Effects of elevated sulfate in eutrophic waters on the internal phosphate release under oxic conditions across the sediment-water interface

Eutrophication in freshwater environments may be enhanced by the elevation of sulfate in waters, through the release of internal phosphorus (P) from anoxic sediments. However, the influence of increasing but modest sulfate concentrations (less than 3000 mu M) on P release under oxic conditions across the sediment-water interface (SWI) in eutrophic freshwater is poorly understood. In this study, the profiles of P, iron (Fe), sulfur (S) and physicochemical parameters were measured in a simulated lacustrine system with varying concentrations of sulfate (970-2600 mu M) in overlying water. The results indicated that elevated concentrations of sulfate increased the soluble reactive Pin overlying waters under oxic conditions across the SWI. A 100 mu M increase of sulfate was found to induce a 0.128 mgm-2d-1 increase of P flux from surface sediments into overlying waters under oxic conditions. Higher sulfate concentrations in the overlying waters increased the concentrations of labile S(-II) in the deep sediments, due to sulfate penetration and subsequent reduction to S(-II). We also found the fluxes of labile Fe (10.34 to 18.33 mgm-2d-1) and P (2.70 to 1.33 mgm-2d-1) from deep to surface sediment were both positive and greater than the corresponding fluxes (Fe, 2.2 to 3.51 and P, 2.6 to 0.39 mgm-2d-1, respectively) from surface sediment to the overlying water, suggesting that reduction of P-bearing Fe(III)(oxyhydr)oxides in deep anoxic sediment acted as a major source of internal P release. In addition, the upward flux of Fe(II) was significantly lower under higher sulfate conditions, indicating that the Fe(II) flux could be mitigated by formation of Fe(II) sulfides in the deep sediment. Under these conditions, less Fe(II) from deep sediments could be re-oxidized and combine with Pin the surface, oxic sediment, thereby reducing the retention capacity for P and leading to higher release of internal P to the water column. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study found that increasing sulfate concentrations under oxic conditions result in higher levels of soluble reactive phosphorus in overlying waters, leading to an increase in the upward flux of phosphorus in eutrophic freshwater systems. Moreover, higher sulfate concentrations also elevate the concentrations of labile sulfur and iron in deep sediments, demonstrating that internal phosphorus release is influenced by sulfate penetration and subsequent reduction processes.