PARTICLE MIXING RATES OF FRESH-WATER BIVALVES - ANODONTA-GRANDIS (UNIONIDAE) AND SPHAERIUM-STRIATINUM (PISIDIIDAE)

Sediment mixing by freshwater suspension feeding bivalves Anodonta grandis (Unionidae) and Sphaerium striatinum (Pisidiidae) was studied by adding illite clay particles with adsorbed Cs-137 as a submillimeter thick layer to the surface of silt clay sediments contained in clear rectangular cells maintained in a temperature regulated aquarium. A NaI gamma detector scanned the sediment column in each cell at 0.2 cm intervals five times over 22 days and recorded changes in Cs-137 activity over time with depth in cells containing 3 A. grandis, 4 S. striatinum, and a control cell containing no bivalves. Sediment mixing by these organisms was diffusional. The diffusion coefficient in the control cell was 0.02 cm(2)/yr, consistent with molecular diffusion of Cs-137 tracer Whole cell biodiffusion coefficients (D-b) for A. grandis and S. striatinum were 0.81-2.11 cm(2)/yr and 0.53 cm(2)/yr, respectively. Adjusting to equal population densities, the 11-27x higher sediment mixing rate of A. grandis was likely due primarily to its larger size. When D-b for similar sized organisms was compared, S. striatinum was found to mix sediments at about the same rate as the marine bivalve Nucula proxima but at a 5x lower rate than the freshwater amphipod Diporeia sp. A. grandis mixes sediments 5-14x more slowly than the similar sized conveyor belt deposit feeding marine bivalve, Yoldia limatula. While deposit feeding organisms are the dominant sediment mixers in the Great Lakes, suspension feeding bivalves can be locally significant.