Structure equation model-based approach for determining lake nutrient standards in Yungui Plateau ecoregion and Eastern Plain ecoregion lakes, China

Predictive nutrient criteria method, combined with expert elicitation and structural equation model (SEM), was proposed in this study for establishing lake nutrient standard in Chinese lakes (Yungui Plateau ecoregion and Eastern Plain ecoregion). Expert elicitation was involved to quantify the probability of attainment of the designated-use (drinking water source) based on monitoring data. The experts scoring results were introduced to SEM to assess predictive relationships between candidate standard variables and the designated-use attainment. In Yungui ecoregion, the standardized effects of chlorophyll-a (Chl-a), chemical oxygen demand and total phosphorus (TP) on designated-use attainment were −0.41, −0.14, and −0.43, respectively. These result demonstrated that the two most predictive indicators for designated-use attainment were TP and Chl-a. In Eastern ecoregion, the standardized effects of TP, total nitrogen (TN), dissolved oxygen and water temperature (T) on designated-use attainment were 0.77, −0.12, 0.13 and −0.02, respectively. The most predictive indicator was TP. The model was further used for estimating the designated-use attainment associated with various levels of candidate standards. TP, TN, Chl-a and Secchi depth (SD) were selected as standard indicators in Yungui ecoregion. TP, TN, and Chl-a were selected in Eastern ecoregion. In order to achieve the 85 % designated-use attainment, standard values of TP, TN, Chl-a and SD in Yungui ecoregion would be 0.02 mg/L, 0.2 mg/L, 1.4 μg/L, and 0.581 m; and standards of TP, TN, and Chl-a in Eastern ecoregion would be 0.039 mg/L, 0.95 mg/L, and 1.75 μg/L, respectively. Finally, the differences between standards in these two ecoregions were also analyzed.