Changes in spatial dissolved Fe and total Mn in a tropical river in Brazil: Influence of reservoirs with different operational dynamics

Brazil has 1,320 hydroelectric plants in operation, distributed throughout its territory. The construction of dams for water storage and energy generation causes significant changes in sediment transport and hydrological dynamics, affecting the biogeochemical cycle of various elements. In the Southeast region, the Paraíba do Sul River Basin (PSR) accounts for approximately 6% of these plants, making it a strategic area for studying the environmental impacts associated with reservoirs. To assess the impacts on physicochemical and elemental dynamics (i.e., Dissolved Oxygen “dO₂”, pH, turbidity, dissolved Fe, and total Mn), this study analyzed three years (2021–2023) of water quality monitoring data along the main course of the river, covering four dams with different operational systems (Santa Branca and Funil: storage; Lavrinhas and Anta: run-of-river). The dO₂ median concentration increased downstream, while turbidity, d-Fe, and t-Mn showed a decreasing trend. On the other hand, pH levels remained relatively stable, with little variation. However, near the dams, a decrease in dO₂, pH, turbidity, and d-Fe concentrations was observed immediately downstream of the Santa Branca and Funil dams. Conversely, the Lavrinhas and Anta dams showed little or no influence on these variables. The upper region of the PSR is characterized by intense industrialization, which contributes to the deterioration of water quality variables in this section of the river. The reservoirs in the basin exhibit distinct dynamics, influencing the levels of these variables in different ways. Santa Branca and Funil are larger storage reservoirs with depths ranging from 20 to 40 meters, where intermediate and deep layers have distinct characteristics from surface waters, promoting hydrogeochemical changes at certain times of the year. These waters are generally more acidic and less oxygenated due to the decomposition of organic matter and the respiration of organisms. Additionally, these reservoirs promote particle deposition, contributing to reduced turbidity downstream. These conditions favor the release Fe and Mn from bottom sediments, increasing their dissolved concentrations in the water column. The availability of Fe and Mn, as well as other nutrients, increases the growth of macrophytes and phytoplankton productivity, generating large areas of eutrophication. This process reduces the dissolved fraction of these elements and consequently increases the particulate fraction. On the other hand, Lavrinhas and Anta are run-of-river reservoirs with shallower depths and little to no stratification in the water column, resulting in less significant changes in parameters downstream. However, even in run-of-river reservoirs, depth can influence variable dynamics. This is evident in the case of Anta, which showed an influence on Fe and Mn levels, highlighting that even smaller run-of-river reservoirs can impact the cycles of these elements. In conclusion, long-term studies on these reservoirs are essential, since over the years we have been reporting high concentrations of cyanobacteria with the potential to produce toxins, which has led to interruptions in water supply to the population.