Use of the 'food vacuole content' method to estimate grazing by the mixotrophic dinoflagellate Gyrodinium galatheanum on cryptophytes

Abstract

We measured in situ grazing rates of the mixotrophic dinoflagellate Gyrodinium galatheanum (Braarud) Taylor 1995 on populations of phycoerythrin-containing cryptophytes in Chesapeake Bay. Rates were estimated from instantaneous food vacuole contents, in situ temperatures, cryptophyte abundances and experimentally determined digestion rates. Laboratory digestion experiments showed that specific digestion rate constants increased sigmoidally with temperature, but were unrelated to the initial food vacuole content when it was <0.46 cryptophytes dinoflagellate-1. These results allowed us to establish an empirical model to estimate in situ ingestion of cryptophyte prey by G. galatheanum. The estimated rates ranged from 0 to 0.26 cryptophytes dinoflagellate-1 day-1, corresponding to daily ingestion of 0-12.29 pg carbon, 0-2.48 pg nitrogen and 0-0.34 pg phosphorus dinoflagellate-1. Estimated daily consumption of cryptophyte biomass by G. galatheanum was equivalent to 0-12% of body carbon, 0-13% of body nitrogen and 0-21% of body phosphorus. Estimated in situ clearance rates for cryptophytes ranged from 0 to 0.27 μl dinoflagellate-1 day-1, representing daily removal of 0-4% of the cryptophyte standing stock. Although G. galatheanum may increase its growth rate through phagotrophy, it appears to have little grazing impact on cryptophyte prey populations.