| Based on the long-term survey of macroinvertebrate communities in the Xiangxi River system, several researches were carried out: 1) The variation and spatial scales of macroinvertebrate communities caused by small hydropower plants (SHPs). 2) Impacts of the cascaded SHPs on the spatial patterns of the macroinvertebrate communities. 3) A temporal patterns of the macroinvertebrate communities under the impact of a typical SHP named Cangpinghe (CPH). 4) Construction of useful macroinvertebrate metrics for analyzing the Xiangxi river ecosystem health. 5) The impacts of small hydropower plants on macroinvertebrate habitat and the ecological water requirement of Xiangxi River. The main results are as follows:1. The ecological impact of SHPs on the spatial distribution of macroinvertebrate assemblages was investigated in three cascade SHPs of the Xiangxi River, and the variation caused by SHPs and the natural changes also were compared in this research. Four sites (site 1 is about 100 m upstream the dam, as the reference site, site 2 is just upstream the dam, site 3 locates about 50 m upstream the outlet, which is several kilometers downstream the dam, and site 4 locates just downstream the outlet.) in each SHP and three samples for each site were selected. 2121 macorinvertebrates were collected and mean dencity was 655 ind/m2. Five biotic indices (Ephemeroptera, Plecoptera and Trichoptera (EPT) richness, abundance, Shannon-Wiener index, evenness, and percentage of the dominant species) were analyzed by means of split plot experiment design ANOVAS across different stations, sites, and samples. Significant variation occurred mainly at the site scale. Station scale also has some influences and no significant differences in sample scale. Multi-Response Permutation Procedures (MRPP) also showed that compositions of the macroinvertebrate among four different sites varied significantly and no significant difference among the stations. Non-metric multidimensional scaling (NMDS) presented much clearer separation of communities among sites than among stations, and it also indicated that velocity, dissolved oxygen, and water temperature were the most important factors affecting the macroinvertebrate communities. This study highlights the fact that some necessary measures should be conducted to control the constructions of SHPs. The results also showes that improving the river hydraulic condition could lighten impacts of SHPs.2. The influence of SHPs on the spatial distribution of macroinvertebrate assemblages was investigated in five cascade SHPs of the Xiangxi River during October of 2005. Five sites (site 1 is about 100 m upstream the dam, as the reference site, site 2 is just upstream the dam, site 3 is just downstream the dam, which is a lentic pool, site 4 is about 50 m upstream the outlet and several kilometers downstream the dam, and site 5 is just downstream the outlet.) in each SHP was selected for sampling. A total of 4656 macroinvertebrates were collected and mean density was 658 ind/m2. The characteristics of macroinvertebrate community structures were analyzed by using richness, abundance, dominant species and functional feeding groups. The results suggested that the construction of the SHPs had no significant impact on the water chemistry, but physical variables (such as velocity and water depth) varied significantly among five sites. All the characteristics of the macroinvertebrate community were affected by the construction more or less, especially the density, filter-collecters (FC) percent and predators (PR) percent which were significantly affected by the stations. According to Hierarchical agglomerative clustering, site 3 was the most severely impacted site which had different taxa composition. Site 2, 3, and 5 had different abundance with site 1 and 4. The results suggested that the sites down of the dam had most different community structures, which meant that obstructing the water completely was harmful for the protection of the macroinvertebrate diversity of the river.3. Five sites in a typical plant CPH were set just like the sites in October 2005 and sampled for every month from December 2005 to May 2006 to explore the temporal variations of the macroinvertebrate communities under the impact of SHP. A total of 12756 benthic macroinvertebrate individuals, representing 13 orders 45 families and 85 genera, were recorded during the sampling time. Aquatic insects were the most dominant group and constituted about 91.8% of the total taxa. Baetis spp. (Ephemeroptera), Glossosoma sp. (Trichoptera) and Epeorus sp. (Ephemeroptera) were the main taxa and relative abundance were 48.6%, 8.2% and 6.5% respectively. According to the temporal variation, these three groups presented three kinds of temporal distributions. Baetis spp. expressed the highest abundance in January; Glossosoma sp. expressed the high abundance in the whole winter; Epeorus sp. evenly distributed in every month. The density and biomass of the macroinvertebrate communities were the highest in the January and then declined. For the functional feeding groups, gatherer-collectors (GC) were the dominant group. Filter-collectors (FC) and Predators (PR) kept constantly in different months. According to factorial ANOVA, both the temporal impact and the SHP impact were significant and the temporal impact was seemed more important for the variance of the macroinvertebrate communities. The Coefficient of Variation (CV%) of most community metrics showed a declined trend from January to May. The physi-chemical factors, discharge and precipitation varied according to the temporal variation and influenced the fluctuation of the macroinvertebrate communities, especially the change of the Coefficient of Variation.4. We selected 28 metrics belonged to richness, composition, tolerance and intolerance, functional feeding groups and habitat index of macroinvertebrates in the research area influenced by SHPs of the Xiangxi River as the candidate metrics. EPT taxa, Trichoptera%, Chironomidae%, BI value, Filterers%, Scrapers%, Gatherers%, Predators%, Clingers%, Evenness index were selected as the benthic metrics to assess the biological condition. All of these ten metrics had significant differences (one-way ANOVA, p<0.05) among five sites, which suggested that these metrics were able to distinguish the differences between reference sites and degraded sites. Pearson Correlations analysis also showed that there were low correlations among these metrics. It indicated that little redundant information was included in the further analysis. These metrics fluctuated according to physical metrics among the different sites. General regression analysis showed that they were negatively or positively related physical variables. All of these results confirmed that the metrics selected were suitable and useful for analyzing the impacts of the SHPs. Then the B-IBI was calibrated and constituted. According to the result, the biotic integrity of the Xiangxi river were not severely impacted by the SHPs. The biotic structure of 93.4% sampling sites were excellent or good and only 6.6% sampling sites were fair. There were significant differences among five sites, and site 1 and 4 were better then the other three sites.5. A typical small hydropower plant"Cangpinghe"was investigated monthly for habitat and the ecological water requirement of Xiangxi River. The weighted usable widths (WUW) were calculated for three dominant orders of the macroinvertebrate (Ephemeroptera, Plecoptera and Trichoptera) at each cross section and necessary comparisons were conducted among each cross section. The results showed that the divertion of the water current caused part of the downstream channel dried up and the WUW of the three orders of the macroinvertebrate declined significantly between the upstream channel and the downstream channel. According to the relationship between the WUW and the river discharge, it was calculated that the minimum ecological water requirement for the main group of the macroinvertebrate in Xiangxi River was 3.8 m~3/s.
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