Chengxin Fan

Heavy metal pollution in sediments is a common environmental issue in small- and medium-sized reservoirs not only in China but also worldwide; however, few interpretations of the pollution pattern exist. Based on the analyses of accumulation characteristics, ecological risks, and source apportionments of eight heavy metals (As, Cd, Cr, Cu, Hg, Pb, Ni, and Zn) in sediments, we derived a paradigm to describe the pollution pattern of heavy metals in sediments of a typical small- and medium-sized Tongjiqiao Reservoir. The results showed high levels of Cd, Hg, and As pollutants in the surface and upper sediment layers of the pre-dam area. Additionally, As, Cd, Hg, and Pb pollutants peaked in the middle layers of the inflow area, indicating a high ecological risk in these areas. The positive matrix factorization results implied that industrial, agricultural, and transportation activities were the main sources of heavy metals. The heavy metal pollution pattern exhibited three distinct stages: low contamination, rapid pollution, and pollution control. This pattern explains the heavy metal pollution process in the sediments and will provide scientific guidance for realizing the green and sustainable operation and development of the reservoir.

Sulfur development in water-sediment systems is closely related to eutrophication and harmful algae blooms (HABs). However, the development of sulfur in water-sediment systems during heavy algae accumulation still remains unclear, especially in hyper-eutrophic shallow lakes. In this study, a quarterly field investigation was carried out for a year in the algae accumulated embay area of Lake Taihu, accompanied by a short-term laboratory experiment on algae accumulation. The results show that hydrogen sulfide and methanethiol dominated the volatile sulfur compounds (VSCs) in the water during non-accumulation seasons, whereas the concentrations of dimethyl sulfides increased during heavy algae accumulation, both in the field and the laboratory. An increase in the acid volatile sulfide (AVS) in the surface sediments was also discovered together with the increase in dimethyl sulfides. The depletion of oxygen in the overlying water and sediment–water interface during the heavy algae accumulation and decomposition was found to be closely related to both the increase in VSCs in the overlying water and increase in AVS in the sediment. The increased concentrations of these reductive sulfocompounds might aggravate the eutrophication and HABs and should be given more consideration in future eutrophication control plans for lakes.

The influence of dredging season on sediment properties and nutrient fluxes across the sediment–water interface remains unknown. This study collected sediment cores from two sites with different pollution levels in Meiliang Bay, Taihu Lake (China). The samples were used in simulation experiments designed to elucidated the effects of dredging on internal loading in different seasons. The results showed that dredging the upper 30 cm of sediment could effectively reduce the contents of organic matter, total nitrogen, and total phosphorus in the sediments. Total biological activity in the dredged sediment was weaker (p < 0.05) than in the undredged sediment in all seasons for both the Inner Bay and Outer Bay, but the effect of 30-cm dredging on sediment oxygen demand was negligible. Dredging had a significant controlling effect on phosphorus release in both the Inner Bay and Outer Bay, and soluble reactive phosphorus (SRP) fluxes from the dredged cores were generally lower (p < 0.05) than from the undredged cores. In contrast, NH₄⁺-N fluxes from the dredged cores were significantly higher (p < 0.05) than from the undredged cores in all seasons for both sites, this indicates short-term risk of NH₄⁺-N release after dredging, and this risk is greatest in seasons with higher temperatures, especially for the Inner Bay. Dredging had a limited effect on NO₂⁻-N and NO₃⁻-N fluxes at both sites. These results suggest that dredging could be a useful approach for decreasing internal loading in Taihu Lake, and that the seasons with low temperature (non-growing season) are suitable for performing dredging projects.

