The effect of grazing and viral lysis on the diel variations of Synechococcus spp. abundance in the East China Sea

Diel variations in the nanoflagellate grazing and viral-mediated mortality of Synechococcus spp. were simultaneously estimated using a dilution and size-fractionation approach in the inner (I-1 and I-2) and outer regions (O-1, O2 and O-3) of the Changjiang River plume in the East China Sea during summer 2014. Synechococcus spp. abundance generally tended to increase during the dark period, followed by a plateau until midnight at all sampling stations. Overall, gross growth rate of Synechococcus spp. ranged from 0.069 h−1 to 0.122 h−1 during the growth phase. Microzooplankton, nanoflagellate grazing, and viral lysis had no effect on the Synechococcus spp. abundance during this phase. Moreover, nanoflagellate grazing was the largest cause of Synechococcus spp. mortality during the loss phase at nighttime. Compared to the predators, viruses exerted only a minor impact on mortality at St. I-1, where we detected some of the effect that this community had on Synechococcus spp.. Little is known about the impact of nanoflagellates and viruses on the short-term dynamics of Synechococcus spp. in the East China Sea. Therefore, this study's characterization of the relative importance of nanoflagellates and viruses may help provide a better understanding of trophic structures and the energy flow within the microbial loop.