Impactos na qualidade da água causados pela antropização da zona ripária em reservatórios no semiárido tropical brasileiro

Abstract

The anthropization of terrestrial ecosystems affects aquatic systems by introducing nutrients from the watershed through surface soil processes. The application of geoprocessing tools can support the sustainable management of these ecosystems, helping guide decisions regarding the protection of riparian zones and aquatic ecosystems. This study aimed to assess the relationship between water quality in stored reservoirs and different land use and occupation categories, and whether the response to anthropization varies with reservoir volume. The reservoirs were grouped into large (n = 6) and medium (n = 11) categories based on their maximum water volume (greater or less than 75 hm³). We hypothesized that larger reservoirs have more extensive anthropization areas due to the size of their riparian zones; however, the impact on water quality is delayed due to the dilution effect from their larger water volumes. The study was conducted in 17 reservoirs located in the semi-arid tropical region, within the watersheds of Apodi-Mossoró (RN), Piancó-Piranhas-Açu (RN/PB), and the Paraíba River (PB) in northeastern Brazil. Land use and occupation in the riparian zone were analyzed using geoprocessing, and the following categories were assessed: agriculture and livestock farming, dense caatinga, sparse caatinga, exposed soil, and urban areas. No significant differences were found regarding stored water volumes across the reservoirs. All reservoirs exhibited high nutrient concentrations and algal biomass. Few correlations were found between land use categories and the limnological conditions of the water, and where present, they were weak to moderate. The history of flooding since the reservoirs' operation may have contributed to nutrient feedback into the water. The tendency toward increased eutrophication was more pronounced in smaller reservoirs. The expansion of agriculture and livestock farming activities and exposed soils, especially during prolonged droughts, exacerbated diffuse pollution. The suppression of native vegetation may reduce the buffering effect of soils and vegetation in the riparian zone. Dense caatinga vegetation remains but may not be sufficient to protect reservoirs from nutrient influx. These reservoirs, located in drainage areas with sparse caatinga vegetation, promoted higher nutrient influx when flooded. Thus, no significant segregation was observed between reservoir volumes regarding trophic state.