Testing aluminum addition as a tool for lake restoration in shallow, eutrophic Lake Sonderby, Denmark

Internal phosphorus (P) loading in shallow lakes may delay recovery from previous high external loading for decades. In Lake Sonderby (8 ha, mean depth 2.8 m), Fyn County, Denmark, this is especially true because there is no run-off of lake water during summer when high lake water P concentrations prevail. External P-loading was reduced in 1983 but still in 2001 total P reached 1.5 mg l(-1) during summer. In the present study, we determined the size of the potential mobile P-pool in the sediment by use of a sequential extraction procedure. The potentially mobile P-pool (P-Mobile): Loosely adsorbed inorganic o-P (P-Water), iron-bound o-P (P-BD), and labile organic P (Org-P-Labile) made up 14.6 g m(-2) and constituted 40% of the Tot-P in the upper 10 cm sediment. Then, we tested if molar binding ratios of 4:1 or 8:1 between aluminum (Al) and potential mobile P could reduce sediment P-release at suboxic conditions. An Al:P ratio of 4:1 was enough (p<0.05; n=5) to trap all P in the sediment during a 39 days experiment with undisturbed sediment cores. This dosage was then tested in enclosures in the lake during a 30 days summer experiment with 3 replicate enclosures (1.1 m(2), 2.1 m high) for each of the 2 treatments (Al addition, and control). o-P and Tot-P dropped from 1.28 mg l(-1) to <0.006 and 0.02 mg l(-1), respectively, in the Al treated enclosures but remained high in both control enclosures and in the lake water. Secchi depth increased from 0.8 m to the bottom upon Al treatment but declined to around 1.5 m after 4 days. By the end of the enclosure experiment, sediment analyses showed increased fractions of P-->NaOH in the surface sediment of Al treated enclosures. We conclude that Al addition was able to combat internal P-loading in the shallow lake when added in a molar ratio of 4:1 relative to the mobile P-pool in sediment and lake water. Meanwhile, temporary suppression of Daphnia grazing may be expected upon Al addition.