Control of nanoflagellate abundance by microzooplankton and viruses in a coastal ecosystem of the subtropical western Pacific

Little is known about the factors controlling variations in nanoflagellate abundance in oceanic environments. In this study, combined size-fractionation with dilution experiments were performed to measure nanoflagellate growth rates, microzooplankton grazing, and virus-mediated mortality collected three times during the summer of 2013. Heterotrophic and pigmented nanoflagellate growth rates varied from 0.39 to 0.49 d− 1 and 0.39 to 0.52 d− 1, respectively. Moreover, PNF and HNF loss rates varied from 0.19 to 0.23 d− 1 and 0.17 to 0.25 d− 1, respectively. In these cases, a significant impact of microzooplankton grazing was detected, highlighting the important role of microzooplanktons in transferring carbon bound in nanoflagellates to higher trophic levels in this study area. Nanoflagellate net growth rate was not increased in the virus-diluted treatments, suggesting that there was no impact of viral lysis on the mortality of nanoflagellates during summer periods. Furthermore, these results showed a decrease of Synechococcus spp. and bacterial abundance with removal of viruses, and a subsequent decrease in nanoflagellate growth. This result implies that viral infection is an important mechanism in nutrient recycling under oligotrophic oceanic conditions in summer.