Trophic linkages between grazers and ultraplankton within the microbial food web in subtropical coastal waters

Abstract

To investigate the trophic linkages among different micrograzers and their preys and the possible occurrence of trophic cascades within the microbial food web, we combined the dilution technique and size-fractionation methods to monitor the grazing rates of different grazer assem- blages on various ultraphytoplankton cells in subtropical waters. The mortality rates of Synechococ- cus and picophytoeukaryotes (<2 µm) were often higher in dilution experiments set up with 5 µm pre- filtered seawater (containing only heterotrophic nanoflagellates (HNFs)) than in the <20 µm treatments (including both HNFs and small ciliates), indicating the effect of trophic cascading. In addition, similar grazing rates on Synechococcus and picophytoeukaryotes can be well related to the biomass of HNFs but not to that of total grazers. Growth and grazing rates of Synechococcus and picophytoeukaryotes mostly fell to a 1:1 relationship, suggesting grazing was the major loss pathway for picophytoplankton. The trophic linkages of micrograzers and their preys in coastal waters do not appear to differ substantially from those observed in open oceans.