Long-term trends in dissolved oxygen and environmental parameters in Jinhae Bay, Korea: A 25-year analysis (1997-2021)

In recent decades, global oceanic dissolved oxygen (DO) levels have been consistently declining, leading to a notable increase in hypoxic zones. However, comprehensive information on the current trends of DO in Jinhae Bay remains scarce. This study investigated the long-term variations in bottom dissolved oxygen (DO) in Jinhae Bay, specifically focusing on recurring occurrences of oxygen-deficient water (ODW) in August, coinciding with the lowest average DO levels with the most stratified water. ODW events predominantly manifested in the inner bay, while the outer bay exhibited fewer instances due to robust water exchange processes. Analysis of the Low DO group revealed higher surface temperature and salinity, indicating diminished oxygen solubility compared to the High DO group. Lower Chemical oxygen demand (COD) and chlorophyll-a (Chl-a) values in the Low DO group suggested enhanced bacterial decomposition and subdued photosynthesis, in contrast to the High DO group. Over the last 25 years, Jinhae Bay has experienced a discernible decline in bottom DO concentrations. Contrastingly, surface temperature has exhibited a consistent upward trend, notably accelerating in the last 12 years, indicating reduced oxygen solubility. Notably, a substantial reduction in bottom DO levels was observed in the outer bay, previously considered less susceptible to hypoxic conditions. Furthermore, a concurrent decrease in pH highlights the urgency for a comprehensive investigation encompassing both summer hypoxia and seawater acidification, crucial for protecting Jinhae Bay's ecosystem from further deterioration.