Grazing impact of heterotrophic dinoflagellates and ciliates on common red-tide euglenophyte Eutreptiella gymnastica in Masan Bay, Korea

摘要

The euglenophyte Eutreptiella gymnastica is a common red tide causative species. However, there have been no studies on the grazing impact of heterotrophic protists on this species. To investigate the grazing impact of heterotrophic protists on £ gymnastica, we measured daily the abundances of £ gymnastica and co-occurring potential heterotrophic protistan grazers in Masan Bay, Korea, in August 2004 when an £ gymnastica red tide occurred. In addition, we tested whether the common heterotrophic dinoflagellates Gyrodinium dominans, Oxyrrhis marina, Pflesteria piscicida, Polykrikos kofoidii, Protoper-idinium bipes, and Stoeckeria algidda and the naked ciliates Strobilidium sp. (30-40 μm in cell length) and Strombidinopsis sp. (70-100 μm in cell length) were able to feed on £ gymnastica. We also measured their growth and ingestion rates on £ gymnastica as a function of prey concentration. Finally, we calculated the grazing coefficients by combining field data on the abundance of the heterotrophic dinoflagellate and ciliate grazers and co-occurring E. gymnastica with laboratory data on ingestion rates obtained in this study. The maximum abundance of E. gymnastica in Masan Bay in August, 2004 was 7575 cells ml~(-1), while those of Gyrodinium spp., P. kofoidii, P. bipes, the naked ciliates (≤50 μm in cell length), and naked ciliates (>50 μm in cell length) were 50, 9, 58, 32, and 3 cells ml"1, respectively. The maximum growth rate of G. dominans on E. gymnastica (1.13 d~(-1)) was higher than that of O. marina (0.81 d~(-1)) or P. bipes (0.77 d~(-1)). However, E. gymnastica did not support positive growth of P. kofoidii, Strobilidium sp., and Strombidinopsis sp. (-0.04-2.8 d~(-1)). The maximum ingestion rates of G. dominans, P. kofoidii, P. bipes, O. marina, and Strobilidium sp. on E. gymnastica (2.1-2.7 ngC predator~(-1) d~(-1)) were similar, but they were much lower than that of Strombidinopsis sp. (156 ng C predator~(-1) d~(-1)). The calculated grazing coefficients for P. bipes, small heterotrophic Gyrodinium spp. (25-35 μm in cell length), naked ciliates (≤50 μm in cell length), P. kofoidii, and naked ciliates (>50 μm in cell length) on £ gymnastica were up to 0.77, 0.61, 0.22, 0.07 and 0.03 d~(-1) respectively (i.e., up to 54%, 46%, 20%, 7%, and 3% of £ gymnastica populations were removed by the population of each of these heterotrophic protistan grazers in 1 d, respectively). The results of the present study suggest that P. bipes, small heterotrophic Gyrodinium spp., and naked ciliates (<50 μm in cell length) sometimes have considerable potential grazing impact on the populations of £ gymnastica.