TROPHIC EFFECTS OF SPONGE FEEDING WITHIN LAKE BAIKAL LITTORAL-ZONE .2. SPONGE ABUNDANCE, DIET, FEEDING EFFICIENCY, AND CARBON FLUX

Endemic freshwater demosponges in the littoral zone of Lake Baikal, Ru ssia, dominate the benthic biomass, covering 44% of the benthos. We me asured in situ sponge abundance and,orating and calculated sponge-medi ated Fluxes of picoplankton (plankton <2 mu m) for two common species, Baikalospongia intermedia and Baikalospongia bacillifera. By means of dual-beam how cytometry, we found retention efficiencies ranging from 58 to 99% for four types of picoplankton: heterotrophic bacteria, Syn echococcus-type cyanobacteria, autotrophic picoplankton with one chlor oplast, and autotrophic picoplankton with two chloroplasts. By using a general model for organism-mediated fluxes, we conservatively estimat e that through active suspension feeding, sponges are a sink for 1.97 g C d(-1) m(-1), mostly from procaryotic cell types. Furthermore, graz ing by these extensive sponge communities can create a layer of picopl ankton-depleted water overlying the benthic community in this unique l ake.