EFFECTS OF WATERSHED LAND OCCUPATION AND LAND USE ON THE TROPHIC STATE OF TWO SHALLOW LAKES

Abstract

The increase in external phosphorus inputs has as main consequence the cultural eutrophication, currently regarded as one of the primary threats to water bodies. Anthropogenic activities involving the watershed use and occupation of land can accelerate this process. Thus, the aim of this study was to assess the effects of land use and land cover on the trophic status of two shallow lakes (lakes Paó and Santa Lúcia, Paraíba state, Brazil). Samples of water and sediments were collected for 6 months at three sampling stations in the lakes. The land-use and land-cover mapping was performed through visual interpretation of satellite images with high spatial resolution. Lake Paó remained eutrophic, with a low euphotic zone (ZEU≤1), while Lake Santa Lucia, oligotrophic and with clear waters (ZEU≥1), throughout the study period. The advanced eutrophication of Lake Paó was related to high sewage inputs from nonpoint sources, since the urban/ built-up area corresponded to 37.63% of its riparian zone (the lakeshore), whereas in Lake Santa Lucia to only 2.43%. Our results evidenced the importance of maintaining phosphorus at low concentrations and a minimum of riparian vegetation for reducing the vulnerability to eutrophication and thus preserving water quality.