Smell of rotten eggs Pond in the Kottenforst has stagnated due to persistent drought
Röttgen · It has hardly rained for weeks. Now the deer pond in the Kottenforst has stagnated. In the future, this could be prevented - but it might prove too costly at the deer pond.
A green, thick layer of algae covers the deer pond in the Kottenforst near Röttgen. It smells like rotten eggs. The pond has turned stagnant, explains Stephan Schütte of the Rhine-Sieg-Erft Regional Forestry Office. The persistent drought is also making itself felt in the Kottenforst.
Stagnant bodies of water, such as the Hirschweiher, can sometimes occur in the summer, says Schütte. In recent years, however, this has not happened. "We had enough rain," explains the expert. In the summers of 2018 and 2019, however, the pond also smelled of sulphur. "That is a sign that the pond does not contain enough oxygen." The pond could also stagnate more frequently in the future if dry summers become more frequent due to climate change.
Other ponds in the Kottenforst are drying out
Many other water bodies in the Kottenforst are lower than the deer pond. The pond, on the other hand, is between three and five metres deep, says Peter Tröltzsch of the Bonn Rhine/Erft Biological Station. "The other ponds dry out in summer, so they can't turn stagnant," he explains. However, if there is no rain for a long time, too much organic material, such as leaves, branches, algae or dead animals, accumulates at the bottom of the pond. When bacteria in the water decompose this material, they draw oxygen from the water. This means that if there is too much organic material in the water, there will eventually be too little oxygen in the water. As a result, the pond stagnates.
The Hirschweiher is located in a very nutrient-rich area of the Kottenforest; organic material has been accumulating in the pond for decades. That is why it is very susceptible to oversaturation.
"Actually, we wanted to renaturalise the lake a long time ago, i.e. remove the organic material with an excavator and then distribute this material around the lake," says Tröltzsch.
At the time, they had examined this as part of the so-called Life+ project. However, the project would have cost too much, despite EU funding for the project. A measurement of the water quality at the time showed that the water was polluted with heavy metals. Therefore, the dredged material would have had to be disposed of as hazardous waste. "The contamination probably comes from ammunition and bomb remnants from the Second World War. But we have not been able to check this exactly yet," says Tröltzsch. This disposal would have cost most of the planned 35,000 euros. So the pond is still contaminated.
As part of the so-called Life+ project, the regional forestry office renaturalised some water bodies in the Kottenforst. The forest is now a species protection area for amphibians of all kinds. Tröltzsch therefore wishes that something would be done soon to protect the deer pond. "The toads' spawn probably died when the lake stagnated," says Tröltzsch.
Animals in the forest look for other watering holes
However, he says it is otherwise harmless to the environment when a pond turns stagnant. "The animals that would normally drink from the pond are smart and go to other bodies of water," Schütte knows. And as soon as the temperatures drop again in autumn and it rains more, the pond regenerates.
Original text: Anton Dieckhoff
Translation: Mareike Graepel
