Gradientes limnológicos em uma cascata de reservatórios do Rio Tocantins

Abstract

The increasing in electricity demands resulted in the alteration of river´s flow around the world by the construction of dams. Construction of isolated reservoirs or cascade reservoirs can be considered one of the greatest impacts on aquatic ecosystems. Tocantins river is a great river in Brazil with 10 hydroelectric reservoirs in operation. In this way, it is important to understand how these ecosystems work to the conservation of aquatic communities and to ensure the aquatic multiple uses of these environments. This study evaluated i) the longitudinal variation of the limnological characteristics in three reservoirs, such as the increase of water transparency and decrease of nutrients concentration in a unidirectional flow and ii) the limnological variations along the reservoirs cascade. Water samples were collected between September 2011 and September 2013. Principal component analysis (PCA) was used to reduce the dimensionality of chemical and physical data. The longitudinal variation of the limnological variables was tested through a linear mixed effects model (LMM). The PCA showed similarity in limnological characteristics of the reservoirs studied and the limnological variability of the tributaries was substantially higher than that found inside reservoirs. The linear mixed effects models indicated that there is no relationship between limnological variables and geographic distance, with the exception of dissolved oxygen in the dry season. Even considering the nonsignificance, the beta values of the models indicated that the turbidity and water transparence showed a substantial longitudinal variability inside the reservoirs, mainly during dry season. The comparative analysis of intercepts revealed that the expected progressive reduction for total phosphorus levels, turbidity values and chlorophyll-a concentrations was not observed. The influence of an upstream reservoir to the downstream one was detected just for the dissolved oxygen concentrations. Probably the shorter water residence time of these reservoirs can explain the lack of pattern in the longitudinal limnological variation. These results are important to understand the ecosystem function of these systems, considering that the aquatic communities respond these abiotic parameters.