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Zhu G.-W.,CAS Nanjing Institute of Geography and Limnology | Zhao L.-L.,Hohai University | Chen W.-M.,CAS Nanjing Institute of Geography and Limnology | Xie C.-G.,CAS Nanjing Institute of Geography and Limnology | And 2 more authors.
Journal of Ecology and Rural Environment | Year: 2011

Effects of low water level on water quality and ecology of the Tianmuhu Reservoir, Liyang, China were evaluated by means of high-frequency in situ monitoring of the water quality and plankton in the reservoir during the period from September 2008 till August 2009, when the water level of the reservoir was lowered down by 1.73 m below the mean water level of the years 2004 - 2008 for reinforcement. It was found that the reservoir weakened its buffering capacity of incoming exogenous nutrients in that year of low water level operation. Peaks of TN concentration was closely related to heavy rainfall. One early spring heavy shower (Feb. 17-27, 2009) immediately led to a sharp rise of the ρ(TN) of waterbody from 0.92 to 2.09 mg • L-1, which returned to normal only after the end of the summer storm season. TP concentration in the waterbody was mainly affected by the biomass of plankton and the content of suspended substances therein and little by the low water level. In the summer of 2009, the proportion of Cyanophyta among the phytoplankton was obviously higher than that in the summer of 2008, suggesting that the entire alga community structure tended towards that under eutrophication. Compared with that in June August, 2008, the mean abundance of protozoa was 1730 L-1 and of rotaria 4188 L-1, in June August, 2009, almost doubled, that of copepod decreased by 21% but of cladocera fluctuated sharply. Moreover, the Chl-a nutrition index of the waterbody increased. In general, low water level makes the whole ecological system more fragile, and the fundamental elements of the ecosystem, such as zooplankton, phytoplankton and nutrients, fluctuate more sharply, and the waterbody more susceptible to changes in environmental conditions. It is, therefore, suggested that it is essential to keep a suitable water level in management of reservoirs in terms of water quality and ecology.

He R.-R.,CAS Nanjing Institute of Geography and Limnology | He R.-R.,Bengbu College | Luo L.-C.,CAS Nanjing Institute of Geography and Limnology | Zhu G.-W.,CAS Nanjing Institute of Geography and Limnology | And 3 more authors.
Journal of Ecology and Rural Environment | Year: 2010

Dissolved oxygen (DO) is one of the key factors affecting aquatic ecosystem and changes with water temperature. A study of spatial-temporal distribution of dissolved oxygen in Tianmuhu, a reservoir 12 km2 in water surface and 14 meters in depth (maximum) , was conducted during 2006 and 2007. It was found that in non-summer seasons the increase in water temperature caused reduction of the concentration and saturation of DO in the surface water layer, but in summer it led to increase in DO concentration and likely to over-saturation of DO due to the activities of plankton. In the middle and bottom water layers, temperature stratification is the key factor affecting DO concentration. When there was a thermocline in the water body, the bottom water was in hypoxia, and when thermocline disappeared, DO restored in the bottom water. As a result, the bottom water followed the alternation of hypoxia-DO restoration-hypoxia in a year. The hypoxia of the bottom water layer in summer caused release of ammonia nitrogen from sediments into water, thus leading to rise of the ammonia concentration in the surface water. The investigation and analysis of ammonia nitrogen concentration in the water body before and after dredging demonstrate that dredging of bottom sediments could effectively reduce the release of endogenous nitrogen in summer, thus mitigating the impact of the variation of DO concentration in the water on the release of nitrogen from sediments.

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