Winter and summer phytoplankton peaks in Gwangyang Bay, South Korea: a physical-biogeochemical model study

Phytoplankton in temperate regions typically exhibit spring and fall bloom patterns driven by interactions between physical and biogeochemical factors. In contrast, Gwangyang Bay, located along the southern coast of Korea, is a prominent bay exhibiting estuarine phenology, characterized by distinct peaks in winter and summer. To investigate the mechanisms underlying these uncommon bloom peaks, we employed a coupled physical–biogeochemical model that integrates the Regional Ocean Modeling System (ROMS) with a low-trophic ecosystem module, incorporating size-structured phytoplankton and zooplankton dynamics. The model simulated the temporal and spatial evolution of phytoplankton biomass and nutrient distributions under forcing averaged over the 2007–2015 period, and a suite of sensitivity experiments was performed to isolate the roles of temperature, light attenuation, shortwave radiation, wind, and riverine nutrient inputs. The model successfully reproduced observed seasonal patterns in temperature, nutrients, dissolved oxygen, and chlorophyll a concentrations. Summer blooms were driven by elevated dissolved inorganic nitrogen (DIN) loading from river discharge and high water temperatures, which collectively stimulated the growth of small-sized phytoplankton. Winter blooms, in contrast, were driven by large-sized phytoplankton growth under moderate nutrient conditions and low grazing of zooplankton due to low temperature. The vertical and horizontal structure of phytoplankton distributions was influenced by stratification patterns, with residence time and light availability playing secondary roles. Sensitivity experiments demonstrated that seasonality of water temperature and riverine DIN supply are critical for reproducing the observed seasonal bloom pattern. The results underscore the importance of size-specific traits and temperature-nutrient interactions in shaping seasonal phytoplankton dynamics in complex estuarine-bay environments.