Influence of different spatial-scale factors on stream macroinvertebrate assemblages in the middle section of Qiantang River Basin

Benthic macroinvertebrates are common,long-lived,sensitive to disturbance,and cost-effective to sample,which make them ideal biological indicators of aquatic system degradation,and they are used in stream biomonitoring worldwide.As biological indicators,macroinvertebrates can provide insight into the current and past conditions of waterbodies and they integrate the effects of cumulative stressors.However,patterns in stream macroinvertebrate assemblages are the result of a combination of processes acting at different spatial scales.Understanding the relative influence of environmental variables at different spatial scales can increase our ability to detect anthropogenic influences on stream integrity,as well as to assess and manage aquatic resources.In this study,we used Qiantang River basin as an example of a human disturbed watershed.Based on environmental and biotic data of 60 sites located in the middle section of Qiantang River basin,the specific aims of our study were:(1) to analyze the stream macroinvertebrate assemblages;(2) to identify the key environmental factors that are linked to variation in macroinvertebrate assemblages;and(3) to partition the additive effects of watershed-scale and reach-scale variables,as well as their interaction,on macroinvertebrate community composition.Environmental variables mainly included land use(urban,agriculture,forest,and the total impervious area),geographical location,elevation,slope,stream order,and the area of sub-basin at the watershed scale,physical habitat conditions and water chemistry at the reach scale.Macroinvertebrate responses were characterized by the relative abundance of 262 taxa.Principal components analysis was used to explore the major environmental gradients,and canonical correspondence analysis was adopted to determine the relationships between environmental variables and macroinvertebrate composition.Variation partitioning was performed using partial canonical correspondence analysis(pCCA) to understand the relative importance of different scale factors in macroinvertebrate variation.Assessment of the main environmental gradients suggested that degradation of the macroinvertebrate communities was mainly due to the increase of anthropogenic land cover,followed by elevated nutrient concentrations,and habitat destruction.At the watershed scale,the most important variables structuring macroinvertebrate assemblages were latitude,altitude,sub-basin area,percentage of forest;at the reach scale,they were total N,total P,the concentrations of Ca2+ and SiO2,and the mean substrate score.The total explained variation was 26.4%.Of this,the results of the pCCA indicated that the relative proportions of explained variation attributed to watershed-scale and reach-scale environmental variables were 50% and 31%,respectively.The results of the pCCA suggested that watershed-scale variables play a more important role in stream macroinvertebrates of this study region than reach-scale variables,which could be important for biodiversity conservation and restoration,land management,and environmental monitoring and assessment.