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Spruce can make clear lakes turn brown

Clear water from the lake Fiolen in Småland. Brown water from a stream that runs through the surrounding forest and into the lake. Photo: Emma Kritzberg
Clear water from the lake Fiolen in Småland. Brown water from a stream that runs through the surrounding forest and into the lake. Photo: Emma Kritzberg
When clear lakes turn brown it is problematic for the quality of drinking water as well as entire ecosystems. New research shows that land use most likely contributes to the so-called brownification.

When acidification peaked in Sweden in the 1980s it marked the start of regularly measuring the quality of the water. Data that has been collected ever since shows that lakes rapidly turn brown.

Until now it was a problem that the quality of the water before the 1980s was unknown. But now Emma Kritzberg, associate professor at the Department of Biology at Lund University, has put together a set of data that stretches back 80 years.

In order to do this, she has used records from the limnological field station in Aneboda in Småland, Sweden. These records go back to the 1930s and have never before been put together into one single set of data.

Using the data Emma Kritzberg shows that water was clearer before acidification than it is today. She can also see that the water started to turn brown long before acidification in the 1980s.

”A lot of people have claimed that brownification is a regression to a more natural condition. But my results show it is not. Instead, I believe land use is a very important factor”, Emma Kritzberg says.

The conclusion is drawn from how the landscape surrounding the lakes at Aneboda started to change more than a hundred years ago. Before the twentieth century spruce trees grew on approximately 15 per cent of the land, but then it changed. Today the corresponding figure is 65 per cent. The same shift in land use can be seen all over northern Europe.

”We know that spruce and pine make water turn brown. The reason why the change in colour happens so long after the trees were planted is probably because it takes several decades to build up layers of organic material in the ground”, says Emma Kritzberg.

If her theory is correct there could be a ”cure” for brownification.

”It is speculation, but it seems likely that if you grow less conifer and more leafy trees close to surface water it would lead to clearer water in due time.”

 

Text: Jan Olsson

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