Application of the size-fractionation method to simultaneous estimation of clearance rates by heterotrophic flagellates and ciliates of pico- and nanophytoplankton

To estimate the clearance rates of pico- and nanophytoplankton by heterotrophic nanoflagellates (HNF) and ciliates, the size-fractionation method was employed in combination with a multiple regression analysis. Apparent growth rate of a specific phytoplankton group was decomposed into three terms, net growth rate, grazing rates by HNF and those by ciliates, and the grazing rates were interpreted as a linear function of average cell concentrations of HNF and ciliates. We produced a set of subsamples from a water sample by size-fractionation using three different pore sizes. By measuring phytoplankton and protozoan cell concentrations, apparent growth rate of a specific phytoplankton group was regressed against the average cell concentrations of HNF and ciliates to obtain the net growth rate of the phytoplankton group and the average clearance rates by these two protozoan groups. The estimated values were within the range of those previously reported for cultured and natural protozoan, which supported the feasibility of the present method. The estimated values also suggested that picophytoplankton abundance in Otsuchi Bay in spring is controlled mainly by HNF through active grazing. The present study rediscovered the utility of the size-fractionation method as a method to estimate the clearance rates of different protozoan groups from natural assemblages in a single experiment.