Integrated evaluation of the marine ecological environment in the Yangtze River Estuary and its adjacent area

Based on monitoring data from 1984 to 2015, we report a trend analysis of dissolved inorganic nitrogen(DIN) and phosphate(PO4-P) in the Yangtze River Estuary and its adjacent area; the reference year of nutrients was determined to be 1987, and recommended reference values were 0.0705 and 0.000751 mg/L for DIN and PO4-P, respectively. Then, frequency analyses were combined to produce the partition thresholds of 0.339 and 1.15 mg/L for DIN, and those of 0.0289 and 0.0530 mg/L for PO4-P. Thus, the study area could be partitioned into three subareas: the inner area of the mouth, transition area, and outer area of the mouth. Considering the distributions of the ecological red line area and pollution sources, the study area was divided into eight comprehensive subzones. An integrated evaluation of the marine ecological environment was developed based on the index system method, which was composed of marine water, surface sediment, and biological diversity. The distribution of the marine water environment was similar to that of the biological and ecological characteristics; environmental indices improved from the inside to outside area. South Branch/North Branch/North Port was slightly better than the outside area where the sediment indices were higher than those of Hangzhou Bay/North Channel/South Channel. The distribution of the marine integrated environment was as follows: subzone IV < subzone V< subzone III < subzone I < subzone II < subzone VI < subzone VIII < subzone VII, which indicated that the integrated environment was obviously improving with time.