Differences in the vertical distribution and response to freshwater discharge between aloricate ciliates and tintinnids in the East China Sea

Marine planktonic ciliates are important components of microzooplankton. Depending on the presence or absence of lorica, planktonic ciliates can be divided into tintinnids and aloricate ciliates. We hypothesize that the ecological distributions of the two groups were different due to their difference in morphology. To test this hypothesis, the spatial distribution of ciliates was studied in spring, summer and autumn 2011 in the East China Sea, a large marginal sea significantly affected by freshwater discharge. We found that tintinnids and aloricate ciliates had different vertical distributions. Tintinnids were distributed mainly near the bottom waters, while higher abundance of aloricate ciliate was found in waters 30-70 m above the bottom. The contribution of tintinnids to the total ciliate abundance decreased with increasing distance from the bottom. In waters with distance from the bottom > 80 m, tintinnids accounted for < 10% of the total ciliate abundance. The Changjiang River discharge in summer enhanced aloricate ciliate abundance compared with spring and autumn, although tintinnid abundance did not change significantly. Tintinnidium primitivum was the dominant tintinnid near the bottom waters. Our study clearly showed that aloricate ciliates and tintinnids had different vertical distributions and responses to freshwater discharge in the East China Sea.