The selection of eutrophication indicator and determination of its reference condition is a key process for nutrient establishment in coastal and estuarine. In this paper, the response variables of eutrophication indictors in the Yangtze River estuary were primarily selected. And then, the Yangtze River estuary and adjacent waters was divided into four sub-areas as follows: transitional zone, Outside Yangtze River Estuary, Hangzhou Bay and Zhoushan sea based on the natural and geographical characteristics of the Yangtze River estuary. Furthermore, the "frequency distribution curve" method was used to determine the reference conditions of response variables in the coastal zone outside of the Yangtze River estuary and Zhoushan Archipelago based on the analysis of the data collected from the Yangtze River estuary and adjacent waters during 2009 to 2010. The "frequency distribution curve" method was carried out by SPSS 13.0, and the upper 75^th percentile was used as a starting point, the lower 25^th percentile was also suggested. What to be paid attention was that as the phytoplankton was difficult to breed in transitional zone and Hangzhou Bay because of the lower transparency and higher turbidity, the two sub-areas would not be considered to determine the reference condition.
In the present study, chlorophyll-a (chl-a) and bottom dissolved oxygen (DO) were chosen as the basic indicators, while, phytoplankton density and permanganate index (COD_Mn) were chosen as the auxiliary indicators. The reference condition values which can be referred as the nutrient criteria values were listed in the order generally as follows. In Outside Yangtze River Estuary, the chl-a reference condition values were 0.87mg/m³ in spring, 1.88mg/m³ in summer, 0.84mg/m³ in autumn; the phytoplankton density reference condition values were 17.44×10³cells/L in spring, 25.96×10³cells/L in summer, 12.10×10³cells/L in autumn; the COD_Mn reference condition values were 0.42mg/L in spring, 0.56mg/L in summer, 0.46mg/L in autumn, the bottom DO reference condition values were 8.36mg/L in spring, 4.22mg/L in summer, 6.95mg/L in autumn. In Zhoushan sea, the chl-a reference condition values were 0.73mg/m³ in spring, 1.00mg/m³ in summer, 0.78mg/m³ in autumn; the phytoplankton density reference condition values were 6.77×10³cells/L in spring, 9.72×10³cells/L in summer, 4.59×10³cells/L in autumn; the COD_Mn reference condition values were 0.51mg/L in spring, 0.37mg/L in summer, 0.55mg/L in autumn; the bottom DO reference condition values were 8.75mg/L in spring, 5.94mg/L in summer, 7.40mg/L in autumn. Compared with the relevant studies about chl-a in the same study area in 1980s, the reference condition values determined in this paper were close to the average chl-a concentration in 1980s. Accordingly, the reference condition values could reflect the nutrient conditions of the Yangtze estuary and adjacent waters.
The results showed that there were distinct differences among different sub-areas, and different seasons. Therefore, it is very necessary to develop suitable management measures for different sub-areas and different seasons in the Yangtze River estuary and adjacent waters. Establishing the reference condition is one necessary element of the criteria development process. Reference condition values were appropriately modified on the basis of the examination of the historical records, expert judgment, as well as consideration of downstream effects.