Responses of phytoplankton functional types to marine heatwaves in China’s marginal seas and adjacent waters

Marine heatwaves (MHWs) are intensifying under anthropogenic climate warming, yet their impacts on phytoplankton community composition at the functional-type level remain poorly characterized. Here, we combine the NOAA OISST v2.1 daily sea surface temperature record (1982–2023) with the AI-driven Global Daily 4-km Phytoplankton Functional Type dataset (AIGD-PFT, 1998–2023) to investigate responses of eight phytoplankton functional types (PFTs) to MHWs across China’s marginal seas and adjacent waters (hereafter CMS). MHW events were identified using a standardized detection framework, and PFT-specific anomalies were quantified through composite analysis. We further evaluate how MHW intensity and duration modulate these responses by partitioning events into intensity- and duration-based categories. The CMS experienced approximately 2.0–4.0 MHW events yr–1, with frequency, duration, and intensity exhibiting pronounced spatial heterogeneity. Composite analysis reveals functionally contrasting responses: small-celled, warm-adapted groups (Prochlorococcus and prokaryotes) showed widespread positive anomalies exceeding +25%, whereas large-celled, nutrient-demanding groups (such as diatoms and dinoflagellates) exhibited predominantly negative anomalies reaching –27% across subtropical and offshore waters. This functional dichotomy was broadly preserved across sub-regions and seasons. In most cases, stronger and longer MHWs appeared to intensify these contrasting responses. Because AIGD-PFT retrieval skill varies among PFTs and is reduced in optically complex coastal waters, individual anomaly magnitudes (particularly for Prochlorococcus) should be interpreted with caution, although the convergent response of the better-constrained prokaryote group supports the overall small-celled signal. Overall, these findings suggest that MHWs likely drive a systematic shift in phytoplankton community structure toward smaller size classes, with potential implications for carbon export efficiency and marine food-web dynamics under continued climate warming.