The Effect And Mechanism Of Environmental Gradients On The Distribution Of Fish Assemblage In Cascade Reservoirs Of The Lancang River

Cascade hydropower development is a primary threat to fish diversity because it changes the aquatic environment and causes habitat fragmentations.Previous studies of the dam impact on fish have majorly focused on the whole basin scale or reach scale.However,the environmental heterogeneity among different water areas within cascade reservoirs and its impact on fish distribution were largely ignored.In this study,we divided the Dahuaqiao-Miaowei reservoir into four zones(i.e.,river section below the Dahuaqiao dam,upper section of the Miaowei reservoir,middle section of the Miaowei reservoir and the section in front of the Miaowei dam)based on their environmental gradients.We investigated the fish resources in these four zones using the traditional sampling and environmental DNA technique(e DNA),revealed fish distribution patterns from the perspective of species composition and functional characteristics,and identified key drivers for these patterns.The main results are as follows:A total of 24 fish species were obtained by the traditional sampling and e DNA techniques.Eighteen fish species,most of them were Cobitidae,were captured by traditional method.And 11 fish species,most of them were Cyprinidae,were identified via e DNA metabarcoding.Five fish species were identified through both traditional method and e DNA techniques,and an additional 13 and 6 species were detected via only traditional method and e DNA techniques,respectively.The differences between the two methods were mainly influenced by the biological characteristics of fish and the design of primers.In terms of fish resources,we captured 1,143 fish individuals.Among them,the number of Triplophysa tenure was the largest,followed by Abbotina rivularis,Glyptothorax zanaensis,Pseudorasbora parva and Schizothorax lissolabiatus.Operational Taxonomic Unit(OTU)abundance of Elteobagrus fulvidraco was highest(1223),followed by Rhodeus sinensis(913),Pseudorasbora parva(170)and Schizothorax lissolabiatus(98).Correlation analysis found a significant correlation between fish biomass and OTU abundance of the five fish species,which were obtained by both traditional methods and e DNA techniques.This finding indicates that OTU abundance data obtained by e DNA techniques can reflect fish biomass.Therefore,the fish data obtained from traditional methods and e DNA techniques were used for subsequent analyses on species composition,functional characteristics and functional diversity.In terms of the distribution of fish species and functional traits,only seven fish species were found in the river section below the Dahuaqiao dam,where had the least number of fish species and samples among the four zones,and S.lissolabiatus and G.zanaensis were the dominant species.In this area,fish body size was larger than other zones and the environment was more conducive to the survival of strong-swimming fish.In the upper section of the Miaowei reservoir where the number of fish species and individuals were the highest compared with other zones,and 22 fish species were obtained.A.rivularis,T.stenura and S.lissolabiatus were the dominant species.The number of T.stenura individuals accounted for about 50% of all fish samples in this area.The fish functional traits were diverse in the middle section of the Miaowei reservoir since 20 fish species were found there.A.rivularis and T.stenura were the dominant species.In front of the Miaowei dam,we obtained 13 fish species where P.parva,Micropercops swinhonis and A.rivularis are the dominant species.The fish functional traits were similar in the upper and middle sections of the Miaowei reservoir where most of the fish were of small body size and benthopelagic.Through the redundancy analysis(RDA),we concluded that the main environmental factors affecting the distribution of fish species and functional traits in different zones of the reservoir area were the flow velocity,nitrate nitrogen,ammonia nitrogen,plankton density and permanganate index.The results of fish taxonomic and functional ?,? diversity demonstrated that the Shannon-Wiener index(1.96)and the Margalef index(2.84)in the middle section of the Miaowei reservoir were higher than other zones,while the Pielou index in this area was0.71.In the river section below the Dahuaqiao dam,the Pielou index was the highest(0.89),while the Shannon-Wiener index(0.97)and the Margalef index(0.51)were the lowest.Functional ? diversity in the upper section of the Miaowei reservoir was the highest,while that in the river section below the Dahuaqiao dam was the lowest.The species and functional ? diversity among different zones were 0.33(±0.14)and 0.26(±0.24),and nestedness was the dominant component of ? diversity,which were 0.29(±0.16)and 0.25(±0.25),respectively.The main environmental factors affecting species diversity were the flow velocity,total nitrogen,total phosphorus and p H,while functional diversity was mainly affected by flow velocity,total nitrogen,total phosphorus,dissolved oxygen and plankton density.In conclusion,cascade hydropower development in the Lancang River has resulted in environmental heterogeneity within the reservoir area.Correspondingly,fish also showed a certain distribution pattern in the reservoir area.The number and biomass of fish species in the upper section of the Miaowei reservoir were high,which is highly related to the environmental characteristics of this confluence area of still water and flowing water.In the middle section of the Miaowei reservoir and the area in front of the Miaowei dam,the water area was large and the nutrient level was high.The number and biomass of fish species were high in these two zones,and fishes were mainly lentic species.The rapid flow velocity and high p H value in the river section below the Dahuaqiao dam caused the lowest number of fish species and the dominance of lotic fishes.This study preliminarily confirms the feasibility and advantages of combining traditional sampling method and e DNA techniques in the investigation of fish resources.In addition,it also reveals distribution patterns of fish under the environmental gradient formed by cascade reservoirs from the taxonomic and functional levels,which provides a reliable scientific basis for the planning of priority fish conservation areas and the management of fishery resources.