Xiaowen Lin

The underwater light environment is important for lake ecosystem function and structure. In the summer of 2023, the Yangtze River basin suffered an extreme drought, and Poyang Lake experienced its lowest water level since 1951. However, there remains a gap in understanding the impact of extreme drought on the underwater light environment. In this study, the underwater light environment in Poyang Lake was quantitatively investigated in late July 2023, and its influencing factors were explored. The results showed that the underwater light environment in Poyang Lake attenuated most drastically within the 0–0.4 m depth range, where the underwater spectrum changes significantly, especially in the visible light range (400–700 nm). With increasing depth, the proportion of red and green light increased, whereas that of blue light decreased rapidly. However, in regions with high Secchi depth (SD), even at 1.2 m depth, blue light still accounted for 2.03–9.15%, forming a distinct underwater light environment. In other regions with low SD and high suspended solids (SS), blue light attenuated severely and red light became dominat. Although the red-to-blue light ratio varied across Poyang Lake, its maximum values were consistently distributed in the west. Attenuation of blue, green, and red light, as well as photosynthetically active radiation, in the four lake regions followed the order: northern > western > southern > eastern, with blue light attenuating the most rapidly. In contrast to previous findings, SD was the key factor driving changes in the underwater light environment of Poyang Lake, followed by SS. Future efforts should prioritize enhanced monitoring of SD and the underwater light environment in Poyang Lake.

Understanding changes in the distribution and coverage of aquatic vegetation (AV) is of great significance for the restoration of lake ecosystems. In this study, the vegetation and bloom indices (VBI) algorithm were used to interpret submerged aquatic vegetation (SAV), floating/emergent aquatic vegetation (FEAV), and algal bloom (AB). The dynamics of AV and their influencing factors in Bao’ an Lake, in the middle reaches of the Yangtze River in China, were studied from 2000 to 2023. The results showed that (1) the VBI algorithm can accurately distinguish AV and AB of different life forms with an overall accuracy of 93% and a kappa coefficient of 0.86. (2) Macrophyte coverage decreases. AV grew vigorously in spring, and SAV was the dominant type within it, whereas AV coverage was low in summer, and SAV had no summer species for a long time. In 2000, the coverage of AV was the highest, reaching 64.5%, but a gradual decrease that followed in the coming years finally led to a coverage percentage of less than 5% by 2023. (3) The correlation between SAV coverage and total phosphorus (p < 0.01), total nitrogen (p < 0.05), and water depth/transparency (p < 0.05) in Bao’ an Lake were 0.23, 0.28, and 0.32, respectively. (4) The SAV species experienced three stages: richness (before 2003), monotonicity (2004–2020), and final disappearance (2021–present). This study shows that the coverage of AV in Bao’ an Lake is too low and the number of SAV species is one (2010–now). Therefore, it is necessary to implement measures to improve vegetation coverage and diversity.

Nutrients, organic matter (OM), and heavy metals (HMs) in lake sediments are critical elements contributing to water pollution. In April 2019, surface sediments from Wanghu Lake were collected, and the nutrient, organic matter, and heavy metal content of the sediments were determined. We mainly evaluated the sediment pollutants through four evaluation methods to assess pollution and provide a reference for pollution control in Wanghu Lake. The results indicated that the averages of total phosphorus (TP) and total nitrogen (TN) were (1045.74 ± 190.17) mg/kg, (945.27 ± 203.56) mg/kg; most of them showed serious pollution and moderate pollution, respectively. OM was (32.31 ± 5.11) g/kg. Among them, TP and OM in the northwestern Wanghu Lake were significantly higher than those in the eastern lake (p < 0.05). It shows that nutrients are greatly affected by historical aquaculture and urban human activities. TP was the most serious in the center of the lake, and the source of pollution was mainly the historical deposition. The average of Cd, Cr, Cu, Ni, Pb, Zn, Hg, and As in the sediments were 2.15, 1.09, 1.93, 1.37, 1.28, 1.49, 2.60, 1.77 times that of the soil background values of Hubei Province, respectively. Hg and Cd were the main factors contributing to the surface sediments, with levels at considerable and moderate risks, respectively.

Based on a field investigation conducted in May 2019, the spatial distribution characteristics of dissolved organic matter (DOM) in sediments of Wuli Lake in the Taihu Lake basin were analyzed using ultraviolet-visible (UV-Vis) absorption spectroscopy and three-dimensional fluorescence spectroscopy. The results showed that: (1) The overall water quality of West Wuli Lake was significantly better than that of East Wuli Lake. The water transparency, measured in Secchi depth (SD), of West Wuli Lake was higher than that of East Wuli Lake, while the suspended solids (SS), total nitrogen (TN), total phosphorus (TP) and chlorophyll a (Chl-a) were higher in East Wuli Lake than in West Wuli Lake. (2) The spatial distribution of a(350), M and SR values of DOM in surface sediment in Wuli Lake differed; a(350) values were highest in the northwestern part of Wuli Lake, while M and SR values were highest in the eastern part. The mean value of SR in Wuli Lake was 2.12 ± 0.67, indicating that the DOM contained more authigenic humus; the mean M value was less than 3.5, indicating that the proportion of humic acid was higher than that of fulvic acid. (3) The ranges of the biogenic index (BIX), the humic index (HIX) and the fluorescence index (FI) of surface sediments in Wuli Lake were 0.64~0.85, 3.79~4.84 and 1.72~1.84, respectively, indicating that surface sediments in Wuli Lake were characterized by authigenic sources, and the humic components of DOM in sediments were characterized by a dual contribution from terrestrial and endogenous sources.