The Characteristics Of The Fish Community Structure In Lakes From Middle And Lower Reaches Of The Yangtze River And The Impacts Of River-lake Disconnection

Many lakes connected with the main stream spreading all over the middle and lower reaches of the Yangtze River historically,and such a river-lake complex ecosystem holds a unique and diverse biota.Since the 1950s,many lakes have been artificially gated for flood-control,agricultural,or water-supply purposes,thus leading to partial to complete disconnection from the Yangtze River.Such river-lake disconnection led to remarkable biodiversity decline of aquatic organisms in these lakes,especially fish assemblages.In order to evaluate how the human-mediated loss of river-lake connectivity affected multifaceted diversity(taxonomic,functional,and phylogenetic)of fish assemblages,fish assemblages of 46 large Yangtze River floodplain lakes during the last 70 years were analyzed in the present study.This study can enhance our ability to detect and evaluate the impacts of floodplain disconnection on biodiversity,which is essential to effective biodiversity assessment and conservation in large river floodplains.The main conclusions are as follows:1.Fish species of lakes in the middle and lower reaches of the Yangtze River have declined since the 1950s,and disconnection is one of the main causes.A total of 155 fish species had been recorded and described from 46 lakes in the middle and lower reaches of the Yangtze River since the 1950s,comprising 11 orders,25 families,and 85 genera.The number in orders,families,and genus showed declined both in connected lakes and disconnected lakes last decades.Moreover,the taxa number of disconnected lakes(Liangzi Lake,Honghu Lake,and Hongze Lake)was significantly lower than connected lakes(P<0.001).Species abundance of lake resident,riverine,river-lake migration,and migratory fishes all showed decline in connected lakes,especially the migratory fishes in the disconnected lake declined obviously.2.Taxonomic diversity of lakes fish experienced a significant decrease last decades,especially in disconnected lakes.Average taxonomic distinctness(Δ~+)in disconnected lakes(average values 74.02±3.093)was significantly lower than connected lakes(average values78.31±0.98;P<0.001).On the contrary,variation in taxonomic distinctness(Λ~+)of disconnected lakes(average values 736.89±33.80)were significantly higher than connected lakes(average values 697.31±25.53;P=0.002).The analysis of temporal variation showed that species richness andΔ~+generally declined,andΛ~+generally increased within representative connected and disconnected lakes.However,Λ~+increased more obvious in disconnected lakes.This means that river-lake disconnection on lake ecosystems led to the decline of taxonomic diversity,and fish species were more concentrated in some taxa resulting in high unevenness and low stability in the community.3.The phylogenetic community structure analysis reveals the complex dynamics of fish fauna of lakes in the middle and lower reaches of the Yangtze River,and river–lake disconnection on lake ecosystems results in reducing stability of phylogenetic community structure.Among the 9 connected lakes,the Net Relatedness Index(NRI)and Nearest Taxon Index(NTI)of 3 lakes show the aggregation of phylogenetic community structure,and the environmental filtering effect played a key role in community assembly.The NRI and NTI of the 6 lakes showed divergent phylogenetic community structure,and the effects of competitive exclusion determined community assembly.Among the 43 disconnected lakes,the NRI and NTI of 13 lakes showed phylogenetic aggregation,and the environmental filtering effect played a leading role.The NRI and NTI of 17 lakes showed phylogenetic divergence,and the effects of competitive exclusion determined community assembly.The NRI and NTI of 13 lakes showed opposite results.It shows that the complexity of the community construction mechanism may be affected by other factors such as lake area,physical and chemical environment,and other tributaries.Temporal changes analysis showed the community structure of disconnected lakes is more unstable and further illustrates the complexity of the fish community construction mechanism,which requires a joint analysis of multiple influencing factors.4.Functional diversity analysis showed that temporal reductions in functional richness(FRic)was found both in connected and disconnected lakes,and the functional characteristics tended to be homogenized in disconnected lakes.Analysis of the temporal changes in the composition of functional traits of fish communities showed that the percentages of herbivorous and planktivorous fishes increased in connected and disconnected lakes,while the proportion of piscivorous fish decreased.The changes were more obvious in disconnected lakes.The percentages of herbivorous(HERB)and planktivorous(PLAN)fish in the disconnected lakes was significantly higher than(P<0.05)connected lakes,and the percentages of piscivorous(PISC)fish in the disconnected lakes was significantly lower than(P<0.05)the connected lakes.It indicated that river–lake disconnection on lake ecosystems has caused homogenization of fish functional characteristics.The functional richness(FRic)of the disconnected lakes was significantly lower(P<0.001)than connected lakes.The functional richness index(FRic)of connected lakes(Poyang Lake and Dongting Lake)showed a downward trend with time,but the functional dispersion index(FDis)showed a downward trend initial stage and then increased.The functional richness index(FRic)of the disconnected lakes(Honghu and Hongze Lake)showed a downward trend over time,and the functional dispersion index(FDis)and functional dispersion index(FDiv)of Liangzi Lake,Honghu,and Hongze Lake decreased over time as a whole.It implied that fish assemblages of disconnected lakes held a relatively low level of functional redundancy and were vulnerable to human disturbances.5.In general,river-lake connectivity plays an important ecological role in fish diversity and community stability,while river-lake disconnection on lake ecosystems is one of the main causes of decline in fish diversity,and reducing the stability of fish communities.Based on our results,we suggested to restore the fish diversity in the middle and lower reaches of the Yangtze River by recovering the connection between the lakes and the Yangtze mainstream,improving the quality of fish habitat through water environment management,and adjusting the fish community structure by artificial enhancement different types of fish species.