Aarhus University Seal / Aarhus Universitets segl

Nr. 382: Guide to conducting sea restoration

Søndergaard, M., Reitzel, K., Jeppesen, E., Egemose, S., Lauridsen, T.L. & Jensen, H. 2020. Vejledning for gennemførelse af sørestaurering. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi,56 s. - Videnskabelig rapport nr. 382. http://dce2.au.dk/pub/SR382.pdf


The water quality and ecological state of Danish lakes are particularly determined by the extent of nutrient loading, of which phosphorus plays the most important role as a limiting nutrient. However, there are some situations where the lakes do not improve satisfactorily even though the supply of phosphorus has been reduced to a sufficiently low level. The reason may be chemical resilience in the lakes due to the release of phosphorus from a mobile phosphorus pool that has accumulated in the lake sediment (internal loading) or biological resilience due to, among other factors, a fish stock dominated by roach and bream.


In the river basin management plans, which describe, among other things, the effort needed to improve the ecological state of the Danish lakes, a number of lakes are planned to be restored to achieve at least a good ecological state. Restoration will be conducted where the phosphorus input to a lake has been sufficiently reduced, but where active intervention is required if the lake is to achieve good ecological status within 1-2 management periods. This report reviews the various options for restoring lakes and provides guidance on how the restoration process should be handled from preliminary investigation to implementation and monitoring. The report is an updated and revised version of a 2015 guide.


The preliminary investigation should include verification of the data on which a restoration initiative is to be based. This typically includes information on the external phosphorus loading, the lake’s equilibrium state, a description of the fish stock and an assessment of the internal phosphorus loading.


Restoration to control the internal phosphorus loading can be done by chemical phosphorus precipitation, which includes either the addition of aluminum salts or the modified clay product Phoslock © whose active substance is lanthanum. Phoslock has not yet been used in Denmark on a large scale, while aluminum treatment has been used in seven lakes since 2001. For both methods, it is important to have a good knowledge of the lake’s alkalinity to avoid toxic or other undesirable effects.


Thus, if use of aluminum is desired, it must be shown during the preliminary investigation that the alkalinity of the lake is higher than 1 meq/l and there must be no risk of the pH exceeding 8.5 in the first year after the treatment. For Phoslock, it is generally possible to treat lakes with an alkalinity lower than 1 meq/l if prior tests have shown that there are no problems with lanthanum concentrations in the lake water. The dosage of both products is calculated in relation to the potentially mobile phosphorus pool in the sediment and the concentration of phosphorus in the lake water. The internal phosphorus loading can also be limited by removing the phosphorus-containing sediment, but this is a relatively expensive method that has only been used once in Denmark in a larger lake.


In deep and stratified lakes, oxygenation of the bottom water has been used as a method to reduce the accumulation of phosphorus in the bottom water and improve the living conditions of a number of deep water organisms. The method has been used in six Danish lakes and is still used in a few lakes. It is a method that requires long-term treatment as well as a surplus of iron to bind the phosphorus, and there is a risk that the increased binding of phosphorus obtained by oxygenation cannot be maintained when oxygenation is ceased. The effect obtained on the general condition of the Danish lakes and the quality of the surface water by oxygenation of the bottom water is uncertain and in most cases limited or even lacking.


Biomanipulation by removal of planktivorous fish is a restoration method that can be used in lakes where zooplankton-eating fish such as roach and bream dominate and where external or internal phosphorus loading is not considered the cause of poor water quality. The method has been used in many lakes in recent decades to increase zooplankton grazing on the phytoplankton and to reduce the stirring of sediment caused by fish searching for food in the lake sediment. There are a number of different removal methods that require adaption to the individual lake. A number of other methods and techniques for lake restoration exist and some of the most relevant have been described. One of them is use of more methods in combination to optimise the overall effect. Climate change may impact the way in which future restorations can be carried out.


For all types of restoration, the ecological state of the lake must be monitored before, during and after the restoration to be able to document and possibly adapt the intervention. The report also estimates from Danish experience what it costs to implement the various types of restoration interventions.