Freshwater discharge from artificial reservoirs is a major driver of environmental variability in semi-enclosed coastal systems. This study investigated the ecological impacts of freshwater release from Bunam Lake, which experienced over 40 years of accumulated pollution, leading to severe odor issues and damage to aquaculture, on plankton dynamics and aquaculture species in Cheonsu Bay by integrating field observations with controlled microcosm and mesocosm experiments. Field observations revealed that increased freshwater discharge during summer significantly reduced salinity and increased nutrient concentrations, leading to enhanced phytoplankton biomass and shifts in community structure. These changes were accompanied by variability in dissolved oxygen, indicating potential risks of hypoxia under stratified conditions. Laboratory-scale microcosm experiments (50mL, 10 L) demonstrated that low salinity and elevated temperature conditions increased mortality and reduced filtration rates in bivalves (oyster and Manila clam), while aquaculture fish species exhibited high sensitivity to bottom water influenced by freshwater discharge. To extend these findings to more realistic environmental conditions, outdoor mesocosm experiments (1 ton) were conducted. The mesocosm results showed that although freshwater inputs initially stimulated phytoplankton growth and altered water quality, biological uptake, mixing, and dilution processes contributed to gradual recovery of ecosystem conditions over time. Overall, freshwater discharge induces strong short-term ecological disturbances across trophic levels, but the coastal system exhibits a degree of resilience through biogeochemical and biological feedbacks. These findings highlight the importance of integrating multi-scale experimental approaches to assess ecological risks and support sustainable coastal management.

Freshwater discharge,microcosm experiment,mesocosm,Plankton dynamics