The trophic transfer via a picoplankton-flagellate-copepod food chain during a picocyanobacterial bloom in Lake Biwa trab:4

Abstract

We examined whether or not the production of autotrophic picoplankton and heterotrophic bacteria (collectively, picoplankton) is funnelled up food web via a picoplankton-nanoflagellate-copepod food chain during a picocyanobacterial bloom in Lake Biwa. The experiment was carried out in late summer when the abundance of picocyanobacteria was of its seasonal maximum (7x10 5 cells ml -1 ) and copepods (Eodiaptomus japonicus) dominated the mesozooplankton assemblage. The grazing rates of a mesozooplankton assemblage on nanoflagellates and picoplankton were determined from changes in apparent growth rates of nanoflagellates and picoplankton with increasing mesozooplankton biomass in incubation bottles. Results indicated that the mesozooplankton assemblage consumed nanoflagellates but not picoplankton directly. Dilution culture experiments revealed that the production of picoplankton was mostly consumed by protozoan grazers in the cell-size fraction of <20 μm (nanoflagellates). Carbon flux estimates indicated that of total carbon channelled from picoplankton to nanoflagellates (630 mg Cm -2 day -1 ), only 2 % was passed to mesozooplankton. The carbon from nanoflagellates accounted for <5 % of the total carbon consumption by mesozooplankton. Thus, the picoplankton-nanoflagellate-mesozooplankton trophic link was extremely inefficient and contributed little to the production of mesozooplankton dominated by copepods during a picocyanobacterial bloom in Lake Biwa.