Phosphorus fractions and their availability in the sediments of eight tropical semiarid reservoirs

Purpose We quantified the phosphorus (P) fractions and oxide content in the sediment of eight tropical semiarid reservoirs to assess how water-level decreases affect the potential P internal loading. Methods Chemical fractionation analyses were carried out to determine the P fractions in the reservoirs' sediment, and their chemical composition was evaluated via X-ray fluorescence. Linear regressions were performed to evaluate how changes in the oxide content in the reservoirs' sediment and percentage of the maximum volume stored (%MVS) affected the P fractions. Results About 35 to 46% of the content of the sediment load of the total phosphorus (TP) was associated with potentially mobile P forms (NaOH-P and BD-P). The chemical composition of the sediments indicated a strong presence of Al2O3 and Fe2O3. The regressions showed that the lower the proportions of Al2O3 content, the lower was both P mobile (Total-P and BD-P) and non-mobile forms (HCl-P and Res-P) in the reservoirs sediment, whereas the proportions of Fe2O3 content were positively related to NaOH-P and Res-P. Moreover, the proportions of SiO2 and CaO content in the sediment were respectively negatively related to NaOH-P and positively related to HCl-P. The decrease in the %MVS in the reservoirs favored both the accumulation of non-mobile forms of P (HCl-P and Res-P) and CaO in the sediments, whereas an opposite pattern was observed for Al2O3. Conclusion The high P load in the sediments, associated with potentially mobile forms and low water level, indicates that an internal loading mechanism may be present in the environments observed.

Automated summary

This study quantified the phosphorus fractions and oxide content in the sediment of eight tropical semiarid reservoirs, and evaluated how water-level decreases affect the potential phosphorus internal loading. The results showed that a significant proportion of total phosphorus in the sediment was associated with potentially mobile forms, and the chemical composition of the sediments played a role in determining the phosphorus fractions.