| 其他摘要 | Aquatic environment of a river basin is not only the fundamental for all ecological processes, but also the necessary condition of water resources bringing their functions into play. Due to the rapid development of social economy, the natural properties and processes of river have been impacting by human activities, of which, dam construction for river regulation is thought to be the most notable and important. After dammed, reservoir becomes the main component of basin (river) landscape structure. Previous studies on reservoir paid much attention on the changes in the hydrological status, silt deposition, fish migration, nutrients retention, greenhouse gas emission before and after impounding of river system. Moreover, most studies were carried out only in an independent reservoir or several independent reservoirs in different drainage basins. However, water environment evolution of the cumulative river-reservoir system after cascaded exploitation is still lacking of understanding, and the research concerning pentaintiong to the carbon biogeochemical cycle in single reservoir is lacking also.
Carbon, as an indispensable element for all life, is closely related to the biogeochemical cycle of other important elements. The feedback between aquatic biological activities and the change of aquatic environment and the relationship between the aquatic system and nutrient loads, as well as the process of mutual influence would provide the basic information about the change of aquatic environment. Biogeochemical cycle of nutrients in reservoirs, the transfer of energy, the dynamics of CO2 and the nutrition status are the key factors affecting the aquatic environment.
Six typical cascaded reservoirs sited in middle-upper reaches of Wujiang River which winds across south-western China were investigated and the main line of this work is the carbon cycle of the river-reservoir system. Water samples were collected in April, July and October 2006 and January 2007.The DOC, POC, and DIC concentrations, as well as stable carbon isotopic composition were analyzed for all the samples, and the concentrations of TN, TP, Chla, as well as the amounts and types of algae were also analyzed in October, 2006. The effects of cascaded exploitation on the carbon cycle in rivers were discussed in detail and the following conclusions were made in this study. These conclusions could be useful to understand and evaluate the impacts of hydropower exploitation on aquatic environment.
1 The impact of cascade damming on basic hydrochemical characters: The water chemistry is mainly characterized as CaHCO3-CaCO3 group Ⅱin this study area. With the rising of the water level in reservoir, the water chemistry in the reservoir shows seasonal features. The temperature of the water column becomes stratified in spring, and stops stratifying in winter. And the temperature and pH value of the water go down because the effects of the reservoir processes. Thus，there exists a big difference of water chemistries between upper and down reaches. Na+,K+,Mg2+,Cl- and SO42- was absorbed or reserved, while Ca2+,HCO3- and NO3- become more during the process passing through reservoir.
2 The reservoir effects on biology: Among studied reservoirs, the major algae in Hongjiadu Reservoir, Yinzidu Reservoir, and Suofengying Reservoir is Green Algae, an index of middle nutrition stage, while for Puding Reservoir and Dongfeng Reservoir, there are more amounts of Diatom，belonging to mild eutrophication stage, and for Wujiangdu Reservoir，major algae is Cyanobacteria, indicating an eutrophication status. A positive correlation can be found between the reservoir age and its nutrition status,i.e. the longer the reservoir impounding time is, the higher the reservoir nutrition status will be. So, the succession of the reservoir primary productivity structure has important influences on the water chemical composition.
3 The reservoir effects on carbon cycle: The concentration of DIC rises and DOC, POC concentration down during the reservoir process. The cascaded reservoirs in the middle and upper Wujiang River make the DIC export rise 22.18%, the DOC and POC export fall down 18.19% and 70.09%. For a brief estimate, the annual export flux of DIC, DOC and POC though Wujiangdu Reservoir to the downstream is 263.64 kt, 12.40 kt and13.86 kt, respectively. The cascaded reservoirs are the source of DIC, and the sink of DOC and POC in the case of runoff equilibrium between the input and output flow, in river-reservoir system.
As a whole, the cascade reservoirs in the study area are the DIC source and the of the lower reach river. In the vertical profile, DIC concentration increases downward, while DOC and POC show a reverse trend. However, DOC and POC contents in bottom water are higher because of the re-suspending of sediment. The carbon isotopic composition of DIC (δ13CDIC) has a good response to reservoir process. δ13CDIC value in the outflow is lower than in the inflow and the in surface water of reservoir, and it is lower in the lower reach reservoir than in the upper reach reservoir. In the water column of reservoir, δ13CDIC decreases with depth and δ13CPOC value has less obvious changes. Hence, δ13CDIC could be a useful tool to trace the response of the water environment to the reservoir reaction processes, while δ13CPOC is not a good parameter to trace that response for its complex sources.
4 River damming and the emission of Green House Gas: Generally, the partial pressure of CO2 （pCO2）in inflow water is lower than that in outflow water, suggesting that the downstream river becomes a source of CO2 to atmosphere. In addition, the pCO2 values increase downward in the water column of reservoir. As the outflow is usually from the hypolimnion of reservoir, a higher pCO2 in outflow can be observed accordingly. In this study we estimated that the CO2 emission flux is 6.51 times that of the reservoir surface, in average. So the degassing of CO2 in outflow water can not be ignored when we study the impact of the reservoir processes on green house gases. |
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