Macroinvertebrates of a fluvial system in Patagonia: altitudinal zonation and functional structure

Abstract

Most investigations of altitudinal zonation of river macrobenthic communities have been conducted in the northern hemisphere. Conversely, South American habitats, and specifically the Argentinean ones, are poorly known. This is the first study of altitudinal zonation of macrobenthic communities in the Patagonian Andes. It was carried out at 14 sampling stations along 51 km of the Esquel-Percy River system over a 1000 m altitudinal gradient. Untreated sewage of Esquel City discharges in the middle section of the system. The main species assemblages of the monitored sites were identified and a functional categorization of the taxa was produced. Population distribution patterns were analyzed throughout a complete annual cycle, and both qualitative and quantitative variations were interpreted as a function of abiotic and biotic variables. The macrobenthic community found along the Esquel-Percy system is rich and diverse, except at middle-stretch stations, which are altered by anthropogenic perturbations. Specific composition, diversity and density were influenced by the topographic gradient, geochemical (conductivity and total alkalinity) and anthropogenic factors (BOD and oxygen saturation), and the seasonality of the system. Similarities with lotic environments of Australia and New Zealand were found in specific composition, density, and community structure. The biomass of shredders and predators decreased, while collectors increased, downstream from the headwaters. Collector-gatherers were the most important group at all sites. Functional organization showed better correspondence with the RCC model if biomass was used instead of macroinvertebrate density. We found that local and regional characteristics produced deviations from the RCC model, but it is still useful as a comprehensive framework for stream ecology in the Patagonian Andes.