Sediments collected from three sites in Harvey Estuary on three occasions all displayed increased phosphorus release with lowering DO concentrations during laboratory incubations. The critical DO concentration to enhance such phosphorus release was about 4 mg L^-1, that is, before oxygen depletion takes place in the overlying water. Application of nitrate to the water column at various doses decreased DO consumption and increased redox potential near the sediment-water interface, from below -200 mV to over 200 mV during 25 days of incubation, thus increasing the potential oxidation of ferrous iron (II) to iron (III) and thereby decreasing water column SRP. Reductions in water column SRP by nitrate application are considered mainly to result from a process including: (1) a decrease in phosphate release from sediment due to increased iron (III) binding near the sediment-water interface; (2) an increase in DO concentration in the water column due to lowered oxygen demand, therefore increased oxidation of ferrous irons and SRP binding substances in the water column.

The nitrate demand to reach an effective reduction in water column SRP may vary widely between sampling sites, and at the same site between different seasons. This variation is largely a reflection of differences in sediment properties. The bioavailable carbon, which may be reflected by the organic matter content of the sediment, is one of the most critical properties relevant to the nitrate application. Higher nitrate doses are generally required for the sediment with higher concentrations of bioavailable carbon, or high oxygen demand. Therefore, for sediments containing higher level of bioavailable carbon, it is difficult to maintain a long-term concentration of nitrate in the water column to stabilise redox potential and prevent anaerobic phosphorus release.

Nitrate application directly into surface sediment increased nitrate consumption and therefore is a less effective way to control SRP concentration in the overlying water. Nitrate application to the water column was more effective than sediment application in this regard.

The effect of sediment resuspension on nitrate application was delicate. Less frequent resuspension increased the rate of nitrate consumption, while frequent resuspension increased DO concentration and therefore had a positive effect in association with the nitrate application in controlling SRP concentration in the water column.
 
 

Acknowledgements

This study was undertaken while T.F.M. was employed in The Botany Department and Centre for Water Research of the University of Western Australia under a grant to R.J.L. from the Commonwealth Tertiary Education Commission. It supplemented an ongoing study of eutrophication in the Peel-Harvey Estuarine System, initiated by the Environmental Protection Authority and the Waterways Commission. Technical support was provided by Geoff Bastyan, Frank Salleo, Christina Blackburn and others.