Analysis on the multi-year change of water quality and pollution causes in Luoyuan Bay

Based on the water quality survey data of Luoyuan Bay from 2004 to 2019, this paper analyzed the multi-year change characteristics of water quality, spatial distribution characteristics and pollution causes. The research results can serve as a reference for water quality improvement and ecosystem restoration in Luoyuan Bay. Our study revealed that there was serious eutrophication during the spring over the past years in Luoyuan Bay. Despite the apparent improvement after 2015, water quality failed to meet the second class of sea water quality standard (GB3097-1997). Specifically, there was a serious imbalance in the nitrogen and phosphorus ratio, with an excess of dissolved inorganic nitrogen (DIN) and that is more polluting than dissolved inorganic phosphate (DIP). The differences in the multi-year spatial distribution of DIN and DIP were small, and the pollution levels displayed a generalized feature. After 2015, the spatial distribution of DIN and DIP tended to be more serious at the top of the bay. The abolition of aquaculture was an important reason for the improvement of water quality after 2015. Land-sourced pollution was the primary source of water pollution in Luoyuan Bay after 2015. The synergistic effect of the reduction of tidal intake caused by reclamation was identified as the primary reason for more serious pollution at the top of the bay.