| River ecosystem provides diverse ecosystem services,such as fresh water supply,biodiversity maintaining,climate regulation,culture and recreation.Our human development shares weal and woe with river,precisely because the important role of river ecosystems.However,frequent anthropogenic activities and pollution emission have caused degradation of some river ecosystems,remedies need to be applied to bend the curve of river biodiversity loss.The concept of metacommunity can offer novel framework to study biodiversity contrast with conventional approaches based on local communities alone.Environmental filtering and dispersal are considered to be two essential processes in determining the metacommunity assembly.Dendritic network and unidirectional flow are specific characteristics of river,which also make river metacommunity study different from metacommunity studies on other ecosystems.Shaying river,which is the largest tributary of Huaihe river basin,has been regarded as China's important agriculture production area and significant strategic energy reserve base.But owing to anthropogenic impacts on it,many degraded rivers located in its basin and its biodiversity loss has impaired ecosystem functions.This study took northern basin of Shaying river as research area,zooplankton with characteristics of small size,high fecundity and sensitive to environmental change was used as model organism,main aim of the study is to assess the relative contribution of local environmental conditions versus dispersal in determining the assembly of zooplankton metacommunity in different seasons.Given the scale effect,the assembly mechanisms were studied both at basin scale and stream scale.It is hoped that this study could offer more comprehensive and scientific support for biodiversity conversation and ecological restoration in Shaying river.The main findings are as follows:1)This study identified a total of 167 species in the study area,including 75 rotifers,43 protozoa,25 cladocera and 24 copepods.The average abundance of zooplankton was 2601.91±576.60 ind./L and the average biomass was 2.29±0.62 mg/L.The tendency of zooplankton in terms of species number,abundance and biomass is: high flow season > medium flow season > low flow season.Rotifers accounted for most of the species number,abundance and biomass,but the proportions of cladocera and copepods are relatively low due to weaker tolerance to pollution.Many dominant species in Shaying river northern basin are betamesosaprobity indicator species,and oligosaprobity dominant species were slightly up in high flow season.Canonical Correspondence Analysis suggested that ORP?TSS and nutrient concentration(COD and NH3-N)were the main environmental factors affecting abundance distribution.2)The average Shannon-Wiener index was 1.160,the average Margalef index is 1.422,the average Pielou index is 0.519.Indices of Shannon-Wiener and Pielou in dry season were significantly lower than in high flow season(p<0.05).The community dissimilarity percentage among three streams in northern basin of Shaying was over 75.2%,the Bray-Curtis index is lower in high flow period than that in other two periods.3)AEM model which was built to represent dispersal along the river flow direction explained better and more pronounced than PCNM model represents overland dispersal in different seasons.The results suggested that zooplankton mainly dispersed by prevailing river flow.4)This study got different results at different scales,in other words,we validated the spatial scaling effect on metacommunity formation again.Spatial variables had higher explanatory power than environmental variables at basin scale,especially in high flow season,though the influence of environment on metacommunity enhanced in low flow season.In contrast,environmental filtering was dominant process on average at stream scale,its explanatory power of total and significant environmental variables was higher than those at basin scale.Further analysis showed that some environmental variables including water depth,transparency,habitat quality,total suspend solids and nutrients concentration existed significant difference between upstream and downstream,so we inferred that.environmental gradient along streams may resulted in environmental filtering becoming dominant.5)The assembly of zooplankton metacommunity in the Qingyi River during periods of mean-flow and high-flow were mainly driven by dispersal process,but switched from dominance of dispersal to dominance of environmental sorting during dry season.Hence,assembly mechanisms in the Qingyi river were more dynamic than those of the other two streams.Branches in the Huaihe river basin were divided into 2 types based on their long-term hydrological data in previous studies and assesments,Qingyi river was classified as intermittent river but the other two were categorized as perennial rivers.Therefore,we speculated that mechanisms dynamic had some relation to intermittent drying-up and hydrological dynamics should be paid more attention in the future study and work. |
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