A brownified lake can be likened to a cup of tea in which the tea leaves colour the clear water. In a lake, the “teabag” consists of organic material in the surrounding land, such as plant residues and soil.
The lakes have become browner over the past few decades, according to Tina Martin, researcher in engineering geology at Lund University and project manager of the FORMAS-funded project “Reducing browning of lake water”.
Lake Bolmen is particularly important
Brownification leads to changes in the ecosystem and impairs drinking water – a challenge that is particularly important to address in the case of Lake Bolmen.
“Bolmen is Sweden’s tenth-largest lake and an important source of drinking water. We must use our combined forces to look after it,” says Tina Martin.
The project’s combined forces comprise researchers, public authorities, municipalities, local businesses around the lake, as well as Southern Sweden Water Supply’s Bolmen research station.
Climate change and ditches are contributing factors
Due to climate change, the winters are shorter and the period of plant growth has increased by three weeks over the past few decades.
Emma Kritzberg, professor of aquatic ecology at Lund University, explains that the warmer and wetter climate leads to transport of more organic material into the lake.
“Extreme weather such as downpours and storms have become increasingly common and heavy rain transports more material from the forest down into the lake water. So, the wetter it is, the browner the water becomes,” she says.
The rapid run-off into the lake is also affected by the ditches in forest land.
“Ditches in the forest make the ground close to the channels drier, making it easier for vehicles to get around the forest. But at the same time, the dikes form direct channels to the lake through which organic material is transported more rapidly into the lake water,” says Tina Martin.
Coniferous forest is taking over
A large proportion of the land around Lake Bolmen was previously wetland, which in addition to having water purifying properties also binds carbon dioxide – something that is important for slowing climate change.
Emma Kritzberg says that a substantial part of the wetlands has been diked and planted with coniferous forest.
“The soil under spruce forest releases more organic carbon than deciduous forest, so the more spruce trees there are close to the lake, the browner the water gets. The organic material accumulates over time, and the older the forest, the more the organic material can leak into the water,” she says.
Method development is proceeding
The ongoing research project has a good picture of the problem and what affects the water in Lake Bolmen, and testing is under way to find methods that may help to reduce browning.
“One of the solutions may be to plant more deciduous trees near the lake. The organic material in deciduous forest and agricultural land breaks down much faster than in coniferous forest, and in contrast to coniferous forest it does not leak water to the same extent,” says Emma Kritzberg.
But this is not the only solution, and much work remains to be done to improve the ecosystem and make the lake water clearer – something that in the long term would mean the water treatment plants use a smaller volume of chemicals in the purification process and that certain algae do not spread to the same extent.
With reference to the work of research colleague Antonia Liess at Halmstad University, Tina Martin comments that one algae type that could be reduced is brown water algaes – to which the algae gonyostomum semen belongs. An algae with slimy properties and a not so pleasant name.
“Taking a dip in brown lake water with gonyostomum semen on the surface is not something I hanker after,” says Tina Martin.