Overview - Restoration
Many of our lakes have poor water quality, with turbid water and large amounts of phytoplankton, little or no bottom vegetation and a fish population dominated by roach and bream. This has been attributed to the long-term input of nutrients (phosphorus and nitrogen) from the drainage basin. In order to improve the situation, it is necessary to reduce the external nutrient load as much as possible. Even if this load has been reduced with waste-water purification and regulation of agricultural practices, only a few lakes have returned to a clear water state.
This inertia may have both biologic and chemical causes. The biologic inertia is often attributed to a “skewed” fish population, dominated by roach and bream. These so-called white fish (or “trash fish”) eat animal plankton, which are thus unable to keep the phytoplankton in check. Thus, the lakes appear greenish and turbid. The chemical inertia is due to the pooling of phosphorous in the sediment, which derives from past accumulations that steadily release phosphorous into lake water.
Once the external load is brought under control, lake restoration may be necessary to accelerate water quality improvements. A distinction is made between biologic restoration methods (also known as biomanipulation) and physical-chemical restoration methods.
The biologic restoration methods may involve intervention in fish populations, such as the removal of trash fish and stocking with predatory fish. Generally, this is accomplished by releasing pike fry or perch to reduce the population of trash fish. Submergent aquatic plants may be transplanted in order to hasten and maintain water clarity, since it stabilises sediment, consumes nutrients and conceal animal plankton.
The physical-chemical restoration methods encompass methods that bind to or completely remove the phosphorous reservoirs in the sediment. This may involve oxygenation of the bottom water and sediment, the addition of aluminium to the lake water or dredging the sediment.
