Objective Current response measures for red tides in eastern Guangdong sea area remain limited to manual tracking and monitoring, which suffer from significant spatial constraints and temporal delays. This study aims to propose recommendations for the regular monitoring and early warning of red tides in eastern Guangdong sea area.

Methods Based on monitoring data from 9 cruises conducted in eastern Guangdong sea area from 2021 to 2023, the distribution of chlorophyll-a concentration and its relationship with environmental factors were analyzed. Multiple sources of information, including river inflows, regional ocean currents, and aquaculture distribution, were integrated to discuss phytoplankton distribution patterns in the region.

Results 1) The average chlorophyll-a concentration follows the pattern: estuary area>aquaculture area>offshore area; summer>spring>autumn; Shantou City>Shanwei City>Chaozhou City>Jieyang City. 2) The dilution effect of river inflow reduced the likelihood of red tide outbreaks in estuary area. Regional ocean currents had a reverse impact on the growth of offshore phytoplankton. Aquaculture area, subjected to dual pressures from river inflows and ocean currents, combined with nutrient enrichment from fish and shellfish farming, were more prone to red tide outbreaks. 3) Red tide outbreaks in Shantou City were attributed to multiple factors. The eutrophication in Zhelin Bay of Chaozhou City was severe but relatively stable, with no red tides reported in the past three years. Phytoplankton growth in Jieyang City and Shanwei City relied more on nutrient inputs from offshore sources. 4) High chlorophyll-a concentration areas were mainly located in Zhelin Bay, the area from Nan’ao Island to Shantou Port, the Lianjiang Estuary, and Jieshi Bay.

Conclusion The four coastal cities in eastern Guangdong should establish a multi-center red tide monitoring and early warning system tailored to regional phytoplankton distribution. Shantou City should prioritize estuary and aquaculture areas, Chaozhou City should address impacts of aquaculture types, Shanwei City should focus on internal dynamics of Jieshi Bay, and Jieyang City should account for ocean current influences. This study provides reference materials for the normalized monitoring of red tide in eastern Guangdong sea area.