Aquatic Coleoptera Distribution and Environmental Relationships in a Large Patagonian River

Abstract

The benthic coleopteran assemblages of the Chubut River basin were studied in order to assess the main factors affecting species composition and distribution along the upper, middle and lower catchments. A total of 13 sampling sites were selected and sampled seasonally. Eight taxa and 1,601 individuals were collected during the study. Richness was higher in the main channel of Chubut River at the upper basin than at the middle basin. Beetles were completely absent at the lower basin. Mean monthly density per sites varied from 0 to 85 ind m–2. Stethelmis kaszabi had a more restricted distribution whereas Hemiosus dejeanii, Austrelmis sp. and Austrolimnius spp. were more frequent and abundant. Austrelmis sp. appears as the most tolerant species, especially to higher TSS, ammonia, and conductivity values. Luchoelmis cekalovici was absent in stations associated with urban areas. A Canonical Correspondence Analysis shows that conductivity, total suspended solids, wet width, water temperature and pH were the most important variables structuring beetle assemblages. Land use related variables such as NH4, TP, and NO3 were less important but still significant. An increase in TSS affected negatively the coleopteran community; this could be related to both hydrogeological characteristics and agricultural activities (including overgrazing). This is the first approach to the knowledge of the ecological range of distribution of the coleopteran species in Patagonian rivers.