| The margin of Qinghai-Tibet plateau is one of the areas with the most complicated and diverse geomorphological and ecological conditions around the world.In this study,macroinvertebrate assemblages have been thoroughly surveyed in the middle and lower reaches of the Yarlung Tsangpo River,the middle and lower reaches of the Nujiang River on south margins of the Qinghai-Tibet plateau,and in the middle and lower reaches of the Bai River on the northeast margin of the plateau.We characterized different types of aquatic habitats by the environmental variables and the macroinvertebrate assemblage indices,and furtherly investigated the patterns of how macroinvertebrate assemblages responded to the typical river geomorphological conditions in these areas,including the developing nick points and the stable barrier lake cascades in deeply incised valleys,and the oxbow lakes on the peat floodplain.Three main habitat types were identified in the middle and lower reaches of the Yarlung Tsangpo River,including the low stream power habitat,the moderate stream power habitat,and the high stream power habitat,which was specially created by the developing nick points.The stream power was found to be the most important physical environmental variable affecting macroinvertebrates in this area with a significantly negative relationship to the density.Different macroinvertebrate taxa showed dramatically distinguishable preferences and adaptabilities to the stream power—around 63.9% of the macroinvertebrate taxa preferred to moderate stream power conditions,and around 27.8% of the taxa were apt to inhabit low stream power conditions,while around 8.3% could adapt to high stream power conditions.Five main habitat types were identified in the middle and lower reaches of the Nujiang River,including the undisturbed main stem habitat,the tributary habitat,and three other disturbed habitats distributed in different parts of the barrier lake cascades.Although the disturbed habitats in barrier lake cascades were found to be relatively low in ?-diversity,they dramatically improved the reach scale ?-diversity of the main stem by providing heterogenous habitat conditions,and around 47.8% of the taxa found in main stem inhabited the barrier lake cascades only.Besides,the well-sorted sandy bars formed as a result of the barrier dam impoundment provided the most fundamental physical conditions for the survival of a newly found and named sand-burrowing mayfly,Behningia nujiangensis.Four main habitat types were identified in the middle and lower reaches of the Bai River,including the sand-bed river habitat,the cobble-bed river habitat,the semi-open oxbow habitat,and the isolated oxbow habitat.Macrophyte biomass was found to be one of the most important factors affecting the macroinvertebrates in this area—it positively related to the macroinvertebrate density,and was controlled by the hydrological connectivity.Additionally,the isolated oxbow lake habitat possessed a much more complicated food web structure compared with other habitats,and it dramatically improved the secondary productivity in this area by restoring much organic carbon,which was 50 times more than the sand-bed river,and 3 times more than the cobble-bed river.In the three study areas,diversity baselines of macroinvertebrates ranked as the Nujiang River > the Yarlung Tsangpo River >> the Bai River.The developing nick point in the middle and lower reaches of the Yarlung Tsangpo River increased the reach scale ?-diversity to a small extent;the stable barrier lake cascades in the middle and lower reaches of the Nujiang River increased the ?-diversity to a comparatively greater extent;while the oxbow lakes in the middle and lower reaches of the Bai River increased the ?-diversity to a fairly great extent.Besides,a multi-metric evaluation system has been established based on the macroinvertebrate assemblage indices,and gave out distinguishable assessments on the ecological qualities of different habitat patches. |
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