Diel variation of limnological parameters in a reservoir in northeastern Brazil (Boa Esperança, Maranhão/Piauí): Rainy period

Abstract

AbstractThe diel variation of limnological parameters, measured in Boa Esperança Reservoir, Brazil during the rainy period, exhibited small temperature gradients characterized by an increase in temperature by midday, followed by a partial homogenization of the water column in the night and the early morning. The result was an increase in the epilimnion to a depth of 17 m. The pH and dissolved oxygen concentrations showed higher values at the surface, with the occurrence of anomalies (concentrations higher than at the surface) of the latter variable in the epilimnion, probably as a result of the activity of primary producers and/or losses of oxygen to the atmosphere. The chemical forms of iron in the reservoir exhibited increasing concentrations with depth, as a result of an increase in the concentration of seston and/or accentuated reduction of the levels of oxygen in the bottom layers. The reduction of Fe3+ to Fe2+ in the bottom water layers in the reservoir, in addition to the primary production process in the epilimnion, could explain the increased phosphorus in the bottom water layer. The concentrations of nitrogen compounds did not exhibit relevant changes or a defined evolution profile, except for nitrate, which showed an outstanding increase in the bottom water layers, suggesting it could be an important nitrogen source for primary producers. Although not unequivocal, these data suggest that ecosystem instability in Boa Esperança Reservoir is the product of short diel thermal variations, and the reduced input of allochthonous materials and nutrients from local catchment. These conditions guarantee the oligotrophic status of Boa Esperança Reservoir for the present time.