Impact of drying on nutrient transformations and water quality in wetlands

Song Qiu

The effects of sediment drying and reflooding on phosphorus and nitrogen transformations in shallow wetlands were studied using sediments from seven wetlands near Perth, Western Australia. Changes in sediment properties on drying and rewetting were investigated, including the properties of phosphorus sorption; phosphorus fractionation; sediment solution properties; iron and aluminium extractability [by oxalate and CDB (citrate-dithionite-bicarbonate)]; molecular weight distribution of sediment humic acid; and planktonic and microbial phosphorus release. Air-drying significantly affected phosphorus sorption and increased phosphorus release from organic-rich sediments during rewetting. Changes in phosphorus sorption in reflooded sediments were correlated with changes in iron extractability, suggesting that changes in iron properties during drying were the primary cause for changes in phosphorus sorption. Air-drying and reflooding of intact sediment cores induced a rapid increase in phosphorus and ammonia release under aerated conditions. The increase in ammonia from reflooded cores subsequently stimulated rapid nitrification in the sediment / water systems. The results strongly suggest an accumulation of 'reactive soluble phosphorus' and ammonia in the sediment during drying. The accumulation of phosphorus and ammonia is likely to be due to stimulation of microbial activity in the earlier stage of drying, especially in organic-rich sediments. The 'reactive soluble phosphorus' was taken up and stored largely in microbial biomass. Drying caused substantial mortality in microbial biomass. Thus, upon reflooding, the phosphorus previously bound to microbial biomass was released into the overlying water. This process could be reinforced by a flush of phosphorus released from sediment due to changes in iron properties during drying. This study suggests that (1) wetland drying can induce a rapid and significant increase in internal phosphorus and inorganic nitrogen loadings during reinundation, (2) microbial activity and phytoplankton productivity can be subsequently stimulated, and the probability of an algal bloom increased through drying and refilling, (3) care must be taken in using dry sediment samples to assess sediment-related nutrient activity in natural wetlands.

Impact of drying on nutrient transformations and water quality in wetlands

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