Changed cycling of P, N, Si, and DOC in Danish Lake Nordborg after aluminum treatment

Loading, retention, and in-lake cycling of phosphorus (P), nitrogen, silica, and dissolved organic carbon (DOC) were studied 1 year before and 3 years after P-inactivation by aluminum (Al) hydroxide in Danish Lake Nordborg in 2006. Simultaneously, external P loading was reduced by 40% via establishment of precipitation ponds in two inlets. After Al treatment, the internal P loading (sediment P release) during summer declined 90%–94%, owing to adsorption to aluminum hydroxide. Also, silicate regeneration from the sediment was reduced by 69%–76%, and sediment oxygen uptake as well as ammonium release declined markedly. Consequently, lake water total P, dissolved inorganic P, silicate, and DOC decreased by 73%, 97%, 87%, and 46%, respectively. The Secchi depth increased in the summer period during the first post-treatment year, but declined afterwards to pre-treatment levels, even though the mean lake-water total P concentration during summer was reduced from ∼240 µg·L–1before treatment to 26–65 µg·L–1in the first three post-treatment years. We conclude that a further reduction in external P loading is needed to obtain the full effect of the Al treatment in Lake Nordborg.