Feeding by the heterotrophic dinoflagellate Protoperidinium bipes on the diatom Skeletonema costatum

Abstract

The heterotrophic dinoflagellate Protoperidinium bipes is a predominant heterotrophic dinoflagellate (maximum density = 137 cells ml -1 ) during diatom blooms. To investigate its role as a grazer in the population dynamics of diatoms, we measured growth and ingestion rates of P. bipes when feeding on the common diatom Skeletonema costatum. We also calculated grazing coefficients by combining field data on abundances of P. bipes and co-occurring S. costatum with laboratory data on ingestion rates obtained in the present study. Specific growth rates of P. bipes increased continuously with increasing concentration of S. costatum. The maximum specific growth rate of P. bipes on S. costa- tum was 1.37 d -1 when data for the growth rate were fitted to a Michaelis-Menten equation. The thresh- old prey concentration (where net growth = 0) was 111 ng C ml -1 (4270 cells ml -1 ). Maximum ingestion and clearance rates of P. bipes on this diatom were 2.9 ng C grazer -1 d -1 (112 cells grazer -1 d -1 ) and 1.0 µl grazer -1 h -1 , respectively. P. bipes exhibited the highest maximum swimming speed (ca. 8.3 mm s -1 ) and maximum volume-specific clearance rate (5.4 × 10 6 h -1 ) among Protoperidinium species so far reported. Calculated grazing coefficients by P. bipes on S. costatum (0.001 to 0.034 h -1 , i.e. 0.1 to 3.4% of S. costa- tum populations were removed by a P. bipes population in 1 h) were much higher than those by co- occurring Acartia spp. (<0.002 h -1 ). The results of the present study suggest that P. bipes sometimes has