Copepod grazing impact on the trophic structure of the microbial assemblage of the San Pedro Channel, California

In August 2002 and March 2003 the trophic structure of the microbial assemblage from the San Pedro Channel, California was studied following the experimental alteration of the number of copepods. Changes in the abundance/biomass of microorganisms <80 m during 3-day incubations were monitored in (i) the absence of metazoa >80 m, (ii) the presence of natural abundances of metazoa and (iii) the presence of an elevated number of copepods. Prokaryotes and small-sized eukaryotes (<4 m) dominated plankton biomass during both experimental months. Diatoms numerically dominated the 10–80 m plankton in August 2002, but ciliate and heterotrophic dinoflagellate biomass generally exceeded diatom biomass on both dates. Ingestion of protozooplankton (predominantly ciliates) contributed substantially to copepod daily carbon rations. The adult copepod assemblage removed 4.6 and 36% per day of the microzooplankton standing stocks (10–80 m size fraction) in August and March, respectively. Elevated copepod grazing pressure on protozooplankton resulted in increased biomass of nanoplankton (<5 m) presumably via a trophic cascade. Accordingly, the copepod–protozoan trophic link appears to be a key factor structuring the planktonic microbial assemblage in the San Pedro Channel.