“Cyanobacteria and Safety in Schultheis-Weiher: How to Avoid Toxins in Bathing Water”

At Schultheis-Weiher, all preparations for the new bathing season have been completed in good time. The fire brigade laid out the chain of buoys in the water, the GBM cleaned the sandy beach and the city health department carried out the first microbiological tests in accordance with the bathing water ordinance. Swimming has therefore been allowed again since May 1st.

However, already at the beginning of the bathing season, there is an increased occurrence of blue-green algae in the Schultheis-Weiher. Contrary to what the name might suggest, these microorganisms are not algae, but bacteria. They are cyanobacteria that drive photosynthesis and are easily recognizable by the typical green streaks on the water surface. Due to the effects of wind, they can temporarily occur more frequently on sections of the bank. The basis for the mass development of cyanobacteria is the accumulation of nutrients in a body of water, the so-called eutrophication. A problem that is exacerbated by the effects of climate change, such as longer periods of heat and less precipitation.

Toxic substances hazardous to human and animal health

Some of the cyanobacteria can produce toxins that can be harmful to humans and animals. Skin contact can cause allergic reactions. Swallowing water with cyanobacteria can lead to stomach and intestinal infections. It is therefore recommended to avoid areas with an increased density of blue-green algae.

According to the recommendation of the Federal Environment Agency, the occurrence of cyanobacteria is assessed in three stages. The first stage has now been reached and increased attention is required. At level two, warnings are posted on site, but bathing is still allowed. Level three means a temporary bathing ban; in the water there are then large areas with a closed layer of framed cyanobacteria (“carpet”) and/or a pronounced green turbidity with visibility depths of less than 0.5 metres.

Plant for eliminating phosphates in the Schultheis-Weiher

The city of Offenbach has been operating a phosphate removal plant on the south bank since April last year. The system counteracts the accumulation of phosphates in the Schultheis-Weiher and has already removed around 30 kilograms of phosphorus from the water in 2022, thereby helping to relieve the situation. At the same time, the nutrients introduced into the groundwater inflow are examined. Since the groundwater is the primary entry path for nutrients into the Schultheis-Weiher, the uses in the groundwater catchment area must be considered accordingly, with the aim of developing measures with which new entries into the water body via the groundwater path can be avoided or reduced in the long term.

While heat and drought often pose a problem for the Schultheis-Weiher ecosystem in the summer months, the heavy rainfall in spring this year contributed to the early mass proliferation of cyanobacteria. In the rainy months of March and April, a rapid rise in the groundwater level in the catchment area of ​​the Schultheis-Weiher was observed. In Offenbach, 207 percent and April 126 percent of precipitation fell in comparison to the long-term average. In the course of this, at the beginning of the vegetation period, more nutrients were introduced into the water body via a larger inflow of groundwater. In addition, fewer taller aquatic plants are growing in the Schultheis Weiher this year, which has reduced the competitive pressure on the blue-green algae.

The bathing season at Schultheis-Weiher lasts from May 1st to September 15th. During this time there is a security service with lifeguards on site.

(Text: PM City of Offenbach)