More poison than before?: Blue-green algae bloom not only dependent on phosphorus

Every year there are massive blue-green algae blooms that poison water bodies.

More poison than before?: Blue-green algae bloom not only dependent on phosphorus

Every year there are massive blue-green algae blooms that poison water bodies. A case in 2014 in Lake Erie caused a sensation. As a countermeasure, Canada and the USA want to curb the phosphorus supply. But that could make things worse.

Authorities should exercise caution when tackling blue-green algae - especially in Lake Erie between the US and Canada. Because the action planned by both countries against the cyanobacteria there could prove to be counterproductive and significantly worsen the toxin problem. A German-American research team led by Ferdi Hellweger from the Technical University of Berlin warns of this in the journal Science.

Cyanobacteria, commonly known as blue-green algae, multiply en masse in many bodies of water in summer. Because they can produce various toxins that endanger the health of humans and animals, bathing bans are also regularly imposed in Germany. In the summer of 2014, the situation in the city of Toledo in the US state of Ohio on Lake Erie - one of the five Great Lakes - attracted particular attention. At that time, residents were not allowed to use tap water for drinking, brushing their teeth or cooking.

In Lake Erie, the blue-green algae Microcystis produced particularly large amounts of the toxin microcystin (MC). In order to prevent this, the USA and Canada have committed to reducing the amount of the nutrient phosphorus (P) in the lake by 40 percent by 2025 - through appropriate regulations, for example on fertilization.

Hellweger's team is now warning that this plan could have unexpected consequences. They first evaluated more than 700 experiments and then tested the consequences of the phosphorus reduction in a new model. In this so-called agent-based simulation, the behavior of various blue-green algae individuals belonging to different Microcystis groups and producing different amounts of the MC toxin is simulated.

According to this, a reduction in the phosphorus supply could reduce the absolute number of cyanobacteria, but not necessarily the concentration of toxins in the lake. On the contrary: because the remaining Microcystis population would then have a much larger supply of light and nutrients, especially nitrogen (N).

Since nitrogen is an important building block for the MC molecule and light can also promote the formation of the toxin, according to the study those Microcystis strains that produce a particularly large amount of microcystin would benefit from this. The result would be even higher concentrations of toxins in the lake - largely independent of the temperature.

"Less phosphorus in the water reduces the amount of blue-green algae and thus the amount of poison, that was the simple formula for water management," Hellweger is quoted as saying in a statement from his university. However, the reality is more complex. "When 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. And nitrogen, in turn, is an important building block for the MC molecule."

Therefore, Hellweger suggests for the management of water bodies: "If you want to reduce the toxins of blue-green algae, you not only have to reduce the input of phosphorus into the lakes, but also of nitrogen, which is also used in large quantities as fertilizer in agriculture. "

In a "Science" commentary, Irina Ofiţeru from Newcastle University and Cristian Picioreanu from the King Abdullah University of Science and Technology in Thuwal, Saudi Arabia, write that Hellweger's team undertook an enormous feat with its complex simulation to obtain experimental data collect and combine. The study shows that advice should also be given on limiting nitrogen intake.

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