Algae‐induced black bloom, a kind of black water phenomenon in some severe eutrophic lake areas, is characterized by a black color and offensive odor and is one of the most serious environmental problems in certain eutrophic shallow freshwater lakes in China. Ferrous iron (Fe²⁺), soluble inorganic sulfides (ΣS²⁻, ΣS²⁻ = S²⁻ + HS⁻ + H₂S), and dissolved oxygen (DO) in the overlying water are presumed to be directly related to the formation of black blooms. In this study, the algae‐induced black bloom in Lake Taihu, China, is simulated in the laboratory by using a large‐scale lake process simulation apparatus. Changes in the characteristics of Fe²⁺, ΣS²⁻, DO, pH, and oxidation reduction potential (ORP) are investigated during the entire black bloom formation period. Results show that black blooms occurr in water columns with high Fe²⁺ and ΣS²⁻, but not in water columns with high Fe²⁺ and low ΣS²⁻, or low Fe²⁺ and ΣS²⁻. During the formation of black bloom, Fe²⁺ increases quickly as DO decreases but starts to decrease before the outbreak of black bloom. ΣS²⁻ concentrations only increase sharply 12 h before the outbreak. Both Fe²⁺ and ΣS²⁻, affected by oxic and redox conditions, respectively, contribute to the formation of black bloom. However, ΣS²⁻ is confirmed to be the limiting factor directly controlling the outbreak of the black bloom.

Because of global concerns regarding pollution and eutrophication in fresh water, China’s Taihu Lake has gained attention both for these issues and as a source of nitrous oxide (N₂O) emissions. In this study, we investigated N₂O fluxes and nitrification and denitrification rates at the sediment–water interface and analyzed monthly the relationships between these processes in different areas of Taihu Lake over a one-year period. Annual maximum nitrification and denitrification rate and N₂O flux were observed during June in an algae-dominated area of the lake and measured 17.80, 235.51, and 31.49 µmol N m⁻² h⁻¹, respectively. The nitrification rate ranged from 0 to 1.18 µmol N m⁻² h⁻¹ at other sampling sites, with less variation. The denitrification rate showed clear seasonal variation, with lower levels between August and January (0.01–8.57 µmol N m⁻² h⁻¹; average = 1.49 µmol N m⁻² h⁻¹) and a rapid increase between February and July (1.03–235.51 µmol N m⁻² h⁻¹; average = 41.73 µmol N m⁻² h⁻¹).The N₂O flux ranged from −0.64 to 1.5 µmol N m⁻² h⁻¹, with little variability except for a much higher rate (31.49 µmol N m⁻² h⁻¹) in June in algae-dominated areas. N₂O flux was significantly positively correlated with nitrification and denitrification rates in most lake zones. By comparing the slopes of the regression equations, we found that N₂O emissions from the sediment–water interface were influenced predominantly by nitrification, suggesting that lower N₂O fluxes from the sediment–water interface in Taihu Lake are caused primarily by lower nitrification rates.

In eutrophic lake restorations, in situ capping is an often considered method to control sediment internal phosphorus (P) pollution for mitigating eutrophication status. Subsequent aquatic macrophyte revegetation can directly derive P from the sediment for growth. However, the effects of capping with clean soils on internal P release from sediments under rooted aquatic macrophyte revegetation are still unclear. In the present study, the influences of sediment P remobilization by P. australis revegetation on P inactivation by capping were investigated based on an entire growth simulation study. Our findings showed during the growth of P. australis, tests conducted on total phosphorous (TP), calcium-bound P (Ca-P), loosely bound P (loose-P), organic P (Org-P), and iron-adsorbed P (Fe-P) found significant changes (p < 0.001). Specifically, the mean contents of TP and Ca-P decreased by 291.1 and 224.2 mg kg⁻¹, respectively, while those of Fe-P increased from 26.4 to 124.8 mg kg⁻¹. In addition, sediment mobile-P contents increased coincidentally with the growth of P. australis during the whole course of experiment. Further analysis indicated calculated diffusion fluxes of soluble reactive phosphorus (SRP) generally increased with incubation time, although capping effectively induced the reduction of SRP concentration in pore water and its release to waters. Therefore, sediment P remobilization by P. australis revegetation was able to enhance P lability in lake sediments, with intermediate activation ability compared to other correlated water bodies. This phenomenon was most likely attributed to solubilization of sediment P by organic acids secreted from P. australis rhizosphere. Overall, sediment P remobilization by rooted macrophytes is unfavorable for capping to control internal P release to water column during eutrophic lake restorations.