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

We measured in situ grazing rates of the mixotrophic dinoflagellate Gyrodin ium galatheanum (Braarud) Taylor 1995 on populations of phycoerythrin-conta ining cryptophytes in Chesapeake Bay. Rates were estimated from instantaneo us 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 t emperature, but were unrelated to the initial food vacuole content when it was <0.46 cryptophytes dinoflagellate (1). These results allowed us to esta blish an empirical model to estimate in situ ingestion of cryptophyte prey by G. galatheanum. The estimated rates ranged from 0 to 0.26 cryptophytes d inoflagellate (1) day (1), corresponding to daily ingestion of 0-12.29 pg c arbon, 0-2.48 pg nitrogen and 0-0.34 pg phosphorus dinoflagellate (1). Esti mated daily consumption of cryptophyte biomass by G. galatheanum was equiva lent to 0-12% of body carbon, 0-13% of body nitrogen and 0-21% of body phos phorus. Estimated in situ clearance rates for cryptophytes ranged from 0 to 0.27 <mu>l 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.