The influence of coastal waters on distributions of heterotrophic protists in the northern East China Sea, and the impact of protist grazing on phytoplankton

Abstract

Spatial and temporal variations in the abundance and biomass of heterotrophic protists and of their grazing impact were investigated during five cruises between July 2006 and February 2009 in the continental shelf waters of the northern East China Sea (ECS). Strongly patchy distributions were observed on all cruises, generally with a higher biomass in the western areas affected by the Changjiang River discharge. An opposite pattern was observed in February when the Kuroshio onshore transport is greatest, with a greater biomass in the eastern area. Small heterotrophic dinoflagellates (,20 mm) were most abundant numerically, whereas ciliates contributed the most to the biomass, accounting for 28–58% of the total heterotrophic protist biomass. Small heterotrophic dinoflagellates were more strongly correlated with phytoplankton biomass than were other types of protists. The total protist biomass was often most strongly related to amounts of particulate organic carbon of non-phytoplankton origin, suggesting that their abundance distribution often depended on prey other than phytoplankton, such as heterotrophic bacteria. Heterotrophic protists consumed 30.1–91.5% of Chl a production (mean 68.2%), with grazing rates increasing with the phytoplankton biomass. The results suggest that heterotrophic protists were the major consumers of primary production, and that their grazing is one of the most important losses affecting the phytoplankton biomass in the northern ECS.