Changing Nutrients, Oxygen and Phytoplankton in the East China Sea

Biogeochemical processes in the East China Sea are substantially affected by anthropogenic nutrient inputs. The dramatic decadal changes in nutrient concentrations were mainly due to the increases of DIN and DIP in the Changjiang (Yangtze) River since 1980s. As a result, phytoplankton abundance increased dramatically between 1958 and 2016 in both the Changjiang Estuary and the East China Sea. Before 1980s, chain-forming diatoms were dominant, while increasing of large-cell dinoflagellates is probably related to increasing DIN/silicate ratio. Increasing nutrient input and phytoplankton abundance greatly impact seasonal hypoxia condition in the East China Sea. Hypoxia is relatively sporadic and patchy before 2013. In 2016 and 2017, hypoxic events were more severe, occurring over larger areas with dramatically lower minimum values of dissolved oxygen. Notwithstanding, occurrences of bottom hypoxia in the East China Sea are highly dynamic and are significantly influenced by episodic events such as wind mixing.