Looming hypoxia on outer shelves caused by reduced ventilation in the open oceans: Case study of the East China Sea

Abstract

The discharge of nitrate and phosphate from Changjiang (Yangtze River) has increased in recent decades. Eutrophication off the mouth of Changjiang has subsequently become a serious problem, as evidenced by the hypoxia area reaching 12,000 km2. This study demonstrates that in the wide East China Sea (ECS) the nitrate and phosphate concentrations in the Kuroshio Intermediate Water (KIW) have also increased, but the dissolved oxygen (DO) concentration has decreased since as early as 1982, most likely owning to reduced ventilation in the North Pacific Intermediate Water (NPIW). Conversely, the Kuroshio Tropical Water (KTW) has decreased in the nitrate and phosphate concentrations yet increased in DO concentration. As KIW contributes substantially to the upwelling, the nitrate and phosphate concentrations in the bottom water on the outer shelf of the ECS appear to have increased as well, but the DO has decreased. Given that the nutrient inputs from both the land and the Kuroshio Current have increased, yet the input of DO from the Kuroshio has decreased, more severe eutrophication and hypoxia may occur in the entire ECS. Similar processes may also affect other shelves that come into contact with NPIW.