Seasonal dynamics at low trophic level and ecological interactions of microbial and plankton communities during...

This study investigates the seasonal dynamics of microbial and plankton communities in Tongyeong (TY) and the Bodol Sea near Yeosu (YS), South Korea, focusing on the interactions during algal blooms and their ecological implications. In May at TY, moderate chlorophyll a (Chl-a) and high copepod abundance suggest top-down control by zooplankton grazing, rather than nutrient limitation, regulates phytoplankton biomass. Microbial communities resembled those during August YS blooms, likely due to enhanced organic matter from grazing. By late summer, reduced nutrients favored Dinoflagellata and Ochrophyta indicating weakened grazing pressure and greater microbial diversity. In August at YS, a harmful Cochlodinium bloom dominated offshore waters, while diatoms prevailed nearshore. Elevated Chl-a, oversaturated oxygen, and low copepod abundance indicated disrupted trophic balance during the blooming. Cochlodinium correlated strongly with bacteria such as Pseudoaltermonas and Vibrio, reflecting responses to high productivity. After the bloom decline in September, Syndiniales (parasitic dinoflagellates) increased, suggesting predator-prey regulation of algal populations. Bacterial remineralization supported nutrient cycling, maintaining ecosystem balance. Contrasting responses to high productivity were observed: TY exhibited strong top-down control without biomass accumulation, whereas YS experienced significant biomass buildup during the bloom. Despite these differences, the microbial communities in both systems were similar and reflected the degree of productivity more accurately than the primary producers themselves. Overall, these results highlight the complex interactions among primary producers, grazers, and microbes, emphasizing the critical role of trophic dynamics in shaping coastal bloom events.