Molluscan assemblages under multiple stressors in a large fluvial lake

Freshwater molluscs can be affected by various environmental and anthropogenic pressures, which consequently determine their distribution, richness, and abundance in freshwater ecosystems. The objectives of this study were to determine the relative influence of water quality, exposure to wind, and emersion on the composition, diversity, and abundance of molluscan assemblages in a large fluvial lake. For this, we sampled molluscs at 14 sites located at increasing distances from agricultural tributaries flowing along the north (exposed) and south (sheltered) shores of Lake Saint-Pierre (St. Lawrence River). We measured physical characteristics (fetch, emersion, current speed, water depth, and transparency), chemical water quality (total suspended matter, dissolved iron, total phosphorus, nitrates, ammonium, dissolved organic carbon), and macrophyte biomass at each site. Abundance and richness of molluscan assemblages were lowest at sites periodically emersed and exposed to prevailing winds, where macrophytes were absent. Sites located downslope and protected from wind and waves were characterized by abundant molluscs and submerged macrophytes. Sphaeriid clams, however, were negatively associated with aquatic vegetation and were most abundant at exposed sites. Our results revealed the overwhelming importance of physical environmental conditions on both gastropods and bivalves either directly or through submerged macrophyte alteration. Poor water quality from agricultural tributaries, however, explained part of the variation in taxonomical composition of gastropods (total suspended matter and dissolved iron) and bivalves (dissolved organic carbon). With climate change, the rising frequency of extreme water levels and unpredictable flood events will likely negatively impact molluscan assemblages, which represent the basis of the trophic network supporting aquatic vertebrates.