Nutrient release from sediments: The effect of short-term anoxia

A total of 56 core samples from the bed of the Swan River estuary were studied under aerobic and anaerobic conditions and the rate of release of nutrients was compared with the particle size distribution and organic content of the sediments. Whilst sediment release of phosphorus is enhanced under anaerobic conditions, total nitrogen release remains unaffected, though its speciation is altered. Contrary to the traditional limnological view, the results indicate that an increase in organic content translates to a decrease in sediment oxygen demand simply because the mean pore space (and hence the rate of oxygen diffusion into the sediment) decreases. However, as diffusion of oxygen into the sediment appears to limit the rate of benthic mineralisation, recently deposited organics in the uppermost layer of the sediment are the most rapidly degraded. The oxygen demand of Swan River sediments decreases with distance from the river mouth due to the high flux of organic matter to the sediment in the lower reaches. Fick's Law is a valid means of estimating nutrient release from sediments provided that tortuosity effects are accounted for and the oxygen conditions are favourable for the release of that nutrient. This study has, in addition, derived corrections for unfavourable oxygen conditions. Consequently, sediment nutrient release from Swan River sediments can be accurately predicted under all conditions.