Organic nutrient availability and extracellular enzyme activities influence harmful algal bloom proliferation in a coastal aquaculture area.

Small-scale but highly destructive harmful algal blooms (HABs) are increasingly observed in coastal aquaculture areas. However, the impact of elevated organic nutrients generated during mariculture production and their bioavailability for HABs are not yet fully understood. This study investigated the population dynamics of HAB-causing species and the hydrolytic dynamics of different extracellular enzyme activities (EEAs) along with variations in environmental inorganic and organic nutrients during a dinoflagellate bloom of Prorocentrum shikokuense adjacent to the coastal aquaculture area in Sansha Town, located in the East China Sea. The results indicated that both inorganic and organic nutrients were enriched in the study area, with organic nutrients comprising more than half of the total nutrient content. The presence of enriched organic nutrients and a high N:P ratio significantly contributed to bloom occurrence. Organic nutrient concentrations played a predominant role in controlling both the density and duration of the P. shikokuense bloom. The significant increase in EEAs during the bloom stages, primarily contributed by P. shikokuense , emphasized the crucial hydrolytic role of extracellular enzymes in nutrient recycling and supply for the bloom. The enzymatic kinetic parameters indicated enhanced efficiency in the recycling of organic nutrients as the bloom progressed. This study highlights the significant role of organic nutrients facilitated by EEAs in determining the occurrence, duration, and magnitude of HABs in coastal aquaculture areas. • Organic nutrients are enriched in waters near coastal aquaculture areas. • Enriched organic nutrients and a high N:P ratio contribute to bloom occurrence. • Organic nutrient levels control bloom density and duration. • Extracellular enzyme activities play a crucial role in bloom nutrient recycling. • Bloom progression enhances the efficiency of organic nutrient recycling. [ABSTRACT FROM AUTHOR]