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Abstract
The structure and function of marine microbial food webs is largely determined by the ways in which phytoplankton die (microzooplankton grazing, viral lysis, and parasitic infection), as these mortality processes can differentially influence carbon and nutrient cycles. Temporal shifts in mortality have often been overlooked, hindering our ability to identify drivers of plankton mortality or include loss terms in ecosystem models. Temporal resolution in plankton mortality is warranted for coastal marine regions, where environmental conditions and phytoplankton biomass often fluctuate. To assess mortality dynamics, we performed a series of dilution experiments in the Skidaway River Estuary (GA), measuring phytoplankton growth, microzooplankton grazing, and viral lysis rates at time scales commensurate with tidal (hourly) or seasonal (weekly) periods. Significant variability in grazing rates were detected between daily tides and seasons, with rates driven by changes in plankton community composition rather than environmental factors (e.g., temperature or chlorophyll). Though accounted for in dilution experiments, significant viral lysis rates were rarely observed at all time scales. Given the inability to estimate bulk parasitic infection rates, we applied 18S amplicon sequencing with network correlation analysis to identify temporal trends of the parasite group, Syndiniales (Alveolata), along with parasite-host interactions within the protist community. Syndiniales were temporally variable, accounting for up to 28% of protist relative abundance in summer-fall, which coincided with increased temperatures in the estuary. Yet, as with microzooplankton grazing rates, Syndiniales abundance and composition were poorly explained by any single environmental factor. Syndiniales sequences represented 20% of all interactions in the network and were positively correlated to a range of plankton groups (e.g., dinoflagellates, ciliates, and diatoms). Parasitic relationships inferred from the network typically involved a single Syndiniales parasite and multiple hosts, implying flexible parasite-host interactions. Our findings of temporal variability in grazing rates and parasite-host relationships further emphasize the heterogenous nature of coastal marine systems, with implications for accurate plankton population monitoring and ecosystem modeling in these locations.