Water temperature regulates picoplankton growth and mortality during the cold season in a subtropical coastal ecosystem

Marine picoplankton populations are influenced by bottom-up control (resource availability, temperature) and top-down processes (e.g. grazing, viral lysis); however, seasonal variability in the bottom-up and top-down controls of picoplankton remains unclear, especially in cold-season studies, which remain scarce due to sampling challenges. To gain insight into the role of bottom-up and top-down control as determinants of picoplankton population abundance in the cold season, we examined bacterial, Synechococcus spp., and picoeukaryotic phytoplankton growth and mortality in a subtropical coastal region during the cold season (<25°C) using modified dilution experiments. Results showed bacterial and Synechococcus spp. production rates were positively correlated with temperature, with growth rates ranging from 0.02 to 4.91 d -1 (bacteria) and 0.29 to 6.28 d -1 ( Synechococcus spp.). Nanoflagellate grazing accounted for 43-80% of bacterial and 64-448% of Synechococcus spp. production, with carbon losses ranging from 2.07 to 18.82 mg C m -3 d -1 (bacteria) and from 1.94 to 92.93 mg C m -3 d -1 ( Synechococcus spp.), while viral lysis was minimal for both but more significant for picoeukaryotic phytoplankton. Moreover, the 3 microbial groups exhibited distinct patterns: (1) Bacterial production was nearly balanced by total mortality (grazing and viral lysis) across all temperatures; (2) Synechococcus spp. contributed significantly to picophytoplankton production but declined sharply below 20°C as grazing exceeded production; (3) Picoeukaryotic phytoplankton production surpassed Synechococcus spp. below 20°C. These findings underscore temperature-driven microbial production and grazing-dominated mortality, suggesting reduced viral lysis due to increased lysogeny.