Factors influencing phosphate exchange across the sediment-water interface of eutrophic reservoirs

The results of a survey of the sediment chemistry of 7 East Anglian reservoirs are presented as part of a regional study on the assessment and control of eutrophication. The influence of water quality (dissolved oxygen, pH, temperature) on phosphate (PO4) adsorption by sediment from hypertrophic Ardleigh Reservoir is also examined. Extractable phosphate-P (extr.-P) varied between 92 and 383 mg kg−1 dry matter. Extractable P varied between 5.3 and 16.6% of the total phosphate-P (Tot. P) content and increased with the concentration of dissolved reactive phosphate-P (DRP) in the overlying water column. Organically complexed iron (organic Fe) was the determinand which correlated most closely with phosphate adsorption capacity, PAC (r = 0.8). Organic Fe was also related inversely to Extr. P. The rate and extent of PO4 adsorption by Ardleigh Reservoir sediment increased with the initial concentration of DRP and adsorption equilibria were reached after 24 h. The equilibrium DRP concentration, [DRP], was 0.7 mg P 1−1 under aerobic conditions indicative of a high potential for PO4 exchange. The rate and extent of PO4 adsorption was greater at 7 °C than at 22 °C PO4 adsorption increased markedly with dissolved oxygen status. Ardleigh sediment exhibited a marked buffering capacity to a change in pH; however, PO4 adsorption was greatest at an equilibrium pH of 5.6 and decreased progressively either side of this pH value.