Phosphorus fractionation in sediments of tropical semiarid reservoirs

The reduction of external loads of phosphorus (P) is the main action to control eutrophication in lakes. However, the elimination of these sources is not always sufficient for lakes' recovery, because the P accumulated in the sediment can be released for decades after it has accumulated. Thus, to restore a lake, it is also necessary to reduce its internal P loading. For this, it is essential to know the composition of P in the sediment to evaluate the potential P availability. In this study, the forms of P in the sediment of two reservoirs in a semiarid tropical region were investigated. The forms of P were determined by fractionation, sorting out the forms in loosely sorbed P (P-Water), reductant-soluble P (P-BD), metal oxide-bound P (P-NaOH), P bound to organic matter (P-Humic), calcium-bound P (P-HCl) and residual P (P-residual). The sediment was collected in September 2016 and sliced in situ. The total amount of P forms in the sediment varied from 5mgkg−1 to 349mgkg−1 in the Gargalheiras Reservoir and from 12mgkg−1 to 371mgkg−1 in the Cruzeta Reservoir. Despite some variation in amounts at different depths, the general range of fractions in Gargalheiras and Cruzeta was: P-BD>P-NaOH>P-HCl>P-water>P-Residual>P-Humic. In both reservoirs, the predominant form was BD, followed by NaOH and HCl. The first two forms are available and released easily, making them bioavailable for eutrophication processes and thus phytoplankton growth. [Display omitted] •P loads from the sediments of 2 reservoirs in the semiarid region was evaluated.•The mobile forms constituted the main P forms in both reservoirs.•The P form was present mainly as P bound to Fe (P-BD) in both reservoirs sediment.•Our results show the high amounts of labile P in the sediment of semiarid.