Biochemical reduction of nitrate has been widely documented in aquatic systems, particularly near the sediment-water interface, where decomposition of organic matter often occurs (Ferguson 1988; Kuenen and Robertson 1988; Boström et. al. 1988). Under aerobic conditions, decomposition proceeds through microbial respiration that uses dissolved oxygen (DO) as an electron acceptor. When DO concentration is too low to support aerobic respiration, microbial decomposition may proceed using alternative electron acceptors, such as nitrate, iron (III) and manganese (IV) oxides. As iron (III) is reduced to iron (II), the chemicals previously associated with them (such as phosphate) may be released, causing increased phosphate concentration in the overlying water column.

The classical models of redox potentials in aquatic systems indicate that nitrate should be reduced before manganese (IV), iron (III) and sulphate reduction. A supply of nitrate may therefore stabilise redox potential, reduce oxygen depletion in the water column and prevent phosphate release from sediments.

Ripl (1976) and Ripl and Lindmark (1978) reported a biochemical oxidation method "Riplox" for lake restoration in Sweden. In a whole lake experiment, Ripl demonstrated that injection of nitrate into sapropelic sediments induced vigorous denitrification in the sediments, resulting in degradation of organic matter, oxidation of iron compounds and inactivation of phosphorus by sorption to the oxidised iron. A survey in summer 1984, nine years after the treatment, indicated that oxygen uptake by the surface sediment was similar to the result in 1976 after the treatment, and the total phosphorus concentration in both interstitial water and the overlying water column were much lower than before the nitrate treatment (Ripl 1985).

This paper investigates the potential of nitrate concentration to regulate sediment phosphorus release from the shallow, eutrophic Harvey Estuary in southwestern Australia. The effect of nitrate application was examined by using reconstituted sediment cores incubated under various experimental conditions. The factors which may affect the results of nitrate application, such as method of application and resuspension of sediment, were also examined.