Diel to seasonal variation in abundance of picoplankton and virus-like particles in subtropical coastal Pacific waters

Abstract

Picophytoplankton and viruses are key regulators of marine microbial food webs and biogeochemical cycling, with their population dynamics shaped by light, temperature, and trophic interactions. Although diel variability has been documented for several microbial groups, most field studies rely on low-frequency sampling, limiting resolution of day-night dynamics across seasonal temperature transitions in coastal environments. We conducted high-frequency day-night sampling for two to three times per week over five months (July to December 2025) at a subtropical coastal Pacific station to quantify diel and seasonal variability of Synechococcus spp., picoeukaryotes, heterotrophic bacteria, and virus-like particles using flow cytometry. Picophytoplankton abundance showed pronounced diel variability, with significantly higher nighttime concentrations during warm periods above 25 degrees C. Bacterial abundance exhibited no consistent diel pattern despite seasonal change. Virus-like particles abundance remained comparatively stable across both diel and seasonal scales and showed no significant day-night differences throughout the study period. These differing temporal patterns indicate that short-term host and viral abundance dynamics are not tightly synchronized under changing thermal conditions. Together, the results show that diel variability in coastal microbial communities differs among functional groups and depends on temperature regime, emphasizing the need of repeated day-night sampling for resolving shortterm ecological variability across seasons.