Utility of Trophic State Index in lakes and reservoirs in the Chinese Eastern Plains ecoregion: The key role of water depth

The Trophic State Index (TSI), based on chlorophylla [TSI(Chla)], Secchi depth [TSI(SD)], total nitrogen [TSI(TN)], and total phosphorus [TSI(TP)], is a popular tool for lake eutrophication assessment worldwide, higher TSI scores implicitly indicate increased intensity of algal blooms. However, the application of TSI across diverse lakes may be questionable since the precondition of SD, TN, and TP as indicators of algal blooms lies in the fact that SD is dominated by algal turbidity; Chla in all lakes respond the same (or very similarly) to changes in TN (or TP) concentrations. Herein, 120 natural lakes and artificial reservoirs with a wide range of morphological, physicochemical, and Chla variables in the Chinese Eastern Plains ecoregion were collected via data mining to elaborate the utility of TSI on a regional scale. Moreover, we analyzed the source of uncertainties of TSI(SD), TSI(TN), and TSI(TP) in indicating algal biomass. Our results demonstrated that the TSI(SD), TSI(TN), and TSI(TP) scores were different with those of the TSI(Chla) (approximately 10 scores deviations). Considering 10 scores represent a doubling in algal biomass, the direct use of TSI may cause the misestimation of algal blooms intensity and under- or over-protection of the lakes and reservoirs. Furthermore, our results revealed water depth (WD, m) is the key inner-regional factor affecting the accuracy of TSI(SD), TSI(TN), and TSI(TP) in indicating algal biomass. For one, WD was closely related to mass ratio of nitrogen to phosphorus (N:P) and non-algal light attenuation (NALA, m−1) in our study lakes and reservoirs (lnNALA = -0.70*lnWD + 1.02, P < 0.001, R2 = 0.59; lnN:P = 0.61*lnWD + 1.88, P < 0.001, R2 = 0.50). For another, Linear relationships of [TSI(Chla)-TSI(SD)] vs NALA (negative, P < 0.001), [TSI(Chla)-TSI(TN)] vs lnN:P (negative, P < 0.001), and [TSI(Chla)-TSI(TP)] vs lnN:P (positive, P < 0.001) were also found. Based on the above equations and results of algal limiting factors, we recommended that TSI(SD) and TSI(TP) should not be used in very shallow lakes (WD < 2 m), and TSI(TN) should be excluded from the eutrophication assessment if WD > 5 m in our study area. On a broader scale, empirical TN (or TP), Chla, and SD relationships of lakes in other ecoregions may also be affected by WD and other unmeasured factors (e.g., zooplankton grazing), the direct use of TSI may be problematical. Thus, this study provides essential information for developing accurate lake eutrophication assessments in both ours and other ecoregions.