폐쇄성 내만에서 빈산소수괴의 시공간적 분포특성

Abstract

To examine spatiotemporal variations of oxygen deficient water mass (<3 mg/L), dissolved oxygen was measured with depth in seawater in the eutrophic Masan Bay, Korea. The oxygen deficient water mass was observed continuously in the Bay form May to September. It extends about 9 km from central part of Masan harbor to the West of Modo and about below 5 m depth in summer. In mid-July, oxygen deficient water mass has occurred in the inner part of Masan Bay, and then has spread to the outer part since late-July. Continuous monitoring results measured using a dissolved oxygen sensor showed that oxygen deficient water mass intermittently began to appear from March in the evening time, and then it persisted until the end of October. The concentration of dissolved oxygen in seawater was lowest between July and August when pollution discharge and stratification intensity was increased. For the conservation and management of marine ecosystems, it is necessary to accurately understand the mechanism and timing of oxygen deficient water mass occurrence. Long-term variations of bottom DO in seawater of Masan Bay from 1997 to 2016 were studied to understand changes oxygen deficient water mass from before to after the introduction of the total pollution load management system (TPLMS). The oxygen deficient water mass occurred 16 times before and 11 times after. The implementation of TPLMS is judged to have resulted in the decrease in the frequency of the occurrence of oxygen deficient water mass. The average annual concentrations of COD and nutrient in the seawater decreased by about 20%, 50% respectively, when compared to those prior to the introduction of the TPLMS in Masan Bay. Therefore, water quality in Masan Bay has improved considerably since the TPLMS was implemented. Concentration of organic matter and nutrient in the inner part of Masan Bay was about twice as high in comparison with the outter part. In order to improve the quality of seawater in Masan Bay special management area, it is necessary to identify the cause of contamination and intensive management of the inner part of Masan Bay. It seems that oxygen deficient water mass in the bottom layer begin as the DO supply from the surface layer to the bottom layer is blocked by the stabilization of water mass due to an increase in the surface seawater temperature. It also seems that oxygen deficient water mass fully develop later in August due to DO consumption as organic matter derived from the land or created in the ocean flows into the bottom layer and decomposes, while the surface layer salinity decreases and the water masses’ stratification is enhanced due to the increase in the amount of rainfall in the region during this period. Therefore, continuous monitoring is needed to improve the oxygen deficient water mass of Masan Bay.