One in five Finnish lakes does not reach good ecological status, with agriculture and forestry placing the greatest pressure on freshwater ecosystems. Researchers are also warning that lakes have become visibly darker since the 1990s, a change whose ecological consequences are still not adequately reflected in Finland’s official monitoring system.
The national classification measures how far a lake has been altered by human activity. It assesses fish, aquatic plants, bottom-dwelling organisms and several chemical indicators, including nutrient concentrations. Lakes classified below good status may suffer from eutrophication, algae blooms, excessive vegetation and declining biodiversity.
According to Finland’s official environmental information service, around 82 percent of the country’s total lake area is currently classified as being in good or high ecological condition. The remaining share falls below the environmental objective set under the EU’s Water Framework Directive. (Vesi.fi)
Agriculture remains the main pressure on Finnish lakes
The condition of a lake largely depends on how the surrounding land is used. Water bodies tend to be in poorer condition in areas with more agricultural land, managed forests and human settlement.
Runoff carries soil, nitrogen and phosphorus into rivers and lakes. These nutrients feed algae and aquatic vegetation, disrupting the balance of freshwater ecosystems and contributing to eutrophication.
Agriculture remains the largest source of nutrient pressure, despite measures such as applying fertilisers at more suitable times and creating buffer zones between fields and waterways. The Finnish Environment Institute (Suomen ympäristökeskus, Syke) describes diffuse pollution from agriculture and forestry as the most widespread factor weakening the condition of Finnish waters. (Syke)
Forestry has also left a long environmental legacy. More than one million kilometres of drainage ditches have been dug into Finland’s peatlands and forests to improve conditions for timber production. These networks allow organic material, sediment and nutrients to move more easily into nearby waters.
Small lakes are particularly vulnerable because relatively limited changes in nutrient or sediment loads can alter their ecological balance.
Finnish lakes are becoming darker
Alongside nutrient pollution, researchers are examining another change that remains less visible in official statistics: the brownification of Finnish lakes.
Finnish lake water has become noticeably darker since the 1990s. Brownification occurs when greater quantities of dissolved organic matter, particularly humus from decomposing vegetation and peat-rich soils, enter the water.
The phenomenon does not necessarily make water cloudy. Instead, it changes its colour and reduces the depth reached by sunlight. Researchers sometimes compare the distinction to a cup of tea: suspended particles may be filtered out, while the colour released by the tea remains in the water.
At Iso-Jurvo, a deep and unusually clear lake near Äänekoski in central Finland, visibility reportedly exceeded five metres during the 1970s. By the 2010s, it had fallen below four metres. The lake nevertheless retains the highest official ecological classification. (Yle)
This discrepancy illustrates a limitation in the current system. A lake may receive a good or high ecological rating even while its water is gradually becoming darker.
Brownification can reshape freshwater ecosystems
Reduced light penetration affects photosynthesis and changes the areas in which aquatic plants, algae and other organisms can survive. As conditions shift, some species may decline while others become more dominant.
In extreme cases, changes in biological production and decomposition can also contribute to oxygen loss near the bottom of a lake. Researchers warn that the cumulative effect may be a reduction in freshwater biodiversity and less stable ecosystems.
Brownification is taking place across many boreal regions of the Northern Hemisphere. In Finland, it is linked to several overlapping factors, including climate change, land drainage and the recovery from acid rain.
Acid deposition previously reduced the amount of dissolved organic carbon entering many lakes. As acid rain declined, part of that organic matter began returning to the water. At the same time, warmer temperatures, heavier rainfall and increasingly snow-free winters can increase runoff from forests, peatlands and agricultural areas.
Recent Finnish research has therefore focused on developing biological indicators that can show how brownification changes lake ecosystems. Syke’s updated classification guidance also recognises the need to distinguish the ecological effects caused by human pressures from broader changes in water colour. (Syke)
Good ecological status does not remove future risks
Finland’s lakes remain in better condition overall than its rivers and coastal waters. Nearly half of Finnish rivers and almost all coastal waters fail to reach the desired ecological level, according to the national assessment reported by Yle.
However, specialists warn that maintaining the present condition of lakes will require continued intervention. A good classification does not mean that a water body is untouched or protected from future deterioration.
Under the EU Water Framework Directive, ecological status is intended to represent the health and functioning of rivers, lakes and coastal waters, taking biological, chemical and physical conditions into account. European countries are required to work towards good status for their water bodies, with the current planning cycle running to 2027. (European Environment Agency)
Local restoration projects can reduce sediment and nutrient runoff through wetlands, buffer zones, small dams and structures placed in drainage channels. These measures may slow eutrophication, but researchers say there is still no established method capable of reversing brownification across Finland’s extensive network of drained forests and peatlands.
The finding that one in five lakes remains below good ecological status complicates Finland’s image as an almost untouched “land of a thousand lakes.” More importantly, the growing evidence on darker waters suggests that ecological deterioration may begin before it becomes visible in the indicators currently used to measure it.
