Experimental addition of phosphate to enclosures in an acidified lake in So
uthern Norway was performed to study the effect on nitrate, pH and labile a
luminium along a gradient of phosphate from 4-19 mug P L-1. Nitrate decreas
ed from 180 mug L-1 to below detection limit after three weeks at P-concent
rations > 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-interva
l 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 i
n moderately acidified lakes, and also increase nitrate retention in strong
ly acidified lakes. In humic lakes, the treatment will be less efficient du
e to light limitation of primary production and the presence of organic aci
ds.