4.2 Reservoir water quality eutrophication studies
Through evaluating the TLIs of five parameters of water quality in three reservoirs in Chuzhou City (Figs. S6 to S9), the results demonstrated that the dominant factors causing eutrophication in the three reservoirs were SD, TN, and TP. The mechanism of its occurrence is mainly when the water body is enriched with excessive N, P nutrient salts, algae will rapidly multiply and form water bloom, which hinders the penetration of light, thus leading to the eutrophication of the water body. Meanwhile, TN and TP have historically been recognized as the main drivers of eutrophication (Havens and Walker, 2002; Sui et al., 2022), this is also consistent with the findings of this study. In summary, water quality in the three reservoirs is generally poor, and the TLI (Σ) does not show a sustained downward trend (Figs. S4 and S5). Consequently, it is still imperative to continuously assess the eutrophication of the three reservoirs in Chuzhou City in the future.
The TLI (SD) was the highest in the evaluation of the trophic status of the reservoir water quality (Fig. S6). This is due to Chuzhou City belonging to the subtropical monsoon climate, long-term seasonal winds will be around the reservoir area of sediment, rocks plant debris, etc., blown into the reservoir, resulting in a gradual accumulation of suspended particles in the water, the refraction effect becomes stronger and stronger, and ultimately impede the penetration of light, greatly improving the TLI (SD) (Saluja and Garg, 2017). Moreover, the pigmented substances secreted by some microorganisms and aquatic plants in reservoirs can also affect water transparency and change the TLI (SD), such as chlorophyll and carotenoids from algae (Qi et al., 2014). The three reservoirs had higher TLI (TN) and TLI (TP) in 2019 than in 2020 and 2021 (Fig. S6), which was primarily attributed to human activities. Throughout nearly all of 2019, industry, agriculture, and commerce have functioned normally in Chuzhou City, with large population movements, giving rise to high levels of eutrophication in the reservoir. However, in 2020 and 2021, when China is experiencing the severity of COVID-19, many businesses have ceased operations and population activity has decreased dramatically, resulting in a downward trend in the nutrient indicators (TN and TP) in water bodies. Among the reservoirs, the TLI (TN) of Shahe Reservoir was abnormal in 2021, mainly because of the better management of the epidemic in the area around the reservoir and the return of population activities to the pre-epidemic period (Liu et al., 2021), thus the eutrophication of the reservoir was again aggravated.
Although the values of TLI (CODMn) and TLI (Chl-a) were relatively low, it does not mean that CODMn and Chl-a do not contribute to the occurrence of eutrophication in the reservoir. Algal overgrowth triggered by CODMn pollution and water blooms caused by high Chl-a concentrations can greatly help improve the reservoir trophic state (Du et al., 2019). In monthly variation, the TLI (CODMn) in all three reservoirs reached a maximum around September and a minimum around April (Figs. S7 to S9), it is primarily related to the storage and drainage functions of the Chuzhou City Reservoir. CODMn is a commonly used indicator of water pollution by organic matter (Li et al., 2018). Reservoir impoundment in September increases the release of organic pollutants from inundated soils and plants, intensifying the nutrient levels in the water body, whereas reservoir dewatering in April causes the opposite result (Zhao et al., 2013). It is well known that Chl-a is an essential indicator for characterizing algae biovolume and measuring the degree of eutrophication. (Li et al., 2021; Saluja and Garg, 2017; Sui et al., 2022). The TLI (Chl-a) from May to September was higher than that of other months (Figs. S7 to S9), mainly because the summer temperature is suitable for phytoplankton growth and reproduction, which promotes the metabolic rate and cell division of phytoplankton, increases the productivity of phytoplankton, and aggravates the eutrophication of the water body. Of these, the anomalies in the Shahe Reservoir and Chengxi Reservoir in November can only be attributed to the input of N and P nutrients and pollutants that were far greater in that month than in other months, which promoted algal growth and led to a rapid increase in Chl-a concentrations.
From the CA results, it can be seen that there is a cluster corresponding to the winter low-flow period in all three reservoirs (Fig. S3), and the TLI(j) of the five parameters is higher during this period than the time corresponding to the other two clusters, which strongly suggests that low precipitation is an important reason for the high probability of eutrophication in the reservoirs. That’s because low precipitation during this period can lead to slow flow and poor exchange in the reservoir, which is extremely likely to contribute to algal growth and thus increase the nutrient status of the water body. (Liu et al., 2021).