Efeitos da poluição aquática sob aspectos tróficos da fauna de peixes: uma abordagem utilizando os isótopos estáveis de carbono (δ13C) e nitrogênio (δ15N)

Abstract

The transfer of the capital of Minas Gerais to Belo Horizonte in 1898, resulted in the expansion of a center of urbanization and industrialization along the Rio das Velhas, the largest tributary in extension of the São Francisco basin. Thus, the region that was once known for its abundance of fish, nowadays is in an advanced process of degradation. The main objective of this study was to use the stable isotopes of carbon (δ13C) and nitrogen (δ15N) to evaluate whether the various anthropogenic activities present in the Rio das Velhas basin (especially urban pollution) affect the trophic structure of fish communities. In addition, other objective was to evaluate whether the isotopic composition of fish and other aquatic organisms are good indicators of the presence and intensity of aquatic pollution. In the four chapters that make up this study, the impacts of pollution from the specific level to the community level were assessed. In the first chapter, it was observed that pollution affects the feeding behavior of an exotic species (Poecilia reticulata) and an endemic species (Phallocerosuai) in a similar way, however the exotic species feeds directly from sewage (and it is found in abundance in polluted places), which may favor the establishment of this species in strongly polluted sites. In the second chapter, it was observed that the nitrogen signature (δ15N) of the periphyton and the cascudos (periphyton consumers) varies temporally and spatially throughout the basin in response to changes in nitrate and ammonia concentrations. And, therefore, both can be used as pollution bioindicators. However, periphyton is more sensitive to changes in nitrate and ammonia concentrations between seasons than fish. In the third chapter, it was observed that pollution promotes changes in the trophic niches of fish communities from different sites of the basin. At polluted points, a large variation in the δ15N axis was observed, which is probably due to the high concentration of nitrogen (especially nitrate) from anthropic activities. At these points, there was also greater δ15N enrichment in detritivorous fish, which is not expected for fish that feed on the base of the food web. In the fourth and last chapter, it was observed that autotrophic resources (algae, macrophytes and periphyton) are the main sources of carbon for the fish of Rio das Velhas. However, at points immediately after the Belo Horizonte Metropolitan Region, sewage is an important food source for fish of all trophic guilds. Metrics indicating trophic diversity were also affected by pollution. Although it was observed a reduction in the number of species in the most polluted sites, there was an increase in trophic diversity, and less redundancy in the items consumed by fish. We conclude that stable isotopes are an important tool in assessing the effects of pollution on aquatic communities. In addition, fish, which alter their trophic behavior in response to pollution, proved to be good indicators of water quality and could be used in environmental monitoring programs not only in Rio das Velhas, but in other watersheds.