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

Growth and ingestion rates of the heterotrophic dinoflagellates Protoperidinium cf. divergens and P crassipes feeding on red tide dinoflagellates local to southern California, USA, were measured in the laboratory. Unialgal diets of the larger dinoflagellates Gonyaulax polyedra and Gymnodinium sanguineum supported population growth, while the smaller dinoflagellates Prorocentrum cf. balticum and Scrippsiella trochoidea did not; G. polyedra was the optimal diet. The maximum specific growth 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 with mean prey concentration, with saturation at approximately 760 to 1500 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 Protoperidinium-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 function 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 concentrations, and dead Protoperidinium were observed. P cf. divergens selected G. polyedra over G. sanguineum. Cannibalism was observed in cultures mainly when Protoperidinium abundances were high, and may be a mechanism for withstanding prolonged starvation. The estimated ingestion rate of Protoperidinium during a G. polyedra red tide is less-than-or-equal-to 4800 prey ingested l-1 h-1, so Protoperidinium may have a considerable grazing impact on some red tide dinoflagellate populations.