The effect of organic matter on sedimentary phosphorus release in an Australian reservoir

Australian reservoirs, compared to much of the world, are subjected to extr eme arid and semi-arid climatic conditions where dam volumes can range from near-empty to full, often with rapid filling events. P-release, after re-f looding of desiccated sediments, can be important to water quality, and can be further influenced by dried macrophyte, exposed as water recedes and in corporated into sediments. P-release from Lake Rowlands (New South Wales, A ustralia) sediments was studied under different aerobic and sterile conditi ons with five carbon source amendments to the sediment (the macrophyte Isoe tes sp. in different stages of senescence and acetate). Sedimentary P-relea se involved a complex array of factors modified by aerobic, biotic and abio tic processes, organic matter breakdown, iron content of sediments and turb ulence. Under aerobic conditions, P-release from sterile non-amended sedime nts and sterile macrophyte-amended sediments was greater than from non-ster ile sediments. Under anaerobic conditions, P-release was maximal from non-s terile macrophyte-amended sediments, probably via pathways involving fermen tative Fe3+-reducing bacteria where electrons are transferred from organic matter to amorphous Fe(OOH) leading to Fe2+ and consequent release of P. Ma crophyte addition (whether fresh or dried) enhanced P-release under anaerob ic compared with aerobic conditions. P-release from acetate-amended sedimen ts appeared to involve acetate aerobes. The re-flooding of sediments, there fore, has the potential to create conditions that are conducive to aerobic sedimentary P-release and should be taken into account in management strate gies adopted for reservoirs where levels are likely to fluctuate.