Sulphate reduction determines the long-term effect of iron amendments on phosphorus retention in lake sediments

Purpose This field study aimed to guide the planning of iron amendments for phosphorus retention by investigating the long-term fate of iron added to two urban lakes (Plotzensee and Gross Glienicker See) in Berlin, Germany. The contributions of iron dosing to improve lake status as well as the relevance of competing processes for management success were evaluated. Methods Sediment stratigraphy, as well as occurrence of iron minerals, and fluxes between water and sediment were examined using geochemical analyses (i.e. element composition, sequential extraction, X-ray diffraction, and pore water analyses). A one-box lake model was used to relate these fluxes to monitoring data from the water column and to sediment inventories. Results In both lakes, the added iron was preserved in the sediment. Whereas phosphorus retention increased following the addition of iron to Gross Glienicker See, sulphur was retained by the excess iron in Plotzensee. This contrasting effect is attributed to significantly different sulphate reduction rates in two lakes (Wilcoxon rank sum test: W = 25, p = 0.008). According to the one-box model, sulphate reduction explained both the decrease in measured sulphate concentrations after iron application as well as the observed increase in sulphur deposition in the sediments. Conclusion Management interventions involving iron amendments to enhance phosphorus retention must consider the competing process of iron sulphide formation during the entire management plan period, and additional iron may need to be applied to account for this effect.

Automated summary

This field study aimed to investigate the long-term fate of iron added to two urban lakes in Berlin, Germany, and evaluate the contributions of iron dosing to improve lake status. The study found that the addition of iron led to increased phosphorus retention in one lake while causing sulphur retention in the other due to varying sulphate reduction rates. Management interventions involving iron amendments to enhance phosphorus retention should consider the competing process of iron sulphide formation.