Long term effects of phosphorus precipitations with alum in hypereutrophic Lake Süsser See (Germany)

Precipitation and inactivation of phosphorus (P) by the addition of aluminum (Al) is a widely applied lake restoration measure. Many studies about short-term effects are reported, but little is known about long-term effects. To reduce this lack of knowledge we investigated the German hardwater lake, Süsser See, which was treated almost annually from 1977 to 1992 with aluminum sulfate (alum). We conducted sediment core experiments in laboratory, as well as in situ investigations of sediment and pore water. The treatment is clearly recognizable in sediment depth profiles as increases of total phosphorus, NaOH-extractable phosphorus, and NaOH-extractable aluminum. The molar ratio of added Al to additionally bound P is approximately 2.1:1. Pore water profiles of soluble reactive phosphorus taken in situ and in sediment core experiments, as well as sorption batch experiments, illustrate that the Al(OH)3 layerʹs sorptive capacity is still not exhausted with further P sorption occurring in different P fractions. The P release of the sediment is affected by the magnitude of the downward flux into the P sorbing layer. However, sediment core experiments and a modeling exercise indicate that a buried P sorbing layer has little or no effect on the P release of the uppermost fresh sediment layers.