Developing numeric nutrient criteria for the Yangtze River Estuary and adjacent waters in China

Developing numeric nutrient criteria for estuaries and adjacent waters has been exceedingly complex due to the uniqueness of this ecosystem and its dynamic properties. To control and improve water quality, based on data from 1984 to 2014, a population distribution approach was applied to establish the numeric nutrient criteria for the Yangtze River Estuary and adjacent waters. Based on the natural geographical characteristics, currents effects and biological community features, the Yangtze River Estuary and adjacent waters are divided into four sea subareas: the transitional zone (segment I), Hangzhou Bay (segment U), the coastal zone outside of the estuary (segment III) and Zhoushan adjacent area (segment IV). The respective recommended criteria for dissolved inorganic nitrogen (DIN) and phosphate (PO4-P) were 1.05-1.24 mg/L and 0.03-0.036 mg/L for estuary segments I and II and approximately 0.2 mg/L and 0.01 mg/L for the adjacent water segments III and IV. The dissolved oxygen (DO) criteria are similar for the four segments, i.e., 6-7 mg/L. The chemical oxygen demand (CODMn) criteria for segments III and IV (0.56-0.59 mg/L) are much lower than those for segments I and II (1.04-1.15 mg/L). The total nitrogen (TN) and total phosphorus (TP) criteria for segments I and II (1.56-1.85 mg/L and 0.060-0.072 mg/L, respectively) are much higher than those for segments III and IV (0.27-0.29 mg/L and 0.023-0.028 mg/L, respectively). The proposed criteria were reasonable and reliable based on validation using nutrient-algal bloom sensitivity and comparison with current sea water quality standards and previous studies.