New, healthy lakes in Denmark

In recent years, many new lakes have been established or restored in Denmark, and according to biologists at SDU, we can learn a lot from them.

They have analyzed all the newly established lakes on which they could find data; ca. 200 lakes. Of these, they have conducted their own, extensive surveys in 30 lakes. The work is published in MDPI.

- This analysis work has given us insight in how to best approach the job of establishing a new lake, explains Theis Kragh, associate professor at the Department of Biology.

According to his research colleague, Sara Egemose, who is also an associate professor at the Department of Biology, they have now acquired so much knowledge that it is possible to predict the water quality in the individual new or restore lake, and this is a major step forward.

Nutrients from the catchment area

Two of the most common reasons for building a new lake are that you want to increase biodiversity or retain nutrients from the catchment area.

In both cases, nutrients from the catchment area are one of the first things to look at, Kragh and Egemose believe.

- We have always known this but based on our collected data on land use in the catchment area, slopes towards the lake, wind and precipitation models, etc., we can now with unprecedented precision calculate, for example, how much nutrients are washed out in a new lake, how it will take place and where in the lake it will stored, says Theis Kragh.

For example, a wind model can predict how nutrient-rich sediment will store itself in the lake, and if you know that, you can remove it afterwards, simply by digging it up and reusing it as fertilizer.

Mini wetlands

You can also install so-called mini-wetlands or other remedial measures in the inlets to the lakes, where the nutrients can be metabolized or accumulated instead of ending up in the lake water.

This approach is valuable when establishing a lake to deal with the catchment area's nutrient runoff - especially nitrogen, but also phosphorus. The difference between success and failure can be very simple conditions, the researchers state:

In Lake Filsø in Jutland, for example, the lake's outlet has not been dug out, allowing the water level to rise and fall with the seasons because it is not artificially kept the same all year round.

- This means, for example, that the plant Baldelia repens is now doing well in the lake. It does not like the same water level all year round but would like to be flooded in the winter - as it does now, Theis Kragh explains.

If you stop fertilizing

Especially the accumulation of the nutrients nitrogen and phosphorus makes it difficult to achieve a good biodiversity in new lakes. Quite often, new lakes are placed on or near previously cultivated lands where nitrogen and phosphorus have been included in the fertilizer.

If you stop fertilizing the area, the accumulated nitrogen will disappear relatively quickly, but it is a different matter with phosphorus: It is released only slowly from soil, and it can take many years before the phosphorus has been taken up by growing plants and thus removed from the earth.

- With our new knowledge, we can now better predict the condition of the new lakes of the future, and thus we can make the necessary measures before the lakes are established. In this way, we can achieve the best possible state of nature in the lakes in the given catchment area, says Sara Egemose.