Gyrodinium galatheanum (Braarud) Taylor 1995 is a common bloom-forming, potentially toxic photosynthetic dinoflagellate in Chesapeake Bay, USA. Abundance of this dinoflagellate achieved densities >4 x 103 cells ml-1 in the mid- and upper Bay during late spring and early summer of 1995 and 1996. Ingestion of cryptophytes by this dinoflagellate was detected in most samples collected from the Bay. During late spring and early summer, mean number of ingested cryptophytes per G.galatheanum was as high as 0.46 for dinoflagellate populations located in surface waters of the mid- and upper Bay where dissolved inorganic phosphorus was low. Observations on the distribution of G.galatheanum in Chesapeake Bay show that populations of this dinoflagellate were usually restricted to waters with salinities ranging from 7 to 18 psu, seasonally progressed up the estuary, and usually co-occurred with cryptophytes. Correlation analysis indicates that abundance of G.galatheanum and incidence of feeding was negatively correlated with dissolved inorganic phosphorus, and that incidence of feeding was positively correlated with abundance of cryptophyte prey. These results indicate that G.galatheanum is an important component of the Chesapeake Bay phytoplankton during the spring and summer. Our results suggest that the phagotrophic capability possessed by this phototrophic dinoflagellate may contribute to its success in a varying-resource environment like Chesapeake Bay.
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