Impacts of precipitation variability on water quality in the Nakdong River Estuary: multi-year (2016–2021) data analysis

Physicochemical and biological parameters (temperature, salinity, pH, dissolved inorganic nutrients, and chlorophyll-a) were analyzed to evaluate the effects of precipitation variability associated with climate change on the water quality in the Nakdong River Estuary, South Korea. Multi-year monitoring data (2016–2021) were collected seasonally (February, May, August, and November) throughout the study period. Extreme rainfall events caused pronounced estuarine freshening (salinity < 1) and sharply enhanced riverine nutrient fluxes, with wet-to-dry season increases of 4–70 times for dissolved inorganic nitrogen, 4–36 times for phosphorus, and 9–740 times for silicate, showing strong positive correlations with precipitation (r² = 0.76–0.82, p < 0.001). Time-series and self-organizing map classifications revealed estuarine that the water quality was strongly controlled by seasonal precipitation and river discharge, whereas offshore waters exhibited weaker but detectable responses. Notably, extreme rainfall events altered the chlorophyll-a distribution, suppressing phytoplankton accumulation in the estuary because of dilution and flushing, while enhancing chlorophyll-a concentrations in offshore waters through nutrient-enriched river plume dispersion. These results demonstrated that extreme rainfall driven by climate change can enhance the terrestrial nutrient input into coastal waters, thereby increasing the potential for eutrophication and harmful algal blooms.