Seasonal Effects of Nakdong River Freshwater Inflow and Coastal Environmental Changes on Phytoplankton Community Structure, Including Harmful Species, in Eastern Jinhae Bay, Korea

Rainfall-induced freshwater influx is a major nutrient source in estuarine and coastal waters, often driving changes in phytoplankton community structure and blooms. In Jinhae Bay of Korea, a critical area for shellfish aquaculture, the interaction between the Nakdong River discharge and the Tsushima Warm Current creates a frontal zone conducive to phytoplankton proliferation. This study investigated the seasonal variation in phytoplankton communities, including harmful and toxin-producing species, in relation to environmental factors from February 2022 to November 2023 in Jinhae Bay. Except for the summer increase in certain dinoflagellates, diatoms, including Chaetoceros spp., Pseudo-nitzschia spp., and Skeletonema spp., dominated the phytoplankton community across seasons. In addition, nutrient influx from the Nakdong River, particularly nitrate + nitrite and silicate (p < 0.001), was a key driver of phytoplankton community structure. Spatially, phytoplankton communities differed between the inner (St. 1 and 4) and outer (St. 2 and 3) areas in the bay, likely due to the influences of seasonal river discharge, the Tsushima Warm Current, and tidal currents. Among harmful algal blooms causative species, dinoflagellate Margalefidnium polykrikoides was correlated with water temperature, exhibiting higher densities in summer. In contrast, Akashiwo sanguinea was mainly observed in winter. In addition, we found that toxin-producing dinoflagellates, such as Alexandrium catenella, Dinophysis acuminata, and Gonyaulax spinifera, were most prevalent in spring and summer, and their appearance was linked to complex interactions among freshwater influx, water temperature, and current dynamics. Our findings underscore the critical role of bay-specific oceanographic conditions, shaped by tidal and current patterns, in conjunction with riverine nutrient inputs, in driving seasonal phytoplankton blooms. This study provides valuable baseline data for understanding harmful/toxic microalgal dynamics in Jinhae Bay and offers key insights for effective coastal ecosystem management and conservation along the Korean Peninsula.