Can phosphate help acidified lakes to recover?

Experimental addition of phosphate to enclosures in an acidified lake in Southern Norway was performed to study the effect on nitrate, pH and labile aluminium along a gradient of phosphate from 4-19 mug P L-1. Nitrate decreased from 180 mug L-1 to below detection limit after three weeks at P-concentrations > 17 mug L-1, due to phytoplankton uptake. pH increased from 4.9 to 5.2, corresponding to a 50% decrease of H+-equivalents from 12 to 6 mu eq L-1 due to algal uptake of H+-ions when assimilating NO3--ions. Due to the increased pH and probably also precipitation with phosphate, concentrations of labile aluminium decreased from 150 to 100 mug L-1 within the P-interval 4-19 mug L-1. Algal biomass increased from 0.5 to 6 mug chlorophyll a L-1 along the same P-gradient. The results suggest that moderate P-addition (< 15 mug P L-1 to avoid eutrophication problems) can improve water quality in moderately acidified lakes, and also increase nitrate retention in strongly acidified lakes. In humic lakes, the treatment will be less efficient due to light limitation of primary production and the presence of organic acids.