Research On Beta Diversity Of Fish Communities Inhabiting The River-Lake Connected Systems In The Middle-Lower Reach Of The Yangtze River

The shallow lakes in the middle-lower reach of the Yangtze River are connected with the Yangtze mainstream forming the unique river-lake connected systems.With the support of hydrologic regimes and lateral connectivity,aquatic organisms,including fish,migrate and communicate among different habitat patches,creating comprehensive freshwater ecosystems.In recent years,the construction of a large number of sluices projects seriously affects the lateral migration of aquatic organisms,which poses a certain threat to the structure and stability of freshwater ecosystems.As a high-trophic-level group of aquatic ecosystems,the diversity pattern of fishes can reflect the health status of freshwater ecosystems to some extent.And the life history of fish is highly dependent on water conditions,therefore the changes of water environments can make them response sensitively.Therefore,it is necessary to clarify the assemble mechanisms and spatiotemporal distribution patterns of fish communities under the influence of natural hydrologic regimes in the river-lake connected systems,so as to provide theoretical reference for the conservation of fish resources in the relevant areas.Measuring the beta diversity of community is one of the important methods to reveal the mechanisms of community assembling and spatiotemporal distribution.Beta diversity can be measured from both taxonomic and functional perspectives reflecting heterogeneous ecological processes.The classic calculation methods of beta diversity are based on incidence distributional matrix and its accuracy is recently questioned because of its rough processing of species data.The abundance-based calculation can distinguish the differences of species abundance,thus reflecting more accurate beta diversity information.The near-natural Yangtze-Caizi connected system was selected as the study area in this study.The taxonomic and functional beta diversity of fish metacommunities inhabiting the Yangtze-Caizi connected system were compared between high and low water level periods,and between lotic and lotic habitats.At the same time,the temporal changes in beta diversity of fish communities with declining water levels were calculated for lotic and lentic habitat respectively,further the core areas and key species that contributed more to the beta diversity pattern dynamics were analyzed.The main findings are as follows:1.During the high-water-level period(HWL),the beta diversity of fish communities in the study area was significantly lower than the beta diversity in the low-water-level period(LWL,P < 0.05),i.e.fish communities inhabiting the Yangtze-Caizi connected system showed homogenized composition and spatial distribution during the HWL.In addition,the diversity of fish beta in lotic habitats was significantly higher than that of lentic habitats during both water level periods(P < 0.05),reflecting that the lotic habitats could accommodate more heterogeneous fish communities.2.The temporal dynamics of fish beta diversity in the Yangtze-Caizi connected system are mainly caused by the loss of species(Dloss > Dgain),and the loss of fish species in lotic habitats is significantly higher than that in lentic habitat patches(P < 0.05).During the period of LWL,some migratory fish species returned to the mainstream of the Yangtze river and some lacustrine fish species migrated to the open water area in the center parts of the lake.In LWL period,the hydrologic conditions of lotic habitat patches were stressful than that of lentic habitat,which may result in a large decrease of fish abundance in the lotic habitat.3.The fish composition in the lotic habitats is significantly unique than that of lentic habitats during the LWL(LCBDLotic > LCBDLentic,P < 0.05),showing that the lotic habitats as key areas to witness the exchange of fish migrations is of greater relative importance in maintaining the higher fish beta diversity patterns.During the LWL,the riverine fishes such as yellow catfishes(Tachysurus fulvidraco),and gudgeons(Saurogobio gymnocheilus)are less important to the beta diversity pattern,while the relative importance of widespread lake residents such as grass carps(Culter alburnus),spotted steeds(Hemibarbus maculatus),and carps(Carassius auratus)increased in both habitats.4.Functional beta diversity has a higher sensitivity compared to the classical taxonomic one in revealing imperceptible changes.Compared with the incidence-based beta diversity,the abundance-based beta diversity possesses more information and applicability in small-scale spatial extents where communities differ mostly in abundance ranks rather than compositions.