Seasonal timing separates algal bloom records from marine heatwave exposure

Abstract

Marine heatwaves (MHWs) are increasingly invoked in explanations of coastal algal blooms, yet shared seasonality can confound apparent MHW–bloom coincidence. We assembled a source‐resolved catalogue of 48 algal bloom records and a satellite‐derived regional daily sea surface temperature series for Sansha Bay, China, to evaluate bloom‐record timing against MHW exposure during 2005–2020. Bloom records peaked in May–June, and regional MHW days peaked in July–September. The weighted circular seasonal centers were separated by 2.73 months, and the monthly overlap coefficient was 0.29. Event‐level classification identified 9 MHW‐related bloom records and 39 records without event‐period or pre‐start MHW exposure. Under the same‐calendar‐month null model, the observed count of 9 fell within the timing‐conditioned distribution centered at 10.84 records (95% interval, 6–16; one‐sided exact p  = 0.799). The same‐year‐month, duration, source, and exposure‐window analyses placed the observed count within their respective timing‐conditioned distributions. Seasonal timing therefore provides the background needed to interpret MHW–bloom coincidence in Sansha Bay. This framework separates temporal co‐occurrence from enrichment above the expected timing of a historical bloom‐record catalogue.