Exploring the potential of cold patches as an indicator for algal bloom occurrence and migration.

Harmful algal blooms-such as green tides and brown algae blooms-frequently occur near the Subei Shoal in the southwestern Yellow Sea. The locations and migration paths of these algae vary each year, and there is a lack of an appropriate oceanographic indicator for monitoring and prediction. We extracted cold patches from sea surface temperature data and used them to fit the spatial and temporal variations in phytoplankton growth. The correlation between cold patches and phytoplankton blooms is reflected in different ways: (1) the green tide migrates from the Subei Shoal through the corridor between cold patches; (2) the boundary of high chlorophyll-a (Chl-a) concentrations closely aligns with the cold patch boundary; and (3) high Chl-a concentrations of pelagophytes-key species in brown algal blooms-overlap with the cold patch center. The correlation is driven by the role of cold patches in separating water masses of differing properties and sources, as well as by the divergent upwelling structures in the cold patches. This study reveals that cold patches exhibit either the aggregation effect or barrier effect in different phytoplankton growth. These findings facilitate the determination of the occurrence and migration of algal blooms, thereby contributing to the management of ecological hazards.