GRAZING OF THE MIXOTROPHIC DINOFLAGELLATE GYMNODINIUM-SANGUINEUM ON CILIATE POPULATIONS OF CHESAPEAKE BAY

In situ grazing rates for the mixotrophic dinoflagellate Gymnodinium s anguineum Hirasaka feeding on nanociliate populations of Chesapeake Ba y were determined in June and October of 1990 using a ''gut clearance/ gut fullness'' approach. Recently ingested prey were digested beyond t he point of recognition at a rate of approximately 23 % h - 1. Estimat es of in situ ingestion and clearance ranged from 0 to 0.06 prey dinof lagellate - 1 h - 1 and 0 to 5.8 mul dinoflagellate-1 h-1, respectivel y, with daily removal of ciliate biomass representing 6 to 67 % of the less-than-or-equal-to 20-mum oligotrich standing stock. Daily consump tion of ciliate biomass by G. sanguineum averaged 2.5 % of body carbon and 4.0 % of body nitrogen with maximal values of 11.6 and 18.5 %, re spectively. Ingestion of ciliates may help balance nitrogen requiremen ts for G. sanguineum and give this species an advantage over purely ph otosynthetic dinoflagellates in nitrogen limited environments. By prey ing on ciliates, these dinoflagellates reverse the normal flow of mate rial from primary producer to consumer and thereby influence trophodyn amics of the microbial food web in Chesapeake Bay.