Benthic macroinvertebrates in the watershed of an urban reservoir in southeastern Brazil

The Ibirite watershed is subject to several forms of environmental degradation such as the presence of a petroleum refinery industry, urbanization of its surrounding landscape, and non treated domestic sewage from over 135,000 inhabitants. Benthic macroinvertebrates represent a useful tool in the evaluation of environmental quality through studies of the structure of communities and their relationship to anthropic activities within the watersheds. The objective of this study was to evaluate the environmental impact to the Ibirite reservoir caused by the petroleum refinery industry, urbanization, and sulphatation. Degradation of the watershed was measured by using the richness, diversity, evenness, and density of benthic macroinvertebrates. Twelve sampling stations were established as follows: five stations on Pintados and Ibirite streams (upstream from the reservoir), six in the reservoir and one station downstream from the reservoir. From 2002 to 2003, during both the dry and rainy seasons (in the tributaries) and the stratified and non-stratified periods (in the reservoir) were evaluated. A total of 289,777 organisms were collected and the most abundant organisms found in the streams were Oligochaeta (60%), Chironomidae (Chironomus, Goeldichironomus, Dicrotendipes, Cryptochironomus, Polypedilum, Parachironomus, Tanytarsus, Tribelos, Tanypus, Ablabesmyia, Cricotopus, Oliveriella) (38%) and Gastropoda (Biomphalaria straminea, Physa sp., Melanoides tuberculatus, Pomacea haustrum) (2%). In the reservoir, the most abundant specimens were Chaoboridae (46%) and Chironomidae larvae (Chironomus, Goeldichironomus, Tanypus, Coleotanypus, Labrundinia) (17%), in addition to Oligochaeta (24%) and Gastropoda (Melanoides tuberculatus) (13%). Evaluation of the structure and distribution of benthic communities showed a rapid environmental degradation process within the studied aquatic systems, in which low values of richness and diversity and high densities of tolerant organisms were observed. This environmental degradation is a result of the intense discharge of domestic sewage into the streams, reducing water quality and contributing to rapid artificial eutrophication.