Mudanças nos usos e cobertura do solo na sub-bacia contribuem para a dinâmica da eutrofização no Rio das Velhas?: um estudo com ênfase no nitrogênio

Abstract

The heterogeneity in the land use and land cover in river basins impacts the chemical characteristics of water bodies by diffusing pollutants in different ways and intensities according to each anthropogenic activity. The discharge of effluents and waste can cause an abnormal enrichment of nutrients and organic matter in the waters, known as eutrophication. This process can alter water quality, as well as modify phytoplankton communities especially by the input of phosphorus (P) and nitrogen (N), increasing primary production, phytoplankton biomass, and the frequency of algal blooms. P is considered the most important nutrient for the productivity of freshwater ecosystems, while N is usually neglected, although studies show its importance for eutrophication. Here, we aim to map the N-total concentrations from 1997 to 2021 in 24 monitoring stations along the Rio das Velhas main channel (Minas Gerais, Brazil), considering temporal aspects, categorizing their concentrations, and evaluating the social and environmental aspects related to N pollution. We carried out the study using a database from the Instituto Mineiro de Gestão das Águas (IGAM), containing information on concentrations of total-P, nitrate, nitrite, total ammonia nitrogen, organic nitrogen, and total-N; and we also used the data from the MapBiomas Project’s Collection 7.0 on land use and land cover in the Rio das Velhas sub-basin. Spatio-temporal analysis revealed high concentrations and increasing N-total concentrations in most of the analyzed river paths, especially in the most urbanized ones, just downstream of Belo Horizonte and in the next river path, still located in the metropolitan region (RMBH). The N:P ratios were often not balanced, with the river water possibly being limited by P and rarely by N due the high values observed. Population growth in the sub-basin and the expansion of urbanized areas may have contributed to the increase of N in the areas closer and downstream of Belo Horizonte. In addition, the analysis of land use and land cover changes revealed that the increase in agricultural, urbanized, and silviculture areas may also be related to the N concentrations observed here. Therefore, the high concentrations found in the river channel and their connection with changes in land use and land cover highlight the importance of integrated monitoring of water quality, along with land use and land cover. This large-scale monitoring can be crucial for understanding the extent of N pollution in the sub-basin and thus contribute to more efficient management.