Sediment respiration contributes to phosphate release in lowland surface waters

High phosphate (PO₄) concentration peaks in lowland rivers occur due to internal loading at low flow rates and low dissolved oxygen (DO) concentrations. However the mechanisms controlling this PO₄ are not fully understood yet. This study was set up to identify additional factors affecting internal P loading, the hypothesis being that sediment respiration varies among sediments and might explain spatial variability in reducing conditions. The sediment of ten rivers was collected for a static sediment-water incubation experiment without aeration, to induce oxygen depletion by sediment respiration. In addition, four out of the ten sediments were selected and amended with mineral-N and OM in a full factorial design, to evaluate the impact of increased respiration rates and subsequent P release. The P release to the overlying water sharply increased if the average CO₂ release rate exceeded 12 mmol CO₂ m^-2 day^-1 over the first 15 days. However, the P concentration remained below environmental limits as long as the molar P/Fe ratio in the oxalate extract of the sediment was lower than 0.12. The P release increased with increasing sediment cation exchange capacity (CEC), which lowers solution Fe(II) and avoids trapping of PO₄ in Fe-minerals. The PO₄ release could be explained by a multiple regression model including CO₂ release, oxalate extractable sediment P, Fe and Al and the CEC (R² = 0.78), the R² was only 0.41 for the molar P/Fe ratio in the sediment. This study shows that internal loading of P is enhanced under eutrophic conditions which boost sediment respiration and which may be attenuated when the CEC and Fe + Al oxide concentrations in the sediments are large.