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Abstract Islands in the tropical Pacific supply elevated nutrients to nearshore waters that enhance phytoplankton biomass and create hotspots of productivity in otherwise nutrient‐poor oceans. Despite the importance of these hotspots in supporting nearshore food webs, the spatial and temporal variability of phytoplankton enhancement and changes in the underlying phytoplankton communities across nearshore to open ocean systems remain poorly understood. In this study, a combination of flow cytometry, pigment analyses, 16S rRNA gene amplicons, and metagenomic sequencing provides a synoptic view of phytoplankton dynamics over a 4‐yr, near‐monthly time series across coastal Kāneʻohe Bay, Hawaiʻi, spanning from an estuarine Indigenous aquaculture system to the adjacent offshore environment. Through comparisons with measurements taken at Station ALOHA located in the oligotrophic North Pacific Subtropical Gyre, we observed a sharp and persistent transition between picocyanobacterial communities, fromSynechococcusclade II abundant in the nearshore toProchlorococcushigh‐light adapted clade II (HLII) proliferating in offshore and open ocean waters. In comparison to immediately adjacent offshore waters and the surrounding open ocean, phytoplankton biomass within Kāneʻohe Bay was dramatically elevated. Members of the phytoplankton community revealed strong seasonal patterns, while nearshore phytoplankton biomass positively correlated with wind speed, rainfall, and wind direction, and not water temperatures. These findings elucidate the spatiotemporal dynamics underlying transitions in ocean biogeochemistry and phytoplankton dynamics across estuarine to open ocean waters in the tropical Pacific and provide a foundation for quantifying deviations from baseline conditions due to ongoing climate change. 

Abstract In oligotrophic oceans, the smallest eukaryotic phytoplankton are both significant primary producers and predators of abundant bacteria such asProchlorococcus. However, the drivers and consequences of community dynamics among these diverse protists are not well understood. Here, we investigated how trophic strategies along the autotrophy‐mixotrophy spectrum vary in importance over time and across depths at Station ALOHA in the North Pacific Subtropical Gyre. We combined picoeukaryote community composition from a 28‐month time‐series with traits of diverse phytoplankton isolates from the same location, to examine trophic strategies across 13 operational taxonomic units and 8 taxonomic classes. We found that autotrophs and slower‐grazing mixotrophs tended to prevail deeper in the photic zone, while the most voracious mixotrophs were relatively abundant near the surface. Within the mixed layer, there was greater phagotrophy when conditions were most stratified and when Chlaconcentrations were lowest, although the greatest temporal variation in trophic strategy occurred at intermediate depths (45–100 m). Dynamics at this site are consistent with previously described spatial patterns of trophic strategies. The success of relatively phagotrophic phytoplankton at shallower depths in the most stratified waters suggests that phagotrophy is a competitive strategy for acquiring nutrients when energy from light is plentiful.