From picoplankton to microplankton: temperature-driven filtration by the unionid bivalve Lampsilis radiata siliquoidea in Lake St. Clair

Size-selective filtration by the unionid bivalve Lampsilis radiata siliquoidea was determined seasonally for Lake St. Clair seston with special emphasis placed on determining retention efficiency of particles less than 1 μm equivalent spherical diameter (ESD). These results allowed us to estimate the aggregate clearance rates of Lampsilis and total unionids in the lake. Retention efficiency for seston in the 0.79–1.00 μm ESD size category was 0.44 whereas 0.07 was the corresponding value for 0.91-μm microspheres in laboratory prepared mixtures of microspheres and cultured algae; respective values for the 0.63–0.79 μm size category were not significantly different from zero. Particle shape may explain the difference between natural seston and microspheres. Particles in the picoplankton size range (<3 μm ESD) were probably an important food resource to the mussels, since 35–49% of the seston volume in Lake St. Clair was smaller than 3 μm ESD. Regression analyses of filtration rate as a function of temperature and food concentration showed that feeding rate was temperature driven and not saturated by available seston concentration. Lampsilis and total community clearance rates were maximum in September, with respective fractions of the water columns swept clear of 0.018 and 0.053∙d−1.