Influences Of Low-Head Dams On Local Fish Assemblages In Headwater Streams Of The Qingyi River

Low-head dam is one of the most common and typical anthropogenic activities in river systems.Recent research has revealed that low-head dams may modify local habitat conditions and alter local fish assemblages.However,to our knowledge,the patterns of how fish assemblages in impounding upstream and plunging downstream areas vary spatially and temporally are not clear.In addition,how fish assemblages in the impounding and plunging areas relate to local habitat and dam characteristics has gotten less attention.In this study,based on the data collected in 39 impounding and 39 plunging areas of low-head dams within the headwater streams of the Qingyi River during July and November 2014,we examined the pattern of how fish assemblages varied seasonally and between the two habitat types and related them to local habitat and dam characteristics,respectively.In addition,using the data from the plunging areas as references,we identified whether the impoundments created by low-head dams result in the homogenization of habitat and fish assemblages.Our main results were as follows:1.Temporal and spatial patterns in fish assemblages within the impounding upstream and plunging downstream areas of low-head dams(1)Fish numeric characteristics: Two-way ANOVA showed that both fish species richness and abundance significantly varied between the impounding and plunging areas(P<0.05),not between July and November(P>0.05).Paired t-test showed that fish species richness and abundance were less in impoundments than that in plunging areas(P<0.05).After stepwise linear regression,we found that local fish species richness were affected jointly by local habitat(e.g.,elevation and water temperature)and dam characteristics(e.g.,dam length).Futher,the results of enter regression showed that local habitat was more important in influencing fish species richness that dam characteristics.(2)Fish assemblage structures: Two-way crossed ANOSIM showed that assemblage structures significantly varied seasonally and spatially(P<0.05).The spatial difference in fish assemblages were resulted from the abundance variations of some species,such as Ctenogobius sp,Zacco platypus,Rhodeus ocellatus,Odontobutis obscura;these fishes were more abundant in plunging areas than in impounding areas.According to the results of RDA,when we used a forward selection procedure to select the environmental variables significantly affecting fish assemblages,we found that fish assemblages in both impounding and plunging areas significantly related to local habitat(e.g.,water temperature,wetted depth,elevation)and dam dimensions(e.g.,dam number upstream and downstream).Further,when we used a automatic procedure to assess how all environmental variables measured affect fish assemblages,we observed that local habitat features were more important in affecting fish assemblages than dam dimensions(with an except of fish assemblages collected in plunging areas during July).2.Influence of low-head dams on the homogenization in local habitat and fish assemblages(1)Habitat homogenization: PCoA showed distinct variation in habitat conditions between impounding and plunging areas,which was supported by PCA showing significant habitat difference between the two habitat types.Further,PERMDISP showed that habitat variation in impounding areas were significantly lower than that in plunging areas.Compared with habitat conditions in plunging areas,obvious habitat homogenization was observed in impounding areas.(2)Fish assemblage homogenization/heterogenization: Using fish assemblages in plunging areas as reference,we examined,at between-tributary and within-tributary scale,whether low-head dams induce fish assemblage homogenization in impounding areas.According to the variation of Jaccard's coefficient of assemblage similarity(?CSJ),we found that assemblage both homogenization(?CSJ>0)and heterogenization(?CSJ<0)occurred in these impoundments.?CSJ showed significantly negative correlation with "initial" similarity coefficient.Homogenization processes occurred between some impounding areas initially showing relatively low similarity in fish assemblages and heterogenization processes were observed between those initially showing relatively high similarity.