Natural inactivation of phosphorus by aluminum in atmospherically acidified water bodies

Atmospheric acidification of catchment-lake ecosystems may provide natural conditions for the in-lake control of P cycling. This process is based on t he elevated transport of aluminum from acidified soils and its subsequent p recipitation in the water body and is described for strongly acidified fore st lakes, acidified and circumneutral reservoirs, and a moderately acidifie d alpine lake. In water bodies with episodically or permanently acidified i nflows a pH gradient develops between lake water and tributaries due to: (i ) neutralization of acidic inflows after mixing with waters with undepleted carbonate buffering system, and/or (ii) the in-lake alkalinity generation dominated by biochemical removal of NO3- and SO42- With the pH increasing t owards neutrality, ionic Al species hydrolyze and form colloidal Al hydroxi des (Al-part) with large specific surfaces and strong ability to bind ortho phosphate from the liquid phase. Moreover, Al-part settles and increases th e P sorption capacity of the sediment. The presence of Al-part on the botto m reduces orthophosphate release from sediments after its liberation from f erric oxyhydroxides during anoxia because Al-part is not sensitive to redox changes. Consequently, the natural in-lake P inactivation may be expected in any water body with elevated Al input and a pH gradient between its inle t and outlet. (C) 2001 Elsevier Science Ltd. All rights reserved.