Influences Of Low-head Dams On Local Habitat，Fish Diversity And Their Assemblage Structures In The Headwater Streams Of The Qingyi Watershed

Dams have been recognized as one of the primary means by which humansalter fluvial ecosystems, including their structures, functions and compositions.Dams may impact stream fishes by diverse pathways, such as directly blocking fishpassage, altering or modifying local habitat, and creating favorable conditions fornon-indigenous species which will impact indigenous fishes via competition and/orpredation. Qualifying and quantifying how dams affect stream fish diversity andtheir assemblage structures are important for the conservation, restoration andmanagement of stream ecosystems and their fish species diversity. However, by acomparison with those of large dams, the ecological effects of low-head dams andtheir influences on stream fish assemblages have received less attention, whichsuggests that how low-head dams affect local habitat and fish assemblages is stillunknown. In this study, within the headwater streams of the Qingyi watershed (atributary of the lower reach of the Yangtze River), three low-head dams wereselected and a total of11sampling sites were set, representing four types ofsampling sites (i.e., two reference sites far from dams upstream and downstream,respectively, and two treatment sites immediately upstream and downstream oflow-head dams, respectively). Based on the data collected seasonally in thesesampling sites during May, August, November2010and February2011, weexamined the influence of low-head dams on local habitat, fish diversity and theirassemblage structures, assessed the relations between local habitat and fishassemblages, revealed the pattern of how low-head dams affect fish assemblagesand species diversity. Our main results were as follows:1. The influence of low-head dams on local habitat. A total of eight habitatvariables were surveyed during each sampling, including wetted width, water depth,current velocity, water temperature, dissolved oxygen, conductivity, pH, andsubstrate size. The result from Pearson’s correlation analysis showed that somesignificant correlations were observed between local habitat variables (P<0.05),such as wetted width positively correlating with water depth, but negativelycorrelating with current velocity and substrate size; water temperature correlatingwith conductivity (positively) and pH (negatively); and current velocity negatively correlating with substrate. In terms of the results from two-way ANOVA testing theeffects of spatial and temporal factors on local habitat variables, significantseasonal variations were observed for three habitat variables (i.e., current velocity,water temperature and pH)(P<0.05), and wetted width, water depth and currentvelocity varied significantly across sampling sites (P<0.05). But no significantinteraction effect on habitat variable was observed for site category and season(P>0.05). Compared with the two reference sites, the two treatment sites showedsignificantly slower water flow (P<0.05). The two treatment sites had significantlyhigher water depth than did the sites far from dams upstream (P<0.05), but notsignificantly different from those downstream (P>0.05). In addition, the wettedwidths of the two reference sites were higher than those of the impounding areasupstream (P<0.05), not those of the plunging areas downstream (P>0.05).2. The influence of low-head dams on fish diversity. During each sampling,fishes were collected using back-electrofishing, identified to species level, and thenrecorded of species composition, richness and abundance. A total of1640specimens were collected in this study, representing17species,17genus,9families and4orders. Species of family Cyprinidae comprised, on average,41.2%of total species collected. Zacco platypus, Ctenogobius sp. and Acrossocheilusfasciatus were the most common and abundant species. Results from two-wayANOVA testing the influences of spatial and temporal factors on fish speciesrichness, abundance and density showed that fish abundance varied significantlynot only across site categories but also across seasons (P<0.05), while speciesrichness only showed significant variation across seasons and density showedsignificant variation across site categories (P<0.05). But no significant interactioneffect on fish species richness, abundance, or density was observed for sitecategory and season (P>0.05). By a comparison with the two reference sites, thetwo treatment sites showed significantly lower fish abundance and density(P<0.05). Based on the stepwise linear regression models testing the effects ofenvironmental variables on fish diversity, both fish abundance and density weresignificantly influenced by only wetted width (P<0.05), while species richnesswere determined by two habitat variables, i.e., wetted width and water temperature(P<0.05). Wetted width had negative correlation with fish species richness,abundance and density (P<0.05), but water temperature was positively related withspecies richness (P<0.05).3. The influence of low-head dams on fish assemblage structures. Usingtwo-way crossed ANOSIM to test both spatial and temporal factors on fish assemblage structures, it was showed that both site category and season did notsignificantly influence fish assemblages, and obvious overlap was observed for fishassemblages across spatial and temporal factors (Global R<0.25; P<0.05). However,when comparing the fish assemblage season by season and site by site, despite thatfish assemblages did not vary significantly across seasons (R<0.25, P>0.05),significant difference in fish assemblages was observed between the impoundingareas and the sites far from dams upstream (R=0.27, P<0.05). The SIMPERprocedure providing information on overall assemblage and species-specificcontributions to the difference between assemblage groups suggested that theimpounding areas showed more lentic species (such as mud loach, top-mouthgudgeon and rosy bitterling) and less lotic species (such as pale chub, goby, flat-finloach, mud loach, rare spotted loach and slippy-lip barbell) than did the sites farfrom dams upstream. In addition, after the Monte Carlo tests in CCA, wetted width,water depth, current velocity and substrate were selected as the habitat variablessignificantly influencing fish assemblages.