A cascade of dams affects fish spatial distributions and functional groups of local assemblages in a subtropical river

Abstract Dams reduce the longitudinal connectivity of rivers and thereby disrupt fish migration and the spatial distribution of species, impacts that remain poorly studied for some Neotropical rivers from mega-diverse basins. We investigated the spatial distribution of fish species with different trophic and movement/reproductive/size characteristics to assess how functional groups have responded to a cascade of dams on the Uruguai River in southern Brazil. Fish abundance, biomass, and species composition were evaluated at eight locations along the longitudinal gradient. The fish assemblage in the upper stretch was mainly characterized by small and medium-sized species at higher trophic levels, whereas the sites located furthest downstream displayed more medium and large-sized species, including many carnivorous species. Species with high fecundity, seasonal migrants, and catfishes with internal fertilization were common in the river´s middle and lower reaches. Detritivorous species dominated areas distant from the dams. Overall, functional diversity of local fish assemblages was greater in lower reaches. The cascade of dams has impacted the distribution of functional groups of local fish assemblages of Uruguai River. The alteration of functional groups in upper reaches of the river has potential consequences for ecosystem processes and services, such as nutrient cycling and fisheries.

摘要 大坝会削弱河流的纵向连通性，进而干扰鱼类洄游与物种空间分布，但针对超多样流域内的部分新热带河流，这类影响的相关研究仍较为匮乏。本研究针对巴西南部的乌拉圭河（Uruguai River）上的梯级大坝群（cascade of dams），调查了具有不同营养级、运动/繁殖/体型特征的鱼类物种的空间分布，以评估各类功能群（functional groups）对梯级大坝的响应情况。研究沿河流纵向梯度设置了8个采样位点，对鱼类丰度、生物量及物种组成开展了调查评估。河流上游河段的鱼类群落以中小型高营养级物种为主要特征，而最下游的采样位点则以中大型物种为主，其中包含大量肉食性物种。高繁殖力物种、季节性洄游物种以及体内受精的鲶形目鱼类（catfishes）在河流中下游区域较为常见；碎屑食性物种（detritivorous species）则在远离大坝的区域占据优势。整体而言，下游区域的本地鱼类群落功能多样性更高。梯级大坝已对乌拉圭河本地鱼类群落的功能群分布造成了显著影响。河流上游功能群的改变，可能对养分循环、捕捞业等生态系统过程与服务产生潜在影响。