Summer temperatures attract people outside, and the bathing season has already begun in Berlin and many other cities. But there is one problem that can spoil the fun of bathing in bathing lakes: cyanobacteria, also known as blue-green algae. Some species can contaminate water bodies with their toxins – sometimes the authorities are forced to close bathing areas.
So far, pesky blue-green algae have been combated by limiting phosphate inputs from agricultural wastewater. A new study in Science now shows that reducing phosphorous can make lakes more toxic and stimulate the growth of dangerous blue-green algae species.
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In analyzing 103 probes with more than 700 experiments, scientist Verdi Hellweger and his research team from the Technical University of Berlin showed that authorities should always reduce nitrogen input.
The researchers applied the model they developed to Lake Erie on the border between the United States and Canada. The scientists simulated the growth of cyanobacteria of the genus Microcystis in scenarios with less nitrogen, less phosphorous, or less than both. Certain species of the genus produce the toxin.
The result: In the scenarios examined, the lake cleared more effectively when both nitrogen and phosphorous were reduced by 40 percent. If only the nitrogen intake had decreased, the lake was also slightly less microcystine and harbored fewer blue-green algae.
“A major turning point in the management of the water body”
Third scenario: With less phosphorous, fewer cyanobacteria form in the lake. But strains of poisonous blue-green algae remained, which had relatively more nitrogen and sunlight available for growth. They poisoned the lake more than algae in the other scenarios.
“If there are fewer blue-green algae, they are also less competitive for other nutrients, chief among them nitrogen, which is also limited,” Hellweger was quoted as saying in a statement from TU Berlin. This is an important building block for toxic microcysteine. As a result, reducing phosphorous leads to an increase in toxins in the lake.
“This discovery represents an important turning point for the management of water bodies,” Hellweger concludes. If politicians want to reduce the toxins of blue-green algae in lakes, they must reduce not only phosphorous, but also nitrogen inputs from wastewater and fertilizers from agriculture.
According to the researcher, this means that all rehabilitation and health maintenance programs for lakes are under scrutiny – because reducing nitrogen input into lakes means a significant extra effort.
Too much nitrogen and phosphorous from agriculture has polluted groundwater, rivers and lakes in Germany for decades. Thanks to improved wastewater treatment plants, inputs through wastewater have fallen sharply since around 1990, but are still high, the Federal Environment Agency writes.
Only every fourth lake is in good condition
That could change if farmers in Germany were to allocate smaller amounts and in a more targeted manner. The farmers’ lobby rejects strict fertilizer regulations and warns of declining crop yields.
The results of the new study come at a time when German waters often remain in poor condition. Only one in four of Germany’s 732 lakes registered is in good or very good ecological condition, data from the Federal Environment Agency shows.
In 2015, the agency rated most recorded lakes as average (34 percent), unsatisfactory (26.2 percent) or poor (10.5 percent). The main reason for assessing poor condition is the high concentrations of nitrogen and phosphorous in the lakes and the resulting algal blooms. The standards are based on the European Water Framework Directive.
According to this directive, the authorities should ensure that all German lakes are in good or very good condition by 2027. Then the blue-green toxic algae should not spoil the pleasure of bathing.