Distribution and Release of Mercury Regulated by the Decomposition of a Pioneer Habitat-Adapted Plant in the Water-Level-Fluctuating Zone of the Three Gorges Reservoir

Pioneer habitat-adapted bermudagrass is prevalent in the water-level-fluctuating zone of the Three Gorges Reservoir area. This study was performed to explore the response characteristics of dissolved organic matter (DOM) qualities to bermudagrass decomposition and their regulation in the distribution and release of mercury (Hg) and methylmercury (MeHg) in the soil-water system. Compared to the control, the bermudagrass decomposition resulted in a great increase in the protein-like components in the water in the initial stages (p < 0.01), but it also greatly reduced the humification degree of water DOM (p < 0.01). However, it accelerated the consumption of protein-like components, the humification rate, and the synthesis of humic-like DOM in the water over time. This changing pattern of the DOM qualities resulted in an initial elevation and a subsequent great decrease in the dissolved Hg and MeHg concentrations in the pore water, which ultimately reduced their release levels into the overlying water by 26.50% and 54.42%, respectively, compared to the control. Our results indicate the potential inhibitory effects of short-term bermudagrass decomposition caused by flooding and how decomposition affects the release of total Hg and MeHg by shaping the DOM qualities, and they have implications for similar aquatic systems in which herbaceous plants are frequently decomposed after submergence.

Nutrient inputs from the leaf decay of Cynodon dactylon (L.) Pers in the water level fluctuation zone of a Three Gorges tributary

Cynodon dactylon (L.) Pers (C. dactylon) is one of the dominant plants in the water level fluctuation (WLF) zone of the Three Gorges Reservoir (TGR) tributaries. However, the leaves of C. dactylon can decay to increase the inputs of nutrients under flood inundation, increasing the risk of eutrophication in the TGR tributaries. Nutrient inputs from the leaf decay of C. dactylon in three interfaces, namely, water-sediment (WS), water-C. dactylon (WC) and water-sediment-C. dactylon (W-S-C), were estimated in a 180 d inundation experiment. The results showed that the kinetic processes of total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) input accorded with the power function equation: y = ax^b for the WS, WC and W-S-C interfaces (R²s > 0.72, p < 0.001). The cumulative TDN input from leaf decay of C. dactylon in the WC interface was 506.44 mg N kg^-1 of biomass, which was significantly higher than that in the W-S-C interface with 422.24 mg N kg^-1 of biomass (p < 0.05). However, no significant differences in TDP input were found between the WC and W-S-C interfaces (p > 0.05). The total amounts of TDN and TDP inputs at the 165-175 m altitude were 21,688.81 and 13,121.68 kg year^-1, respectively, which were approximately 3.17 times those from the 145-155 m altitude of the WLF zone. The amounts of TDN and TDP inputs from the leaves of C. dactylon for the whole WLF zone were 49,261.65 and 29,803.17 kg year^-1, respectively, which were 0.1 and 2.7 times the annual permissible discharge amount of pollutants calculated from a municipal wastewater treatment plant with the peak flow of 60,000 m³/d according to Class I (A) of the Wastewater Discharge Standard (GB18918-2002) in China. Thus, the aboveground part of this perennial herb should be harvested in a timely manner before reflooding, especially at the higher altitudes of the WLF zone to decrease eutrophication risk.

Influences of the alternation of wet-dry periods on the variability of chromophoric dissolved organic matter in the water level fluctuation zone of the Three Gorges Reservoir area, China

Dissolved organic matter (DOM) is a crucial driver of various biogeochemical processes in aquatic systems. Thus, many lakes and streams have been investigated in the past several decades. However, fewer studies have sought to understand the changes in DOM characteristics in the waters of the Three Gorges Reservoir (TGR) areas, which are the largest artificial reservoir areas in the world. Thus, a field investigation of dissolved organic carbon (DOC) concentrations and of chromophoric dissolved organic matter (CDOM) properties was conducted from 2013 to 2015 to track the spatial-temporal variability of DOM properties in the TGR areas. The results showed that the alternations of wet and dry periods due to hydrological management have a substantial effect on the quantity and quality of aquatic DOM in TGR areas. Increases in DOC concentrations in the wet period show an apparent "dilution effect" that decreases CDOM compounds with relatively lower aromaticity (i.e., SUVA₂₅₄) and molecular weight (i.e., S_R). In contrast to the obvious temporal variations of DOM, significant spatial variability was not observed in this study. Additionally, DOM showed more terrigenous characteristics in the dry period but weak terrigenous characteristics in the wet period. Furthermore, the positive correlation between SUVA₂₅₄ and CDOM suggests that the aromatic component controls the CDOM dynamics in TGR areas. The first attempt to investigate the DOM dynamics in TGR areas since the Three Gorges Dam was conducted in 2012, and the unique patterns of spatial-temporal variations in DOM that are highlighted in this study might provide a new insight for understanding the role of DOM in the fates of contaminants and may help in the further management of flow loads and water quality in the TGR area.

Dynamics of Rodent and Rodent-borne Disease during Construction of the Three Gorges Reservoir from 1997 to 2012

OBJECTIVE

To investigate the impact of impoundment and active public health interventions on rodent populations and rodent-borne diseases in the Three Gorges reservoir region from 1997 to 2012.

METHODS

Surveillance data from 1997 to 2012 were extracted from the Public Health Surveillance System of The Three Gorges established in 1997. Temporal changes in the incidences of hemorrhagic fever with renal syndrome (HFRS) and leptospirosis, rodent density, pathogen-carrying rates, and their correlations were analyzed.

RESULTS

The average indoor and outdoor rodent densities decreased overall from 1997 to 2012. The average densities decreased by 47.72% (from 4.38% to 2.29%) and 39.68% (from 4.41% to 2.66%), respectively, after impoundment (2003-2012) compared with before impoundment (1997-2002). The average annual incidence rates of HFRS and leptospirosis were 0.29/100,000 and 0.52/100,000, respectively, and decreased by 85.74% (from 0.68/100,000 to 0.10/100,000) and 95.73% (from 1.47/100,000 to 0.065/100,000), respectively, after impoundment compared with before impoundment. Incidences of HFRS and leptospirosis appear to be positively correlated with rodent density in the reservoir area.

CONCLUSION

This study demonstrated that rodent density and incidences of rodent-borne diseases decreased and were maintained at low levels during construction of the Three Gorges dam. Measures that reduce rodent population densities could be effective in controlling rodent-borne diseases during large-scale hydraulic engineering construction.