Major controlling factors affecting spatiotemporal variation in the dissolved oxygen concentration in the eutrophic Masan Bay of Korea

Eutrophication-induced coastal hypoxia conditions have been developed frequently and spread widely in Masan Bay over recent decades, but little information is currently available. Thus, the spatiotemporal variations in dissolved oxygen (DO) and its major environmental controlling factors were investigated in Masan Bay. We measured the DO of seawater with depth in the bay. In mid-July 2014, oxygen-deficient water ( 3 mg L−1) occurred in the inner bay and spread to the outer bay by late July. Oxygen-deficient conditions at the bottom persisted into mid-September but formed mainly below 5 m in depth. Continuous monitoring data showed that oxygen-deficient conditions began to appear intermittently in evenings from March onward and persisted until the end of October. The concentration of DO in seawater was lowest between July and August when stratification was most intense. The major factors controlling the bottom DO variations in Masan Bay included biogeochemical factors (including inorganic nutrient concentrations, Secchi depth, and chlorophyll-a concentration) and the physical factor of seasonal water stratification in this study. Based on these controlling factors, a regional DO algorithm was developed for Masan Bay, and the algorithm-derived DO concentrations were in good agreement with field data, but they still require further improvement. To better conserve and manage marine coastal ecosystems, it is necessary to understand the development of oxygen deficient water (ODW) and continuously monitor DO concentrations in Masan Bay and in other southern coastal regions of South Korea.