The Effects of Urbanization on the Food Web of a Mid-order Stream in Rio de Janeiro, Brazil

Stream degradation caused by point source and non-point source pollution, loss of riparian vegetation, erosion and sedimentation can alter aquatic food webs. Due to the lack of proper sewage treatment, the main polluter in urban streams of Brazil is raw sewage. The purpose of this study is to determine if sewage-derived carbon and nitrogen are incorporated into the aquatic system and how this may affect the food web. We collected food web components at four sites along an urban stream, Rio Paineiras, and a reference stream, Rio Andorinha. Samples were analyzed for natural abundance of 13C and 15N. Raw sewage signatures were distinct and enriched compared to primary sources. Riparian vegetation and CPOM samples became significantly more enriched in 15N along the urban stream, compared to the reference stream. Periphyton 15N and 13C signatures were not significantly different between streams. Fish (Astyanax hastatus and Poecilids) showed some incorporation of sewage-derived 13C and 15N, in distinct trophic groups. Macroinvertebrate Hetaerina sp. reflected some incorporation of sewage-derived 15N. We also observed a reduction in functional feeding groups in the sewage-influenced reach of Rio Paineiras. Collectors, gatherers, and shredders were absent as stream degradation increased. Downstream communities were composed mostly of fish.