Desvendando a composição de fósforo em sedimentos de mananciais do Semiárido Tropical

Abstract

The insertion of macronutrients into aquatic ecosystems, mainly nitrogen (N) and phosphorus (P), is the focus of extensive discussions, showing positive relationships between their dynamics and the problem of eutrophication. Therefore, studies mainly about internal loading, are essential for the restoration of a eutrophic system, as the release of P from the sediment to the water column can contribute to the degradation of the water quality of the water body. This work aims to analyze and quantify the fractions of P, as well as to investigate the chemical composition of the sediments of Boqueirão and Caldeirão reservoirs - Parelhas / RN, Brazilian semiarid. In January 2016, a sample was collected with a Kajac sampler/corer from each reservoir, close to the water intake, which were subsequently frozen and lyophilized. P fractions were obtained using the fractionation methodology proposed by Paludan & Jensen (1995), adapted by Cavalcante et al. (2018), performing the sequential chemical extraction of the P forms from the sediment, using a sample from each reservoir, in triplicate. The fractions are classified as P weakly adsorbed to the surface of minerals and dissolved in the interstitial water (P-Water), P associated whith Fe and Mn oxides (P-BD), P bound to Al oxide (P-NaOH), P bound to organic matter (P-Humic), P bound to calcium (P-HCl) and residual P (P-residual). The quantification of the Aluminum oxides (Al2O3), Calcium (CaO), Iron (Fe2O3), Magnesium (MgO), Manganese (MnO) and Silica (SiO2), was obtained by X-ray Fluorescence Spectrometry (FRX). The total amount of P in the sediments was 712.17 mg.kg-1 in Boqueirão, its largest/biggest fraction was P-NaOH (256.63 ± 13.75 mg.kg-1), and the lowest was P-Water (7.40 ± 0.25 mg.kg-1). In Caldeirão, the total was 568.45 mg.kg-1, with the largest fraction P-BD (208.28 ± 2.55 mg.kg-1), and the lowest P-Water (19.74 ± 0.37 mg.kg-1). As for oxides, the largest percentage was linked to Iron, corresponding to 44.04% in Boqueirão, and 42.02% in Caldeirão, followed by Silica, representing 26.22% and 27.37%, and lastly Aluminum 17.85% and 19.26%, respectively. In both reservoirs, labile fractions were predominant, mainly P-BD and P-NaOH, also showing relevant concentrations of oxides bound to Fe, Al and Si, reinforcing the prevalence of mobile fractions. Therefore, the dominant fractions of the sediments have a great potential to be released to the water column, that is, P can be easily released, triggering a continuous supply of the nutrient in the ecosystem and, consequently, a possible increase in the deterioration of water quality. Thus corroborating studies that demonstrate the influence of internal loading regarding the water quality of reservoirs, although little discussed.