Council investigates causes Hundreds of animals perish in Rheinauen lake
BONN · The Bonn city council is removing dead fish and birds from the lake in the Rheinaue. A specialised institute was commissioned to investigate the cause for the many deaths.
The council staff members are using nets to remove several hundreds of dead animals from the water since Wednesday. On Wednesday alone they picked 250 dead fish and six dead ducks out of the lake, on Thursday another 30 dead fish and eleven dead ducks.
„According to the environmental agency we have to suspect that toxic areas in the lake are causing the wild animals to die. The cause could be organic deposits in the water“, explains Stefanie Zießnitz, press officer for the city council. Those deposits can appear when the animals get fed by walkers. „People think that they are doing the animals a favour by feeding them - but they don’t“, adds Michael Mäckel, who runs the boat rental place at the Rheinaue lake.
Because of additional feed sources the animals breed more than under natural conditions. The consequence: The waters get soiled by the faeces of the animals; worst case scenario: The lake might turn toxic. Additionally, falling leaves of the surrounding trees sink to the bottom of the lake. In warmer temperatures the oxygen levels of the lake decrease because of escape through the surface. At the same time the sediment and mud below start to ferment and also to extract oxygen from the water, then bacteria begin to multiply.
„The lower the oxygen levels the higher the risk of poisonous substances appearing“, explains graduate biologist Peter Schmidt of the Biological Station Bonn/Rhein-Erft. Methane, hydrogen sulphide and botulinic toxins can appear.
Probes are being analysed
To determine the exact cause why the animals perished, the city council commissioned a specialised institute to investigate. The Friedrich-Löffler-Institut, which is the Federal Institute for Scientific Reserach, has been contacted. But poisoning symptoms are hard to prove, apparently. Other than in swimming lakes and flowing waters, lakes in parks do not get checked regularly by water probes being taken. The city council now wants the experts to examine water probes.
To find dead water birds at the lake’s shores is not necessarily unusual, says Alexander Heyd, chairman of the Nature Conservation Association (Nabu) Bonn, especially during breeding season in June, but only if they are young animals. „If the dead animals are older, their dying is caused by different reasons.“
The lake gets supplied with fresh water from the cooling system of the Post Towers by a pumping station since 2011. The system in turn gets its water from a well. Both pumps feed about a million cubic meters of water into the lake, annually. The council department of green areas has also installed two pumps.
Mid-April the council had ordered the lake to be „mowed down“ with special boats to contain the algae production. A similar action is scheduled for the autumn period. Two possibile solutions are being examined currently: Removing the sediment level or installing ventilation cabins. In addition, the city council will put up signs to instruct visitors of the Rheinaue to not feed the animals and to warn of the consequences. „It has not been considered to drain the entire lake yet“, said Zießnitz.
One option might be to flatten the banks of the lake in order to improve the water quality, according to Heyd. In natural shore areas, water plants could be domiciled there, which would filter the pollutants from the water and improve the oxygen levels in the lake.
(Original text: Sabrina Bauer / Translation: Mareike Graepel)
