Categorizing numeric nutrients criteria and implications for water quality assessment in the Pearl River Estuary, China

Coastal eutrophication, the over-enrichment of water with nutrients, has become a global ecological problem. As coastal waters are subjected to great pressure due to anthropogenic influences and climate change, establishing numeric nutrient criteria for coastal waters has been exceedingly complex at present. To control and improve the water quality of the Pearl River Estuary (PRE), based on the data from 2015 to 2020, the nutrient criteria of the PRE and adjacent waters were established using frequency statistical analysis. Based on the spatiotemporal salinity patterns, the coastal waters of the PRE were divided in three subareas namely freshwater (Zone I), mixed (Zone II), and seawater (Zone III) using cluster analysis. The recommended criteria values of dissolved inorganic nitrogen (DIN) were 0.573, 0.312, and 0.134 mg center dot L-1 in Zones I, II, and III, respectively. The total nitrogen (TN) criterion for Zone III (0.222 mg center dot L-1) was much lower than those for Zone I (0.902 mg center dot L-1) and Zone II (0.885 mg center dot L-1).The dissolved inorganic phosphorus (DIP) criteria were different for the three Zones, ranging from 0.004 to 0.009 mg center dot L-1, and the total phosphorus (TP) recommended criteria in Zones I, II, and III were 0.039, 0.028, and 0.020 mg center dot L-1, respectively. In the water quality assessment, the categorizing numeric nutrients criteria can be referred and applied into fresh, mixed, and seawater zones of PRE. The results of this study provide a new nutrient reference condition in the PRE, which could be helpful in establishing integrated land-ocean unified nutrient criteria and water quality assessment, and implementing effective coastal eutrophication control in the future.