| Zooplankton,as the important link in the food chain of aquatic ecosystems,with the characteristics of small size,large quantity,strong metabolism and short generation cycle,is sensitive to the environmental variation and thus is a group of effective indicator organisms in studying the ecological quality of aquatic environment.However,biodiversity formation mechanism and its response to human activities are still unclear.Then it is of great significance to study zooplankton community ecology for the stability and sustainable development of water ecosystems.Huaihe is one of the seven major river basins in China which is an agricultural production base and the national important transportation corridor.In recent years,human activities have caused a large area of water pollution,and the deterioration of river hydrological conditions and habitats are seriously threatening the health and function of river ecosystems.In our study,zooplankton samples were collected at 217 sampling sites in the Huaihe River basin during the intermediate season,150 of them were repeatedly collected during the wet season,and 47 samples in the Yinghe river were repeatedly collected during the dry season,with simultaneous determination of physicochemical factors.Our study tried to explore how river connectivity,directionality,dam barriers and water environment determine the community structure of zooplankton assemblage in the wet and dry seasons.And the effects of human disturbance,water environment,spatial dispersal limitation and biological factors on the formation of riverine zooplankton community were further discussed from the perspective of functional traits and phylogenetic structure.The main findings are as follows:1)A total of 321 species(including subspecies)of zooplankton were recorded in the Huaihe river basin,including 269 rotifers,14 copepods and 38 cladocera.Of all taxa,Trichocerca,Lecane,Brachionus,Lepadella,and Cephalodella species richness were high,accounting for 44%of the total species.The average abundance of zooplankton was 4480 ind./L,of which rotifers accounted for more than 90%.Polyarthra sp.was the most dominant species in all water systems.The abundance of zooplankton(4913 ±812 ind./L)and community dissimilarity index(0.75)in the intermediate season were higher than those in the wet season(4047±509 ind./L,0.69).2)Zooplankton communities in the main stream,old path of the Yellow River and the 5 major river systems were quite different.The accumulative species richness of zooplankton in Yishusi and Yinghe rivers were obviously higher than that in old path of the Yellow River(respectively 118,108 and 68 species),with the lowest in the main stream.In terms of abundance,it was the highest in the Yishusi river system(7742 ind./L),followed by old path of the Yellow river(5793 ind./L),and with the lowest in the Yinghe river(1372 ind./L).The species richness(112 species)and abundance(7308 ind./L)of the tributary systems in the middle reaches were about twice as much as those in the upper and lower reaches.Polyarthra sp.was dominant in all water systems,and was the most abundant in the main stream,Yinghe river and upper,middle and lower reaches of the river system.Keratella cochlearis was the most dominant species in the Yishusi river system,and Brachionus angularis was the most dominant in old path of the Yellow river.3)The species richness and abundance of zooplankton increased first and then decreased with the rise of the river order(from 1st to 6th river order,more and more pollutants from tributaries flow into the river,and river width and water flux become larger and larger)and comprehensive water quality type(class ?-inferior ?).The species richness and abundance were highest in the 2nd or 3rd river order(165/157 taxa and 5069/4048 ind./L in the wet/intermediate season),and lowest in the 6th river order(respectively 22/13 species and 900/708 ind./L in the wet/intermediate season).Species richness of rotifera and crustacea were highest in the water quality class ? and ?.Zooplankton recorded in our study were divided into four groups according to the saprobity degree:oligosaprobity,o-beta mesosaprobity,beta-mesosaprobity and beta-alpha mesosaprobity.The species richness and abundance of the first two groups were mostly concentrated in class? and ?,and those of the latter two groups were more abundant in class ?,? and above.4)The abundance of zooplankton showed an inverted U-shaped distribution with the increase of farmland or impervious surface area proportion in the riparian zone.The predominant species were classified according to the above saprobity degree.With the increase of the farmland proportion,the abundance of some groups showed a U-shaped distribution,eg.the oligosaprobity group(Trichocercapusilla and Anuraeopsis fissa),and the o-beta mesosaprobity group(Synchaeta monostyla).While some other groups showed an inverted U-shaped distribution,eg.the beta mesosaprobity group(Polyarthra vulgaris)and the beta-alpha mesosaprobity group(Brachionus angularis and Filinia longisela).The dominant species were classified according to saprobity degree but no regularity was found along the impervious surface proportion gradient.The possible reason was that the distribution of dominant species along this gradient were not mainly affected by the concentration of pollutants,but by other factors such as temperature(heat island effect).5)Cluster analysis was used to divide the zooplankton communities in Yinghe river into 4 groups respectively in the wet and dry seasons,and the community characteristics of each group were obvious.Redundancy analysis and classification regression tree analysis showed that DO was the key environmental factor determining the composition of zooplankton in the wet season,followed by transparency and current velocity.Water temperature was the main influencing factor in the dry season,followed by pH and total nitrogen.In addition,riparian land use was also an important factor,especially the proportions of impervious surface and farmland,which were an important discriminant variable to classify the community types in the two seasons.Further variation partitioning analysis showed that the effect of riparian land use on the species composition of zooplankton was significant in the wet season,with a single explanation rate of 35.4%community variation,and an interactive explanation rate of 10.2%with water environmental factors.While in the dry season,the effect of the land use was much weak.This reflects that land use in riparian zone not only reflects the quality of water environment in the wet season,but also reflects the river hydrological conditions.6)River connectivity,directionality and barrier effects played a significant role in structuring zooplankton metacommunity.The effect of river connectivity was significant in the dry season,with a single interpretation rate of 7.3%,and most of the explanation rate interacted with directionality(17.6%).Directionality was more important in the wet season,with a single explanation rate of 27.6%.Barrier effects were significant in the dry season,with a single explanation rate of 11.2%.Further,considering the influence of water environment on community assemblages,it was found that the single explanation rate(13.6%)in the dry season was much higher than that of the spatial factors(niche theory),while in the wet season,the variation explained by the environment was less than that of spatial factors(neutral theory).For different zooplankton functional groups,spatial factors had a more significant effect on the rotifera communities with smaller size and weaker motility,and the single explanation rates reached 14.3%and 4.7%respectively in directionality and connectivity during the wet and dry seasons.While for crustaceans with larger size and stronger motility,the environment played a key role in explaining the community variation,with an explanation rate of about 40%in the wet season,but in the dry season,the variations explained by both spatial and environmental factors were low.7)25 functional traits of zooplankton showed significant phylogenetic signals(p<0.01),that is,the closer the species were,the more similar the functional traits were.With the increase of human disturbance intensity(the proportions of impervious surface and farmland in the riparian zone),the phylogenetic spacing of zooplankton community increased first and then decreased(p<0.05),that means the phylogenetic structure of zooplankton community dispersed first and then gathered along the disturbance gradient.With the increase of the impervious surface proportion,the functional structure also presented the trend of dispersing first and then gathering,but with the increase of farmland proportion,it presented the trend of dispersing.Spatial factors dominated in the formation of phylogenetic structure and functional structure in the wet season,while environmental factors dominated in the dry season.Interaction between organisms(competition)had little effect on the two structures.The study helps to systematically understand the relative role of local environmental factors and spatial factors(such as dispersal)in structuring community assemblages,and the relationship between functional structure,phylogenetic structure and community structure,which is also helpful to improve the research system of river zooplankton diversity.Our study provides specific case support for the niche process-neutral process continuum and related theories,and supplements the deficiencies in the related fields of watershed ecology researches. |
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