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.status: Published onlin