Evaluation of spatial and temporal distribution of water quality and eutrophication in Bohai Bay during 2006－2020

In this paper, the fluxes of total nitrogen (TN), total phosphorus (TP) and chemical oxygen consumption (COD) into the sea and the water quality of the Bohai Bay were analyzed during the 15 years from the implementation of the total pollutant control plan in 2006 to the end of the comprehensive treatment of the Bohai Sea in 2020. The eutrophication status of the Bohai Bay was evaluated by pressure-state-response method. In the past 15 years, the annual fluxes of TN, TP and COD into the sea in Bohai Bay have fluctuated and decreased by 48%, 55% and 32%, but the interannual fluctuation amplitude of fluxes into the sea is large, with relative standard deviations (RSDS) of 35%, 66% and 27%, respectively. Accordingly, the average annual concentrations of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) in Bohai Bay decreased by 44% and 35%, and RSDS were 25% and 24%, respectively, in the past 15 years. Under the influence of the river input into the sea, the pollutant concentration basically presents the distribution characteristics of high in the estuary area at the bottom of the bay and low in the middle and estuary of the bay. The concentrations of chlorophyll a (Chl a) and bottom dissolved oxygen (DO) decreased by 32% and increased by 14%, and RSDS were 16% and 7%, respectively. In recent 15 years, the eutrophication in Bohai Bay was relatively serious, however, in the five years from 2015 to 2020, the eutrophication degree in Bohai Bay has eased somewhat, but it still fluctuates between “poor” and “medium” levels. This indicates that water pollution and eutrophication in Bohai Bay have improved since the implementation of the total amount of pollutants from land into the sea, but the results are not stable and need to be consolidated.