Effects of organic carbon burial on biomarker component changes in contamination in northeast Dianchi watershed.

The upper reaches of the Yangtze River have experienced increasing anthropogenic stress. Quantitative tracing of carbon (C) sources and ecological risks through biomarkers i.e., polycyclic aromatic hydrocarbons (PAHs) and n-alkanes is significant for C neutrality and sequestration. Here, source and sink patterns, and factors influencing C burial and biomarker components in a small catchment of Dianchi Lake were explored. The sediment core covered the period 1855-2019. Before 1945, the organic C accumulation rate (OCAR) ranged from 0.71 to 5.12 mg cm-2 yr-1, and the PAHs and n-alkanes fluxes were 106.99-616.09 ng cm-2 yr-1 and 5.56-31.37 µg cm-2 yr-1. During 1945-2005, the OCAR, PAH, and n-alkane burial rapidly increased from 3.19 to 16.17 mg cm-2 yr-1, 230.40 to 2538.81 ng cm-2 yr-1, and 11.63 to 61.90 µg cm-2 yr-1. During 1855-2019, deposition fluxes of PAHs and n-alkanes increased 13.01 and 9.14 times, resulting in increased C burial, driven by environmental changes. A PMF model and the diagnostic ratio indicated that PAHs from coal combustion and traffic emission increased from 22.32% to 65.20% during 1855-2019. The PAH concentrations reflected normal-moderate contamination and potential risks to the aquatic environment. The results facilitate a comprehensive understanding of anthropogenic-driven interactions between increasing OC burial and ecological risks.

Evaluation and prediction of water quality in the dammed estuaries and rivers of Taihu Lake.

Proper evaluation of water quality is pertinent to estuarine habitat restoration. Identifying the degrading factors of the water environment and predicting the trend of eutrophication are key to restore the habitat. Through trophic level index (TLI), water quality index (WQI), modified Nemerow pollution index (NPI), and the Random Forest (RF) model, water samples collected from various estuaries of Taihu Lake from 2017 to 2019 were evaluated. To predict the water quality development, four scenarios were set viz. S1: add or remove an ecological buffer, S2: increase or reduce the external nutrients, S3: open or close the dam/gate, and S4: increase or decrease the internal release. In Wuli Lake, the nutrient concentrations in the river regions were higher than in the lake regions, while a contrary trend was observed in Gonghu Bay. The estuarine water quality in the dry season (WQI = 40.91, NPI = 1.73) was merely worse than that in the wet season (WQI = 47.27, NPI = 1.67). On the other hand, the eutrophic status in the wet season (TLIWet = 57.93) was worse than that in the dry season (TLIDry = 57.23). The estuarine water quality of Taihu Lake has improved from 2017 to 2019 but still belongs to medium level. The principal component analysis (PCA) revealed that dam construction, land use types, unstable hydrodynamic conditions, and trumpet-shaped estuary were the main factors that aggravated the water quality degradation. The RF model has strong forecasting capabilities for estuarine water quality. When the estuaries are close to residential and industrial districts, controlling the surface runoff and improving sewage treatment efficiency are the most effective measures to improve the water quality. In the estuaries, the sediments are usually disturbed by the wind-waves. Conclusively, reducing sediment disturbance and internal contamination accumulation via biological and engineering measures is the key to estuarine restoration.

Response of the nitrogen load and its driving forces in estuarine water to dam construction in Taihu Lake, China.

To regulate the water level and minimize the occurrence of water eutrophication in shallow lakes, dams and gates are often constructed in rivers. However, this practice may result in a deterioration of water quality in some estuaries. In the present study, using the correction of Nemerow pollution index (CNPI) and a redundancy analysis (RDA), water samples from different dammed rivers around Taihu Lake were compared to assess the pollution risk and identify the factors responsible for water eutrophication. The average total nitrogen (TN), total phosphorus (TP), total organic carbon (TOC) concentrations, and chemical oxygen demand (CODMn) were 2.45 ± 2.28, 0.08 ± 0.06, 43.01 ± 18.75, and 10.78 ± 4.86 mg L-1, respectively. The CNPI values indicated that approximately 76.47% of the estuarine water was moderately polluted (1 < CNPI < 7.28). A positive correlation was observed between dam construction and nutrient concentrations (e.g., rTN = 0.38, p < 0.05; rTP = 0.89, p < 0.01). Under the effects of dam construction, land use change, estuary shape, and meteorological conditions, there was a clear spatial variation of the TN concentrations. Dams that were closed all year round accelerated the TN accumulation in the water around them. The pollution risk in a trumpet-shaped estuary was higher than that in other regions (t = 2.92, p = 0.02). Endogenous release of pollutants was an important factor that may have a priming effect on algal blooms and should be given more attention. In Wuli Lake, exogenous pollution was the dominant pollutant source. A total of 74.49% of the nitrogen losses with the runoff into the estuarine water in 2018 were derived from urban domestic sewage and constructed land, with the load being 4.40 times higher than in 2000. The RDA results revealed that dam construction was the main factor (43.70%) affecting water quality, while meteorological conditions, land use types, estuary shape, and other factors contributed 56.30%. Scientific regulation and control of dam operation is important to protect the water environment of Taihu Lake.