Evaluation of aluminum treatment efficiency on water quality and internal phosphorus cycling in six Danish lakes

If dosed sufficiently relative to the pool of excess phosphorus (P) in the lake sediment and lake water, aluminum (Al) is hypothesized to block sediment P release and thereby improve water clarity in eutrophic lakes. Aluminum has been used in > 200 lakes worldwide, but in some 30% of the known cases water clarity did not improve beyond the first year, likely because of underdosing or continued high external P loading. We evaluate the viability of the aluminum dosing hypothesis for six Danish lakes that received Al in a molar ratio of < 2-10 relative to the potentially mobile P pool. Lake water total P declined to values that could be predicted from the external P loading but increased again in the two lakes receiving the lowest Al doses. Lasting (> 2-3 years) improvement of water clarity was only observed in two lakes. Sediment P release rates, which were measured in four lakes, declined significantly but then increased gradually again in the two lakes receiving the lowest doses. We conclude that if external P-loading is sufficiently low, lasting improvement of water quality can be obtained if Al is dosed 10:1 or higher relative to the size of the mobile P pool.