OCCURRENCE OF THE PARASITIC DINOFLAGELLATE AMOEBOPHRYA-CERATII IN CHESAPEAKE BAY POPULATIONS OF GYMNODINIUM-SANGUINEUM

Chesapeake Bay populations of the red-tide dinoflagellate Gymnodinium sanguineum were regularly infected by the parasitic dinoflagellate Amoebophrya ceratii during the summers of 1988-1991. Infections developed inside the nucleus of G. sanguineum and were always lethal to the host. Parasite generation time was similar to 40 h at 23 degrees C, with the intracellular, trophont phase lasting 39.5 +/- 0.3 h, and the extracellular, vermiform stage persisting for similar to 20 min. Near surface accumulations of G. sanguineum sometimes exceeded 1,000 cells/ml; however, host abundance was relatively low when integrated over the surface mixed layer of each station (mean = 12.2 cells/ml +/- 2.96 SE; n = 60). Parasitized hosts were encountered in 75% of the samples where host abundance was greater than or equal to 1 per ml, and epidemic outbreaks (20-40% hosts infected) were observed on several occasions. Epidemic infections were generally located several meters below surface accumulations of G. Sanguineum and were always restricted to a narrow region near the pycnocline. Consequently, integrated station values for parasite prevalence were low, with an average 2.7% (+/- 0.31 SE; n = 60). Parasite induced mortality removed up to 8% of G. sanguineum populations per day, but averaged < 2% of host biomass throughout the Bay. Thus, parasitism by A. ceratii does not appear to be a major factor regulating G. sanguineum bloom in the main stem of Chesapeake Bay.