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

Sediment mixing by freshwater suspension feeding bivalves Anodonta gra ndis (Unionidae) and Sphaerium striatinum (Pisidiidae) was studied by adding illite clay particles with adsorbed Cs-137 as a submillimeter t hick layer to the surface of silt clay sediments contained in clear re ctangular cells maintained in a temperature regulated aquarium. A NaI gamma detector scanned the sediment column in each cell at 0.2 cm inte rvals 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 ce ll was 0.02 cm(2)/yr, consistent with molecular diffusion of Cs-137 tr acer 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. Ad justing to equal population densities, the 11-27x higher sediment mixi ng rate of A. grandis was likely due primarily to its larger size. Whe n D-b for similar sized organisms was compared, S. striatinum was foun d 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 size d conveyor belt deposit feeding marine bivalve, Yoldia limatula. While deposit feeding organisms are the dominant sediment mixers in the Gre at Lakes, suspension feeding bivalves can be locally significant.