Watershed studies that account for nutrient loadings are crucial for suitable river basin and estuarine management. This study aims to contribute to the environmental planning and decision making regarding nitrogen and phosphorus loadings for the watershed of the Vitória Bay Estuarine System (VBES; 1925 km2) in southeast Brazil. Here, we estimate that the VBES had annual loadings (in 2016) from ten tributary river basins of 5480 and 10,784 t for P and N, respectively, based on emission factors for natural and anthropogenic sources. The main contribution sources were related to livestock farming (4801 t of P and 8000 t of N) and domestic wastewater (492 t of P and 1706 t of N). However, urban loadings have higher expressions when considering the input by watershed area (0.25 and 0.87 t km-2 year-1 of P and N yields, respectively), which are mainly due to wastewater since 70% of the sewage is untreated. Urban emissions play a prominent role and have a tendency to be aggravated due to Brazilian population growth (0.8% annual rate). If the current wastewater emissions continue, P and N loadings will likely increase by 1.316% to totals of 713 and 2474 t year-1 in 2050 for P and N, respectively. Considering that the wastewater tertiary treatment cost is US$ 0.50 m-3, and that in the future 70% of all wastewater will be collected, the P and N loadings will drop to 18.4 and 424.1 t year-1, respectively, with a treatment cost of 25 million USD year-1. The model holds well for non-regulated watersheds. However, for regulated basins, such as the Santa Maria de Vitória, the model overestimates the loadings mainly due to the damming systems in these watersheds (accounting for 49% retention of P and 0.13% retention of N).
Keywords: Environmental flows; Nitrogen; Nutrients; Phosphorous; Sewage.