Extreme rainfall events are becoming more frequent under climate change, yet their short-term impacts on
microbial community dynamics in coastal ecosystems remain poorly understood. This study investigated how
rainfall modulates the effects of environmental conditions on microbial communities at a subtropical Pacific
coastal station, based on two high-frequency (daily) time series totaling nearly 200 days. The abundances of
viruses, bacteria, Synechococcus spp., nanoflagellates, ciliates, and diatoms were analyzed in relation to fluctuations
in hydrology and nutrients. Heavy rainfall significantly reduced salinity and stimulated nutrient enrichment,
resulting in distinct microbial responses that were substantially different from those observed on nonheavy
rainfall days. At low precipitation (<39 mm day ^-1), temperature and nutrient concentrations (phosphate,
PO4; nitrate, NO3) primarily governed the growth of bacteria and Synechococcus spp., whereas under
intense rainfall (>39 mm day^-1), salinity became the dominant environmental driver. Viral abundance increased
following storm-induced nutrient pulses, indicating an enhanced infection potential during ecosystem recovery
after storm disturbance. These findings demonstrate that rainfall-induced hydrological variability exerts rapid
and cascading effects on microbial food webs, with implications for coastal biogeochemical cycling and
ecosystem stability under a changing climate.

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