The Xiamen Symposium on Marine Environmental Sciences (XMAS 2027)

Marine heatwaves (MHWs) are increasing under ocean warming, but their relationship with harmful algal blooms (HABs) remains difficult to resolve in managed coastal systems where bloom development depends on species composition, nutrient conditions, water-column structure, and event timing. This study examined how intensified MHW exposure was reflected in recorded HAB occurrence and event characteristics in Xiamen Bay during 1990–2019. Daily sea-surface temperature was used to detect MHWs, and taxonomically identified HAB records were used to evaluate annual occurrence, HAB days, monthly distribution, functional-group structure, MHW–HAB timing relationships, species composition, duration, and affected area. MHW exposure increased over the study period, especially in MHW days and cumulative intensity, with the strongest decadal exposure in the 2010s. Recorded HAB occurrence did not follow the same trajectory, and both HAB frequency and HAB days peaked in the 2000s. At the event scale, 23 of 56 identified HAB events were classified as MHW-related, but these events included concurrent, antecedent, and post-start thermal exposure rather than a single pre-bloom MHW sequence. MHW-related HABs were mainly diatom records, and stronger pre-HAB MHW exposure was not associated with longer duration or larger affected area. These findings show that annual MHW intensification, temporal proximity, and antecedent bloom forcing should be separated when interpreting MHW–HAB relationships from long-term coastal records.