Lake Ormstrup, Denmark
Biomanipulation and sediment dredging with P-recycling.
Site description
Lake Ormstrup is 11 ha large lake with a mean depth of 3.2 m and a maximum depth of 5.5 m situated in central Jutland, Denmark. The hydraulic (and external nutrient) loading is low and absent most summer periods. The lake is highly eutrophic with blooms of cyanobacteria, high chlorophyll concentrations, low Secchi depth and very high phosphorus concentration (around 0.5 mg/l). The phosphorus mainly originates from a sediment pool which partly was created when the lake 30-40 years ago was used for stocking and feeding of up to 3,000 mallard ducks each year. Paleo-ecological research has documented that the lake used to be mesotrophic or even oligotrophic.
Restoration measures applied
The lake has been restored by biomanipulation by removing mainly roach (Rutilus rutilus) in 2021-2024 and in 2024 by removing the upper 20 cm of the sediment. Effects of the biomanipulation has been few, although submerged macrophytes have now started to spread and grow in shallow parts of the lake. In the summer of 2024, the lake has mainly been turbid and with frequent blooms of cyanobacteria.
Current goals of restoration at the site
The make the lake clears and in at least good ecological quality as required by the European Water Framework Directive. The investigate the results from the restorations and transpose the learnings to other Danish lakes.
Sectors involved in lake basin management
Local landowner, used by schools and students for teaching. Danish EPA and local municipality involved.
Further reading
Davidson, T. A., Søndergaard, M., Audet, J., Levi, E., Esposito, C., Bucak, T., and Nielsen, A. 2024: Temporary stratification promotes large greenhouse gas emissions in a shallow eutrophic lake, Biogeosciences, 21, 93–107, https://doi.org/10.5194/bg-21-93-2024.
Søndergaard, M., Nielsen, A., Skov, C. et al. Temporarily and frequently occurring summer stratification and its effects on nutrient dynamics, greenhouse gas emission and fish habitat use: case study from Lake Ormstrup (Denmark). Hydrobiologia 850, 65–79 (2023). https://doi.org/10.1007/s10750-022-05039-9.
