IN-SITU GRAZING ON PLANKTON LESS-THAN-10 MU-M BY THE BOREAL SPONGE MYCALE-LINGUA

Ultraplankton, heterotrophic and autotrophic plankton < 5 mu m, are th e most abundant food source in the world's oceans, yet their role as a food source for macroinvertebrates is largely unexamined. We quantifi ed in situ feeding on heterotrophic and autotrophic plankton < 10 mu m by the boreal sponge Mycale lingua using measurements that quantified sponge feeding efficiencies, pumping rates, and abundance to determin e the contribution of plankton < 10 mu m to sponge carbon intake. Usin g dual-beam now cytometry we identified 5 populations of plankton < 10 mu m: heterotrophic bacteria, Prochlorococcus, Synechococcus-type cya nobacteria, autotrophic eucaryotes < 3 mu m, and autotrophic eucaryote s 3 to 10 mu m Mycale lingua nonselectively grazed on all types of pla nkton < 10 mu m. Prochlorococcus was filtered with the highest efficie ncy (93%), followed by Synechococcus-type cyanobacteria (89%), autotro phic eucaryotes 3 to 10 mu m (86%), heterotrophic bacteria (74%), and autotrophic eucaryotes < 3 mu m (72%). We conservatively estimate that M. lingua al naturally occurring densities can obtain 29 mg C d(-1) m (-2) feeding on plankton < 10 mu m, with 74% resulting from ultraplank ton, suggesting that ultraplankton are an important overlooked compone nt of benthic-pelagic coupling.