GROWTH AND GRAZING RATES OF THE HETEROTROPHIC DINOFLAGELLATES PROTOPERIDINIUM SPP ON RED TIDE DINOFLAGELLATES

Growth and ingestion rates of the heterotrophic dinoflagellates Protop eridinium cf. divergens and P crassipes feeding on red tide dinoflagel lates local to southern California, USA, were measured in the laborato ry. Unialgal diets of the larger dinoflagellates Gonyaulax polyedra an d Gymnodinium sanguineum supported population growth, while the smalle r dinoflagellates Prorocentrum cf. balticum and Scrippsiella trochoide a did not; G. polyedra was the optimal diet. The maximum specific grow th rates of P cf. divergens and P crassipes on a G. polyedra diet were 0.484 and 0.308 d-1, respectively. Specific growth rate increased wit h mean prey concentration, with saturation at approximately 760 to 150 0 cells ml-1. Maximum ingestion and clearance rates of P cf. divergens and P crassipes fed on G. polyedra were 0.2 and 0.08 prey Protoperidi nium-1 h-1, and 0.67 and 0.47 mul Protoperidinium-1 h-1, respectively. For a G. sanguineum diet, the pattern of specific growth rate as a fu nction of mean prey concentration was quite different from that of G. polyedra. Maximum specific growth rates were 0.246 and 0. 107 d-1 for P cf. divergens and P. crassipes at mean prey concentrations of 530 to 1100 cells ml-1. Growth was negative at higher G. sanguineum concentr ations, and dead Protoperidinium were observed. P cf. divergens select ed G. polyedra over G. sanguineum. Cannibalism was observed in culture s mainly when Protoperidinium abundances were high, and may be a mecha nism for withstanding prolonged starvation. The estimated ingestion ra te of Protoperidinium during a G. polyedra red tide is less-than-or-eq ual-to 4800 prey ingested l-1 h-1, so Protoperidinium may have a consi derable grazing impact on some red tide dinoflagellate populations.