Trophic effects of sponge feeding within Lake Baikal's littoral zone. 2. Sponge abundance, diet, feeding efficiency, and carbon flux

Endemic freshwater demosponges in the littoral zone of Lake Baikal, Russia, dominate the benthic biomass, covering 44% of the benthos. We measured in situ sponge abundance and grazing and calculated sponge‐mediated fluxes of picoplankton (plankton <2 µ m) for two common species, Baikalospongia intermedia and Baikalospongia bacillifera. By means of dual‐beam flow cytometry, we found retention efficiencies ranging from 58 to 99% for four types of picoplankton: heterotrophic bacteria, Synechococcus‐type cyanobacteria, autotrophic picoplankton with one chloroplast, and autotrophic picoplankton with two chloroplasts. By using a general model for organism‐mediated fluxes, we conservatively estimate that through active suspension feeding, sponges are a sink for 1.97 g C d −1 m −2 , mostly from procaryotic cell types. Furthermore, grazing by these extensive sponge communities can create a layer of picoplankton‐depleted water overlying the benthic community in this unique lake.