ABSTRACT. 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 ~ 40 h at 23° C, with the intracellular, trophont phase lasting 39.5 ± 0.3 h, and the extracellular, vermiform stage persisting for ~ 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 > 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.

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ABSTRACT. Gymnodinium sanguineum, Gyrodinium uncatenum, and Ceratium furca are large phototrophic dinoflagellates that commonly form red tides in the mesohaline portion of Chesapeake Bay during the summer. Examination of protargol‐stained specimens revealed that these dinoflagellates also feed heterotrophically as indicated by the presence of food vacuoles containing partially digested prey. Ingested prey were generally identified as nanociliates (≥20 μm) belonging to the oligotrich genera Strobilidium and Strombidium; occasionally other small ciliates (e.g. Balanion sp. and Mesodinium sp.), dinoflagellates, and diatoms were observed in early stages of digestion. the percentage of these mixotrophs that had ingested prey was usually less than 20%, but approached 30% in some samples. Occurrence of food vacuoles in Gymno. sanguineum was positively correlated with ≤20 μm oligotrichous ciliate density; limited data for Gyro. uncatenum suggests a similar relationship, but C. furca feeding was not related to nanociliate densities.

Favella panamensis Kofoid and Campbell, 1929 is seasonally abundant in meso-to polyhaline waters of Chesapeake Bay and Indian River, Florida, USA, where it reaches densities of 103 cells 1-1 . During the summers of 1986-1992, F. panamensis populations of the two estuaries were commonly infected by the parasitic dinoflagellate Duboscquella aspida Cachon, 1964. The intracellular phase of the parasite reached maturity in -21 h (30 ~ and consumed -35% of the host's biomass. Infections were not typically lethal to F. panamensis, but sometimes forced the host from its lorica. Several D. aspida were found in the cytoplasm of many hosts, and the number of parasites infection -1 was directly related to infection level. Parasite prevalence averaged 24.0 and 11.5% with mean number of parasites infection -1 being 1.5 and 1.3 for Chesapeake Bay and Indian River samples, respectively. D. aspida was estimated to remove up to 68% of host standing stock d -1 with a mean of ~ 10% for all samples. The average impact of parasitism on F. panamensis populations was somewhat less than would be expected from copepod grazing.