Hundreds of dead fish and birds had to be removed from the lake by the staff of the City Parks Department last summer. They died of botulism, which gets its name from the nerve poison botulinum toxin, which is also used in Botox syringes. Bacteria are excreted when there is a lack of oxygen in the water. Animals that ingest the toxin are paralyzed, which eventually causes them to drown or die on land.

Certain factors accelerate the growth of the poison, for example, rotting bread residues extract oxygen from the lake. The same applies to algae, which spread rapidly at warm temperatures. "The situation in the Rheinaue is currently stable," says Markus Schmitz of the city press office. The oxygen content of the water has improved as a result of the low temperatures and reduced sunlight. "The last two cases of avian botulism were found in two out of three waterfowl examined at the beginning of October. The Parks Department regularly inspects the Rheinaue lake.

According to a citizens' petition, which will be discussed in a local citizens' participation committee on Thursday, January 24, three dead swans were found in the last week of December. "We do not know of cases of waterfowl botulism at the Rheinaue lake during this time," explains Schmitz. The petition criticizes the actions of the administration. "One could improve the situation substantially with limited resources and little effort," it says. It is suggested that the sludge in the shallow corners of the lake be removed and the reed bed replanted in the spring. Further, the barbed wire around the bird island should be removed so the birds are not injured. Partially paralyzed birds could still reach the higher shore via small ramps in the water, which would prevent their carcasses from floating in the lake.

But the plans for the city look different - or are not finished yet. Originally, measures were to be taken at the end of 2018. These included the removal of sediment layers or creating a ventilation system in the lake, neither of which was carried out. Instead, signs were erected on the shores prohibiting the feeding of the animals. In addition, the city wanted to grow a green zone around the lake so that the birds would no longer stop there. "But the hot summer has caused the reeds to recede," says Schmitz.

Before a decision is made on restoration methods, those responsible want to observe the lake over a longer period of time - throughout all seasons. "These analyses have begun and will continue into the third quarter of 2019. Among other things, a phosphorus balance must be carried out because phosphorus influences the growth of aquatic plants. "In addition, surveys on blue-green algae are necessary, as these can release toxins in relation to the species, which can be a possible cause of animal deaths," said Schmitz. After the studies are completed, the researchers hope to develop concepts that will be implemented in the second half of 2020. The money for this will be included in the 2019/2020 budget.

(Orig. text: Nicolas Ottersbach, Translation: Carol Kloeppel)