Hydrodynamic habitat influences suspension feeding by unionid mussels in freshwater ecosystems

Abstract

Summary Benthic suspension feeders play significant roles in aquatic ecosystems, but the influences of abiotic, especially hydrodynamic, factors on many of their activities remain largely unknown. This is especially true for freshwater unionid mussels, which are important inhabitants of rivers and lakes, often forming multispecies assemblages in the former. Our goal is to determine whether and how seston flux affects the suspension feeding of freshwater unionid mussels from different hydrodynamic habitats (i.e. lentic and lotic). Fluorometric measurements of algal depletion in a recirculating flow chamber were used to determine the clearance rate (CR) of four freshwater mussel species: Elliptio complanata, Elliptio dilatata, Fusconaia flava and Strophitus undulatus. Clearance rate varied with algal flux (J = UC, where U is velocity and C is algal concentration) for all species examined, resulting in a 41‐fold increase in CR for some species, compared with no‐flux controls. Importantly, the results from the no‐flux controls were consistent with published CR values obtained under static conditions. Elliptio dilatata from the fastest flowing lotic system (Grand River, Ontario) cleared up to four times more algae than any other species, including its conspecifics from a slower flowing river (Ausable River, Ontario) and the species from the lentic habitat (E. complanata; Lake Opeongo, Ontario). Differences in CRs were also found among E. dilatata, F. flava and S. undulatus from the same lotic habitat at the highest algal flux examined, indicating that species may specialise to different hydrodynamic conditions. The impact of suspension feeding by unionid mussels has probably been underestimated, especially under lotic conditions with high seston flux. Mussels may have ecotypic responses to their hydrodynamic habitats, which affect their CR. Moreover, specialisation within a single habitat type is indicative of resource partitioning that could reduce competition in multispecies assemblages.