Diel dynamics of bacterial and picophytoplankton growth, grazing and viral lysis at different depths in the tropical South China Sea in spring

To examine the relative importance of nanoflagellate grazing and viral lysis on prokaryote abundance—including heterotrophic bacteria, Synechococcus spp., and Prochlorococcus spp.—in the tropical ocean, a modified dilution technique combined with light-dark experiments was used during a South China Sea cruise in late April 2025. Results show that during the daytime, the combined carbon loss from viral lysis and nanoflagellate grazing (0.40 and 0.20 µgC L⁻¹ h⁻¹, respectively) accounted for nearly 100% of bacterial production in surface waters. In the deep chlorophyll maximum layer, viral lysis alone removes almost 80% of the bacterial production during the daytime. The carbon losses of Synechococcus spp. to nanoflagellate grazing were approximately 0.06 µgC L⁻¹ h⁻¹ in the daytime and 0.04 µgC L⁻¹ h⁻¹ at night, which represent 55% and 33% of total Synechococcus spp. production, respectively. These results strongly suggest that nanoflagellate grazing is the primary factor controlling the daily fluctuations in Synechococcus spp. abundance. The carbon flows among the components of target organisms and the two top-down forces constructed by this research can be indicative for future studies in this and other tropical seas.