Causes of COD increases in Gwangyang Bay, South Korea

Water quality, the carbon isotope ratio of particulate organic matter (POM) and sediment, and the nutrients limiting phytoplankton growth were investigated to determine the cause of organic matter increase and to determine an effective countermeasure for chemical oxygen demand (COD) increase in Gwangyang Bay, South Korea. The sources of most NO3-N and SiO2-Si entering Gwangyang Bay seem to be land-based, and the primary source of P appears to be industrial complex and/or domestic wastewater. The major cause of the COD increase in Gwangyang Bay was phytoplankton growth. Phytoplankton growth was limited by N at high salinity and by P at relatively low salinity. Phytoplankton growth was more limited by N in Gwangyang Bay than in similar bays because of a strong point source of P in Gwangyang Bay. In the rainy season, phytoplankton were able to massively grow in Gwangyang Bay after heavy rainfall events because of the high input of N from runoff, input of P and Si, and increasing sunlight after the rainy season. The peak chlorophyll a concentration observed in winter may have resulted from mixing N from the lower layer and because Eucampia grew well at low water temperatures. To improve COD levels in Gwangyang Bay, it is important to control the phytoplankton growth in the rainy season, particularly by limiting the input of NO3-N from outside the bay.