Mechanical resuspension of autochthonous calcite (Seekreide) failed to control internal phosphorus cycle in a eutrophic lake

The attempt to restore the highly eutrophic Lake Arendsee (Germany) by means of artificial resuspension of calcareous mud (Seekreide) and capping of the natural bottom sediments in 1995 did not show the expected effects of decreasing the phosphorus (P) content in the water body. To understand this, the sorption characteristics and the mechanisms of P uptake of Seekreide were studied with laboratory experiments. These experimental results were combined with model simulations at field conditions to evaluate the efficiency of Seekreide for controlling the P cycle in the lake. The littoral deposits of Seekreide in Lake Arendsee mainly consist of calcite (70–87% dw) and are characterised by low contents of organic matter (5–8% dw), total P (0.13–0.47 mg g−1 dw) and iron (3.2–3.8 mg g−1 dw). Phosphorus uptake under laboratory conditions could be described with Langmuir sorption isotherms. A substantial portion of this uptake was not due to sorption to the CaCO3 surfaces but to iron oxihydroxides, since most P was found in the reductant-soluble fraction (BD-P). Although the theoretical maximum of sorbed P ranged between 0.19 and 0.22 mg g−1 dw, no substantial P uptake occurred under field conditions. The model predicted that resuspension of about 25,000 tons of littoral sediment would transiently remove not more than 0.3 tons of P from the water column (1.3% of TPlake) during sedimentation. After deposition of resuspended Seekreide, anaerobic conditions prevented any additional P retention. The close agreement of laboratory based simulations with field observations indicates: (1) capping of the lake sediments did not significantly prevent P-release, and, therefore (2) the -negligible- total effect of the large scale treatment and the failure to decrease P concentration in Lake Arendsee may be explained by and predictable from P uptake experiments, only.