|Other Abstract||Since the impoundment of Three Gorges Reservoir – a world-famous hydroelectric project, water depth in the outlet of Xiangxi River has increased from about 20 m to 70 m, water current velocity has decreased from 0.43-0.92 m/s to 0.0020-0.0041 m/s, and the lower reach of this river has been in lentic condition, which now is called as Xiangxi Bay. In this Bay I used zooplankton as bioindicator to explore the impact of large-scale hydraulic engineering (Three Gorges Reservoir) on aquatic environment, in terms of zooplankton species composition, their temporal and spatial distribution, diel vertical distribution and migration and their response to phytoplankton bloom. As the upstream of this river is still a lotic habitat, I also carried out the ecological research of riverine zooplankton. The followings are the main results obtained from this research.
1. From July, 2003 to June, 2005, investigations of temporal and spatial distributions of rotifer species were carried out in Xiangxi Bay. Here, 13 sampling sites (including two transects) were selected. 81 rotifer species were recorded, belonging to 15 families and 29 genera. There were 14 dominant species, which were Polyarthra vulgaris, Keratella cochlearis, Keratella valga, Synchaeta tremula, Synchaeta stylata, Trichocerca lophoessa, Trichocerca pusilla, Brachionus angularis, Brachionus calyciflorus, Brachionus forficula forficula, Ascomorpha ovalis, Conochilus unicornis, Ploesoma truncatum and Anuraeopsis fissa. During the first year of impoundment, rotifer community was dominated by ten species, and one year later it was dominated by eight species; the co-dominant species between the two years were only four. Percent similarity index (PSC) of the same months and the same sites between the two years were low; but PSC of the adjacent months in the second year were higher than those in the first year. This indicated that during the study period rotifer community in Xiangxi Bay was dissimilar, and the community in the second year was more stable and more similar than that in the first year. In spatial distribution, rotifer exhibited patchiness distribution. Significant heterogeneity along the longitudinal axis was observed. However, there was no transverse heterogeneity of rotifer community in Xiangxi Bay. PSC, density and Shannon-Wiener index of rotifers in each transect (left, middle and right) were not statistically different.
2. I investigated the diel vertical migrations (DVM) and distributions of rotifers in summer and autumn of 2004, in winter and spring of 2005, in Xiangxi Bay. Water temperature, pH, conductivity, and phytoplankton were closely related to rotifer vertical distribution, while dissolved oxygen had no relationship with the vertical distribution of rotifers. The species composition and population density of rotifers changed significantly between seasons. However, rotifer vertical distributions among seasons were similar. They aggregated at specific depths in the water column. All the rotifer species inhabited the upper strata (0.5-5 m). Generally, the rotifers did not display DVM except for Polyarthra vulgaris (in summer), which performed reverse migration. The reason that rotifers did not perform DVM may be explained by high density of food resources and the low abundance of competitors and predators at the upper strata.
3. Monthly investigation of rotifers indicated that their temporal dynamics were significant in Xiangxi Bay; and the succession of dominant species was the main cause. Inasmuch as the reproductive time of rotifers is very short (about several days) the monthly interval of sampling frequence was too crude to detect the mechanism of temporal dynamics. And at the same period of time, in Xiangxi Bay severe phytoplankton bloom occurred in spring. Rotifers are the main grazer of phytoplankton. Therefore, from 22 February to 28 April, 2005, I made a daily vertical investigation of rotifers in Xiangxi Bay. The results indicated that the vertical distributions of environment were even; there was less change in the temporal dynamics. However, the vertical distributions of phytoplankton (Chl a) and rotifers were aggregated in the special strata. During the initial period of the sampling date rotifer donimated the zooplankton community and rotifer density increased sharply after the bloom of phytoplankton. In April dominant species of phytoplankton changed from small size (thus edible to rotifers) to large size (thus inconsumable to rotifers) and large crustaceans increased significantly in the water column, both of factors causing the decrease of rotifer proportion.
4. From February 2006 to January 2007, monthly samples were taken in the upstream Xiangxi River (lotic habitat). The results indicated that rotifers dominated the zooplankton community. The dominant species of rotifers were Colurella colurus, Colurella uncinata, Polyarthra vulgaris, Phiodina erythrophthalma, Rotaria tardigrada and Keratella cochlearis. The dominant species of cladoceran was Bosmina longirostris. The dominant taxa of copepod were Cyclopidae and various nauplia. The density of zooplankton in the upstream Xiangxi River was low, the max value was about 60 ind. /L and the mean value was about 10 ind. /L, which was far lower than that in the Xiangxi Bay. The spatial distributions of richness, Shannon-Wiener index and densities of zooplankton were significant and their temporal dynamics were also significant. Generally, the downstream had more zooplankton than the upstream because of having more niches.
5. Xiangxi River is abundant in water resource. Many small hydropower plants were built along its reach. From November 2005 to June 2006, monthly sampling were made in a special reach (of Xiangxi River) affected by a small hydropower plant. The results of our study indicate that: 1) zooplankton richness in Xiangxi River was high (about 56 taxa), but the density is low; the temporal dynamics of zooplankton were significant. 2) the existence of the intake dam (of the plant) had significant effect on zooplankton community. The outlet (of the plant) also had significant effect on zooplankton but to a less extent. In long time drought or dry season the effect of the plant was more intense. But the downfall or the connectivity of channel can attenuate the effect of the plant. Zooplankton in Xiangxi River can reflect the impact of a small in-channel dam quickly. It was a good indicator of the effect of small dam on aquatic organism.
6. Although it is well recognized that huge impounded reservoir had impact on riverine zooplankton, the extent of this impact is still not well known. Quantitative study about the impact of huge impounded reservoir on riverine zooplankton is still scarce. In this dissertation I quantificationally examined the impact of impounded reservoir on aquatic environment by the bioindicator of zooplankton and contrasted the difference of zooplankton between lentic habitat and lotic habitat. The results indicated that some lotic species such as Lecane hamata and L. bulla disappeared but the lentic species for instance Synchaeta stylata and Ploesoma truncatum arosed because of the impoudment of the reservoir. There was significant difference on zooplankton richness, PSC similarity, Shannon-Wiener index, evenness and density between the lotic zone and the lentic zone. Evidentally a huge impounded project has severe impact on aquatic environment and zooplankton.|