed that the EHAA is the best candidate of the sediment nutrient of deposit feeders in the field. The results of this study proved tentatively that the growth and ingestion rates of Capitella sp. I can be used as the bioassay approaches to estimate the sediment nutrients. The combination of the bioassay approaches and the relevant chemical analyses allows us to determine the bioavailability fraction of sediment to the deposit feeders.     

Cholinesterases from the marine mussels Mytilus galloprovincialis Lmk. and M. edulis L. and from the freshwater bivalve Corbicula fluminea Müller

In order to improve the molecular basis for the use of bivalve cholinesterases as a reliable biomarker for aquatic pollution, the polymorphism and characterization of these enzymes in Mytilus edulis, Mytilus galloprovincialis and Corbicula fluminea were investigated. All results are consistent with the presence of only one pharmacological form of cholinesterase in each species. The molecular masses were 180 kDa for the two marine mussels and 240 kDa for C. fluminea. The cholinesterases are anchored to the membrane by a glycosyl inositol phosphate like the G^a form (type I) described in vertebrates. Surprisingly, these cholinesterases were poorly inhibited by organophosphorous compounds compared to enzymes from other sources. This suggests that these bivalves could be used as a biomarker for acute rather than chronic contaminations by anticholinesterase insecticides.