The impact of a polluted stream across a tropical reservoir branch to its central body

Freshwater ecosystems provide various ecosystem services, such as water supply, but they are threatened by eutrophication. The Billings Reservoir, a tropical one, has suffered from eutrophication for decades, and the continuous input of domestic and industrial sewage exacerbates this deterioration. This study analyzed the influence of the Grota Funda Stream on Central Body I of the Billings Reservoir. Five water samples were collected at five points, from the Grota Funda Stream (GF) to Central Body I (P4). Physical and chemical parameters, chlorophyll-a, cyanobacteria, total coliforms, and Escherichia coli were analyzed. A clear spatial gradient was observed from GF to P4, with dilution and partial self-purification along the longitudinal axis. GF showed the strongest correlations with variables related to pollution and eutrophication (EC, TP, N-NO3, N-NH4, coliforms) and had the highest concentrations of nitrate, ammoniacal nitrogen, phosphorus, and orthophosphate. In contrast, downstream sites exhibited improved water transparency and higher phytoplankton biomass, particularly cyanobacteria, consistent with the reservoir’s hypereutrophic condition. These findings highlight that spatial variability outweighed seasonal effects, evidencing the dominant role of sewage inflows in controlling water quality and phytoplankton dynamics in this tropical reservoir.