Phosphorus inactivation by aluminum in the water column and sediments: Lowering of in-lake phosphorus availability in an acidified watershed-lake ecosystem

Acid precipitation has been shown to reduce phosphorus (P) outputs from terrestrial sources. We present held evidence that the in-lake P cycle may also be disrupted in atmospherically acidified watershed-lake ecosystems. Plesne Lake in the Bohemian Forest is an acidified forest lake with epilimnetic pH of 4.5-4.9, total P concentration of similar to 10 mu g L-1, and pronounced anoxia above the bottom during winter and summer stratification periods. The water chemistry of the anoxic zone develops in a predictable way: After depletion of dissolved oxygen, concentrations of nitrate and sulfate decrease while concentrations of ammonium, iron, alkalinity, and pH increase; the only exception to the general pattern is a permanently low concentration of dissolved reactive P (<1 mu g L-1) above the sediment. This in-lake P inactivation can be explained by the acidification of soils in the watershed and subsequent precipitation of Al in the lake. The suggested mechanism is as follows: atmospheric deposition of strong acids has resulted in strong acidification and nitrogen saturation of soils in the Plesne Lake watershed and, consequently, in high terrestrial losses of Al (0.78-1.04 mg L-1), SO42- (4.5-7.7 mg L-1), and NO3- (0.48-1.15 mg L-1 NO3-N) to the lake. Biological removal of NO3- and SO42- by both assimilatory and dissimilatory reduction contributes greatly to the total in-lake alkalinity generation and to the increase in water pH. Under higher pH, ionic Al species hydrolyze and form colloidal Al hydroxides (Al-part), which coagulate dissolved organic carbon and adsorb orthophosphate in the water column. The Al hydroxides settle and increase the P sorption capacity of the sediment. The elevated content of fresh Al oxyhydroxides in the surface sediment reduces P release to the water column after orthophosphate liberation from ferric oxyhydroxides during anoxia, because Al-part complexes are not sensitive to redox changes. The example of Plesne Lake shows that atmospheric acidification of a watershed-lake ecosystem may provide the natural conditions for in-lake P inactivation similar to the lake restoration technique based on the artificial addition of Al salt to the water column.