Rates of phytoplankton growth and mortality are poorly defined over short‐time scales (hours to days), despite these scales being relevant over the daily tidal cycle in coastal marine areas. To assess the influence of tidal variability on phytoplankton rates, we performed eight, same‐day dilution experiments at high and low tide (6‐h intervals) in the Skidaway River Estuary, Georgia, measuring phytoplankton growth, microzooplankton grazing, and virus‐induced mortality. Chlorophyll and group‐specific biomass (Synechococcus spp., picoeukaryotes, and nanoeukaryotes) varied between tides and multidimensional scaling clustering of tidal biomass revealed separation in community composition based on tide. Rates also varied significantly over the tidal cycle, especially microzooplankton grazing, which was higher at low tide in most experiments with rates ranging from 0–3.77 d−1 at low tide to 0–1.51 d−1 at high tide. Virus‐induced mortality rates were rarely detected, only being observed in Synechococcus spp. in three experiments (0.3–0.8 d−1) and picoeukaryotes twice (~ 0.36 d−1). The differences in grazing and growth rates observed between the two tides were not explained by temperature, salinity, or sunlight (i.e., time of day) and grazing was only weakly explained by initial chlorophyll concentration (R2 = 0.36), highlighting the influence of community composition on rate measurements. These results suggest that within tidally influenced areas, short‐term sampling over the tidal cycle is essential to accurately characterize daily phytoplankton dynamics and reliably monitor and predict shifts in primary production and coastal ecosystem health.

Transparent exopolymer particles (TEP) are the central mechanism by which carbon is shuttled from the surface to the deep ocean. Despite the importance of these particles, the magnitude and drivers of temporal variability in the concentration and production rate of TEP in the ocean are not well resolved, especially in highly dynamic and productive regions like estuaries. Here, TEP dynamics were evaluated across weekly, tidal, and diel time scales within the Skidaway River Estuary (GA, USA) and adjacent coastal waters in the South Atlantic Bight. No significant trends in TEP concentration or production rates were observed over weekly time scales, though over tidal cycles, TEP concentration varied between tide stage and TEP:chlorophyll ratios were always lower at low relative to high tides. Over sequential diel cycles, TEP concentrations were two times higher at night relative to midday. Different biological and environmental variables were correlated with TEP dynamics (Spearman ρ) depending on the time scale considered, reinforcing the importance of time-specific drivers of TEP. These results emphasize the importance in considering the temporal variability of field-based TEP measurements, with implications for accurate assessments of carbon cycling in coastal ecosystems and the incorporation of TEP into carbon export models.