Summer temperatures lure people outside, the bathing season has already begun in Berlin and many other cities. But one problem can spoil the fun at bathing lakes: cyanobacteria, also known as blue-green algae. Certain species can pollute water bodies with their toxins – and authorities sometimes have to close bathing areas.
So far, the annoying blue-green algae have been combated by limiting the phosphate input from wastewater and agriculture. A new study in the journal Science now shows that reducing phosphorus can actually make lakes more toxic and stimulate the growth of dangerous blue-green algae species.
In an analysis of 103 investigations with more than 700 experiments, the scientist Ferdi Hellweger and his research team from the Technical University of Berlin have shown that authorities must therefore always reduce the nitrogen input.
The researchers applied the model they developed to Lake Erie on the border between the USA and Canada. The scientists simulated the growth of cyanobacteria of the genus Microcystis in scenarios with less nitrogen, less phosphorus, or less of both. Certain species of the genus produce the toxin microcystin.
The result: in the scenarios examined, the lake was most effectively decontaminated when both nitrogen and phosphorus were reduced by 40 percent. If only the nitrogen input decreased, the lake was also slightly less poisoned with microcystin and harbored fewer blue-green algae.
Third scenario: With only less phosphorus, fewer cyanobacteria formed in the lake. But particularly toxic blue-green algae strains remained, which then had relatively more nitrogen and sunlight available for growth. They poisoned the lake even more than the algae in the other scenarios.
“If there are fewer blue-green algae, they also have less to compete for the other nutrients, the most important of which is nitrogen, which is also limited,” Hellweger is quoted as saying in a statement from the TU Berlin. This is an important building block for the poison microcystin. As a result, a phosphorus reduction leads to more toxins in the lake.
“This finding represents an important turning point for the management of water bodies,” concludes Hellweger. If politicians want to reduce the toxins of blue-green algae in lakes, they not only have to reduce phosphorus, but also the nitrogen input from wastewater and fertilizer from agriculture.
According to the researcher, this means that practically all programs for the rehabilitation and health maintenance of lakes are under scrutiny – because reducing the nitrogen input into lakes means a significant additional effort.
Too much nitrogen and phosphorus from agriculture has been polluting groundwater, rivers and lakes in Germany for decades. Thanks to better sewage treatment plants, the inputs through wastewater have been declining sharply since around 1990, but are still high, as the Federal Environment Agency writes.
That could change if farmers in Germany would fertilize less and in a more targeted manner. The farmers’ lobby rejects strict fertilizer regulations and warns of poorer and shrinking crop yields.
The new study results come at a time when German waters are still often in a poor condition. Just one in four of the 732 German lakes recorded is in good or very good ecological condition, as data from the Federal Environment Agency show.
In 2015, the authority rated most of the lakes recorded as moderate (34 percent), unsatisfactory (26.2 percent) or bad (10.5 percent). The main reason for poor status assessments are high nitrogen and phosphorus concentrations in the lakes and the resulting algal blooms. The standards are based on the European Water Framework Directive.
According to this directive, the authorities must ensure that all German lakes are in good or very good condition by 2027. Then toxic blue-green algae should hardly spoil the bathing fun.