Determining reference conditions for TN, TP, SD and Chl-a in eastern plain ecoregion lakes, China.

Establishing the nutrient reference condition (baseline environmental condition) of lakes in an ecoregion is a critical consideration in the development of scientifically defensible aquatic nutrient criteria. Three methods were applied to determine reference conditions in the Eastern plain ecoregion lakes with respect to total phosphorus (TP), total nitrogen (TN), planktonic chlorophyll a (Chl-a) and Secchi depth (SD). The reference condition value for the lakes in the Eastern plain ecoregion by the trisection method is TP of 0.029 mg/L, TN of 0.67 mg/L, Chl-a of 3.92 mg/m3, SD of 0.85 m, and the reference condition range by the lake population distribution approach is TP of 0.014-0.043 mg/L, TN of 0.360-0.785 mg/L, Chl-a of 1.78-4.73 mg/m3, SD of 0.68-1.21 m. Additionally, empirical models were developed for estimating the reference Chl-a concentration and SD successfully for lakes in the Eastern plain ecoregion. Overall, the data suggest that multiple methods can be used to determine reference conditions and that in Eastern plain ecoregion lakes the reference condition corresponds to a mesotrophic status.