Feeding by the heterotrophic dinoflagellates Gyrodinium dominans and G. spirale on the red-tide dinoflagellate Prorocentrum minimum

Abstract

To investigate the roles of Gyrodinium dominans and G. spirale, which have been reported to be abundant heterotrophic dinoflagellates in many coastal waters at different stages of red tides dominated by the dinoflagellate Prorocentrum minimum, we measured their growth and ingestion rates when feeding on P. minimum and calculated grazing coefficients by combining field data on abundances of G. dominans, G. spirale, and co-occurring P. minimum with laboratory data on ingestion rates obtained in the present study. In addition, the grazing coefficients of G. dominans and G. spirale on P. minimum were compared to those for co-occurring copepods, Acartia spp. Specific growth rates of G. dominans and G. spirale increased rapidly with increasing mean prey concentration before saturating at the P. minimum concentrations of ca. 150 and 80 ng C ml-1, respectively. The maximum specific growth rate of G. dominans on P. minimum (1.13 d-1) was higher than that of G. spirale (0.79 d-1). Threshold prey concentrations (where net growth = 0) were 15 ng C ml-1 for G. dominans and 23 ng C ml-1 for G. spirale. Maximum ingestion and clearance rates of G. dominans on P. minimum (1.2 ng C grazer-1 d-1 and 0.9 μl grazer-1 h-1, respectively) were much lower than those of G. spirale on the prey (13.6 ng C grazer-1 d-1 and 5.3 μl grazer -1 h-1, respectively). Calculated grazing coefficients for G. dominans on P. minimum (up to 0.066 h-1, i.e. 6.3% of P. minimum populations were removed by a G. dominans population in 1 h) or those by G. spirale (up to 0.231 h-1, i.e. 39 % of P. minimum populations were removed in 1 h) were much higher than those for co-occurring Acartia spp. (up to 0.001 h-1, i.e. 0.1 % of P. minimum populations were removed by Acartia spp. populations in 1 h). The results of the present study suggest that G. dominans and/or G. spirale sometimes have considerable grazing impacts on populations of P. minimum and are the most effective zooplanktonic grazers on the prey.