2.3 Trophic assessment
This study used the TLI method for water quality eutrophication evaluation. It is widely used, and the evaluation results have high accuracy, most of the reservoirs in China have chosen this method to evaluate the nutrient status of water quality (Liu et al., 2021) According to the Technical Provisions on Eutrophication Evaluation Methods and Grading of Reservoirs developed by the China Environmental Monitoring General Station, taking Chl-a as the benchmark parameter, selecting TP, TN, SD, CODMn and other parameters with significant correlation with Chl-a to be evaluation factors, and carrying out weighting analysis to get the water quality evaluation results. The following formula is used to calculate the TLI( Σ ) value:
\(\text{TLI}\left(\Sigma\right)\ =\ \sum_{j\ =\ 1}^{m}{Wj\times TLI(j)}\)(1)
Where TLI( Σ ) represents the composite trophic level index, TLI (j) represents the trophic level index of the jth parameter, and Wj is the weight associated with the trophic level index of the jth parameter. Using Chl-a as the base parameter, the normalized correlation weight of the jth parameter is formulated as:
Wj = (2)
Where \(r_{\text{ij}}\) is the correlation coefficient between the jth parameter and the benchmark parameter Chl-a; m is the number of evaluation parameters. The correlation between Chl-a and other parameters in Chinese reservoirs \(r_{\text{ij}}\) and\(r_{\text{ij}}^{2}\) (Table 1).
The nutritional status indices for the five indicators were calculated as follows:
TLI (Chl-a) = 10 (2.5+1.086 ln Chl-a) (3)
TLI (TP) = 10 (9.436+1.624 ln TP) (4)
TLI (TN) = 10 (5.453+1.694 ln TN) (5)
TLI (SD) = 10 (5.118 − 1.94 ln SD) (6)
TLI (CODMn) = 10 (0.109+2.661 ln CODMn) (7)
Where: the unit of Chl-a is mg/m3, the unit of SD is m; and the units of other indicators are mg/L.
To illustrate the eutrophic status of the reservoir, a series of consecutive numbers from 0 to 100 was used to grade the trophic status (Table 2) (Li et al., 2021; Wu et al., 2017).