Despite all the pandemic-related restrictions, the Offenbach residents have just been able to look forward to a bathing season at the popular Schultheisweiher, when the health department has issued a precautionary bathing ban at the weekend.

The reason: residents had discovered a number of dead fish on the surface of the water, which experts described as a shallow lake.

It is a maximum of three meters deep and has neither inlet nor outlet, it was once created through gravel mining.

The pond is mainly fed by groundwater and precipitation and is part of a 27 hectare nature and bird sanctuary of European importance.

Jochen Remmert

Airport editor and correspondent Rhein-Main-Süd.

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    As Heike Hollerbach, head of the Offenbach Office for Environment, Energy and Climate Protection, explains, the strong temperature fluctuations in the past few days have resulted in large parts of the previously lush aquatic plants in the eleven hectare body of water having died off in a very short time.

    As a result, the oxygen content of the water has been reduced so much that a number of fish did not survive, as Hollerbach explained.

    According to a city spokeswoman, volunteer fire brigades and technical aid organizations were able to slightly improve the oxygen content from one milligram per liter to three to four milligrams by using powerful pumps.

    Eight milligrams per liter are considered a good value for a life-friendly environment.

    As Hollerbach further explained, with the help of the local fishing club it was possible to save many fish, including large catfish.

    "We are dealing with a marathon here"

    However, the head of the Environment and Climate Protection Office warned in an interview that with the current success, the rescue of the Schultheisweiher was far from over.

    “We're running a marathon here,” she said.

    Especially since the quick ventilation measures from the weekend, which prevented worse things from happening, remained on the surface in the sense of the word and could not reach the deeper layers of water, which are covered by warmer ones.

    According to Hollerbach, an essential step towards the permanent preservation of the pond and thus also of the diverse fauna and flora that has formed around the water has been taken with the decision to install a phosphate elimination system there.

    However, there are no such systems "off the shelf"; they are tailor-made one-offs.

    The city does not expect the system to be delivered and installed until autumn at the earliest.

    The device should also work through in winter.

    Put simply, it works like a vacuum cleaner that picks up phosphate-containing soil with a large hose, filters out the phosphate and pumps the cleaned material back into the lake.

    The phosphate content in the water of the lake increases mainly due to bird droppings.

    The effect is also reinforced by aquatic plants that die off because the immigrant American marsh crabs eat them.

    Dead plants increase the phosphate input

    The phosphate in turn stimulates the massive development of cyanobacteria. These have a fatal effect on the oxygen content of the water, which can cause fish to die. The cyanobacteria, also called blue-green algae, multiply very quickly at high temperatures and prevent the photosynthesis of the aquatic plants, i.e. the production of oxygen and glucose with the help of light, water and carbon dioxide. Due to the dead plants, the phosphate input increases further.

    But in the past week, according to the information, it was not heat, but sharply fallen temperatures that worsened the situation in the Schultheisweiher.

    The blue-green algae die at lower temperatures, and there is a massive entry of phosphate into the water.

    In the end, there is again a greatly increased consumption of oxygen, which is fatal for fish.

    For Hollerbach, the head of the environmental department, there is no question that the artificially created Schultheisweiher ecosystem, which has now been declared a nature reserve, cannot be preserved without constant human support.

    In the specific case this means: not without the installation of a phosphate filter system.