Frações de fósforo e sua disponibilidade em sedimentos de reservatórios no semiárido tropical brasileiro

Abstract

The Phosphorus (P) stored in eutrophic lake sediments can be an important source of this nutrient for the enrichment of the water column and contribute to the persistence of eutrophication in these environments, even after the reduction of external sources. Thus for the restoration of these water bodies to be successful, it is also necessary to control their internal load. Therefore, it is essential to know the chemical composition and the mobile load of P present in the sediments. In artificial lakes (reservoirs) in the tropical semiarid region, the internal loading mechanism is especially important during periods of drought, where external input from P is reduced due to scarcity of rainfall. Thus, this study aimed to quantify the P fractions and the chemical composition in the sediments of eight eutrophic reservoirs located in the tropical semiarid region of northeastern Brazil, in order to evaluate the internal load available for the maintenance of the trophic status of these water bodies. For this, sequential fractionation extractions were performed to determine the fractions of P present in the sediments and the X-ray fluorescence (XRF) to analyze their chemical composition. The total phosphorus (TP) content in the sediments varied from 593,17 to 1222,12 µg g-1 DW (dry wet), and 35 to 46% of this content was associated with potentially mobile forms of P. The most abundant fractions were P-NaOH (25% to 37% of TP), representing non-reducible forms of P-Fe and -Al oxides, and P-BD (23% to 30% of TP), which is related to forms of P sensitive to reducing conditions, mainly Fe and Mn oxides and hydroxides. The greater abundance of these two fractions is consistent with the chemical composition that points to a large presence of Al2O3 (25,27% ± 2,19%) and Fe2O3 (22,56% ± 2,95%) in the sediments. In general, the P fractions presented the following decreasing order in most reservoirs: P-NaOH > P-BD > P-Humic > NRP > P-Res > P-HCl > P-Water. The high P pool in the investigated sediments associated with potentially mobile forms and common environmental characteristics in these water bodies in the semiarid region, that normally favor their release, are factors that indicate that the internal loading mechanism may be acting in these environments.