Coastal phytoplankton communities exhibit pronounced spatiotemporal heterogeneity that conventional low-frequency sampling and laboratory-based taxonomy cannot adequately resolve. Here we present a multi-year, high-resolution observational framework that couples a compact in situ Phytoplankton Optical Detection (POD) sensor — capable of discriminating six phytoplankton functional groups via 15-wavelength excitation fluorescence spectroscopy — with voluntary observing ship (VOS) platforms operating along repeat transects in Xiamen coastal waters, southeastern China.
Over 4 years of VOS observations (2022–2026), the POD sensor yielded continuous, taxon-resolved chlorophyll-a profiles spanning estuarine, coastal waters. The resulting dataset reveals several key patterns. First, phytoplankton community composition exhibits strong seasonality, with diatoms dominating in spring and winter, while dinoflagellates and cyanobacteria increase in relative contribution during summer stratification periods. Second, fine-scale spatial gradients in community structure are consistently observed across the estuary-to-ocean continuum, closely tracking salinity fronts and freshwater plume boundaries, suggesting that physical transport rather than in situ growth largely shapes taxonomic distributions at sub-mesoscale. Third, interannual comparison of algal bloom  events reveals significant variability in bloom onset timing, peak magnitude, and duration, with diatom-dominated spring blooms showing a lengthening trend and dinoflagellate blooms becoming more frequent .
These results highlight how ship-of-opportunity platforms, equipped with autonomous optical sensors, can provide sustained, taxon-resolved observations at spatiotemporal resolutions unattainable by traditional methods, fundamentally advancing our mechanistic understanding of plankton dynamics in dynamic coastal ecosystems under changing environmental conditions.

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