The coupling of oligotrich ciliate populations and hydrography in the East China Sea: spatial and temporal variations

Variations in the spatial and temporal distribution of oligotrich ciliate populations in the East China Sea were investigated during four cruises of the R/V Ocean Researcher I between December 1997 and October 1998. Over the entire continental shelf, a seasonal cycle was found with a distinct 3–5-fold increase in the abundance of oligotrich ciliates in summer. This increase appeared to be induced by the tremendous summertime runoff from the Changjiang. A radial-type spatial distribution pattern also was observed in summer, with population densities higher toward the Changjiang plume but highest of all in the margins of the plume. In spring and fall, the spatial distribution of the oligotrich ciliates was closely correlated to the abundance of cyanobacterium Synechococcus. In summer in the plume region, mixotrophic ciliates accounted for over 50% of the total ciliate population, compared to less than 30% outside the plume or that in other seasons. We propose a model in which these ciliates constitute part of the pathway through which the particulate and dissolved organic carbon in the runoff water is incorporated into the oceanic food web.