Effects of oxygen concentration on phosphorus release from reflooded air-dried wetland sediments

Intact sediment cores from North Lake (Perth, Western Australia) were air-dried for 40 days, reflooded with lake water and incubated at 20°C. Under aerated conditions, air-drying increased phosphorus release. When aeration was stopped, air-dried sediments continued to release P into the water, but the rate was much slower than that in the early stage of the aeration. The results suggest a gradual depletion of labile phosphorus from the dried sediments. Under anaerobic conditions, the phosphate release was also markedly higher for air-dried sediments than for the 'wet' controls. The accumulation of soluble inorganic phosphorus during air-drying, owing to breakdown of organic material and a drying-induced decrease in phosphate sorption, may be the cause of release on rewetting. The results suggest that drought-induced sediment dehydration in natural wetlands may be followed by a significant increase of internal phosphorus loading under both aerobic and anaerobic conditions. Care must therefore be taken in using drawdown as a lake management technique to improve water quality.