Published on: 2.12.2019

Information produced by Finnish Environment Institute

Is a lake thriving or not?

The waters of a lake look clear and blue. Does this mean that the lake is in a good condition? How is the status of a lake or river actually defined?

There are many types of lakes and rivers as well as coastal waters. While some lakes can be naturally clear, the waters in another may look brown because there are many mires and other peatlands in its catchment. The water in a river flowing through clay soils often looks grey and turbid.

The status of water bodies is today assessed on the same criteria based on their natural condition across the EU. In practice this means that a lake with brown waters or a turbid river can also be classified as having a good ecological status if the stain or turbidity occur for natural reasons.

The essential point is anthropogenic impacts on aquatic environments. Have wastewaters caused eutrophication in a nutrient-poor lake? Is a dam built in a river a barrier that prevents fish migrations? Has the benthic community lost diversity or disappeared altogether because of oxygen depletion driven by pollution?

If changes of this type have not been observed or they are mild, the ecological status of the lake or river can be considered good, whereas major changes indicate a less favourable status.

Image: © Erkki Kettunen, Vastavalo