3.1 Numerical indication of water quality parameters
Based on the annual mean WT values (Fig. S1a), Huanglishu Reservoir (19.03°C) was higher than Shahe Reservoir (18.51°C) and Chengxi Reservoir (18.61°C). All three reservoirs exhibited weakly alkaline environments (Fig. S1b), with Chengxi Reservoir reaching the highest pH value of 8.28, followed by Shahe Reservoir at 8.14, and lastly, Huanglishu Reservoir at 7.98. SD values ranged from 0.40 to 1.79 m (Shahe Reservoir), 0.40 to 1.50 m (Huanglishu Reservoir), and 0.42 to 1.50 m (Chengxi Reservoir), with relatively significant differences (Fig. S1c). DO concentrations were high overall (Fig. S1d), particularly in Chengxi Reservoir (9.26 mg/L), then Shahe Reservoir (9.00 mg/L), and finally Huanglishu Reservoir (8.88 mg/L). The range of CODMn concentration changes for the three reservoirs was markedly greater in 2020 than in 2019 and 2021 (Fig. S1e). BOD concentrations in Shahe Reservoir and Chengxi Reservoir showed a gradual increase from 2019 to 2021, while Huanglishu Reservoir was not characterized by this change (Fig. S1f). NH3-N concentrations displayed the biggest range of fluctuations during 2019, which ranged from 0.04 to 0.34 mg/L (Shahe Reservoir), 0.04 to 0.40 mg/L (Huanglishu Reservoir), and 0.04 to 0.39 mg/L (Chengxi Reservoir), respectively, with relatively small ranges of fluctuations in the remaining two years (Fig. S1g). TN concentrations in Chengxi Reservoir decreased annually throughout the study period, whereas the other two reservoirs did not present this feature (Fig. S1h). TP concentrations varied within a narrow range, ranging from 0.01 to 0.05 mg/L in Shahe Reservoir, 0.02 to 0.05 mg/L in Huanglishu Reservoir, and 0.02 to 0.04 mg/L in Chengxi Reservoir (Fig. S1i). Chl-a concentrations were dramatically higher in Shahe Reservoir and Huanglishu Reservoir (4.33 mg/m3 and 4.11 mg/m3) than in Chengxi Reservoir (3.19 mg/m3) during the study period (Fig. S1j).
Based on the monthly average WT values, it was found that three reservoirs showed a changing pattern of high summer (June - August) and low winter (December - February) water temperatures (Fig. S2a). The pH varied more significantly at different times but exceeded 8 most of the time (Fig. S2b). SD changes were not regular and showed instability (Fig. S2c). For example, the maximum mean for both Shahe Reservoir (1.79 m) and Chengxi Reservoir (1.50 m) occurs in April, yet Huanglishu Reservoir presents the minimum monthly mean (0.42 m) in this month. DO concentrations showed a general downward trend from February to October. In addition, the DO levels in Chengxi Reservoir were higher than in the other two reservoirs for a prolonged period (Fig. S2d). CODMn concentrations in all three reservoirs are characterized by an overall increase followed by a decrease (Fig. S2e). BOD concentrations in Shahe Reservoir and Huanglishu Reservoir peaked at 2.6 mg/L and 3.0 mg/L, respectively, in September, while Chengxi Reservoir peaked at 2.7 mg/L in August of each year (Fig. S2f). Similar to SD, there was no regularity in the variation of NH3-N concentrations, with Shahe Reservoir and Huanglishu Reservoirs, exhibiting the lowest concentrations in multiple months (Fig. S2g). Regarding the variation of TN concentrations, Shahe Reservoir displayed the maximum average concentration (0.87 mg/L) in October that was higher than that of Huanglishu Reservoir and Chengxi Reservoir (0.78 mg/L and 0.78 mg/L), and Chengxi Reservoir presented the minimum average concentration (0.39 mg/L) in May that was lower than that of Shahe Reservoir and Huanglishu Reservoir (0.54 mg/L and 0.43 mg/L) (Fig. S2h). As for TP, Shahe Reservoir was higher than the other two reservoirs for almost the whole year time, followed by Huanglishu Reservoir, while Chengxi Reservoir was consistently the lowest (Fig. S2i). The Chl-a concentration in Chengxi Reservoir was the lowest nearly all year, with only May and November showing anomalies that were higher than the other two reservoirs (Fig. S2j).