Two years after golden algae killed hundreds of thousands of fish, snails and mussels in the Oder River, Polish authorities have in recent weeks battled to prevent a repeat of the ecological disaster. 

Hydrogen peroxide, a chemical more commonly associated with hair bleaching, has been pumped into a section of the Oder's tributaries to contain a recurrence of the golden algae — also known as prymnesium parvum — that have again killed tons of fish. 

Poland is not alone in facing this environmental challenge in its Oder River, which rises in the Czech Republic and flows down to form the Polish border with Germany.  Globally, algae blooms have increased significantly in frequency and intensity, according to research published last year in the science journal Nature.  

What is an algae bloom and why do they occur? 

Algae are microscopic organisms present in oceans, lakes and rivers. 

They are vital to aquatic ecosystems. But when they bloom rapidly out of control, certain types — such as that found in the Oder — produce toxins that can be harmful to fish, people and other parts of the ecosystem. 

Algae blooms occur due to a mixture of environmental factors including warmer temperatures, lots of light, and increased nutrients in the water caused by pollution from agriculture and sewage systems.  Some researchsuggests blooms are likely to intensify with rising temperatures due to climate change.

The Oder fish die-off in 2022 has been called a "man-made environmental disaster," as researchers found that a key driver of the golden algae blooms was high salinity in the water caused by discharges from coal mines and other industries into the river. 

Prymnesium does not produce obviously visible blooms. The first indication of something wrong can be frenzied or dead fish. 

What environmental impact could the hydrogen peroxide treatment have? 

The hydrogen peroxide has killed 90% of the golden algae targeted in the junction between the Gliwice Canal and the Klodnica River, which are connected to the Oder, according to a Polish climate and environment ministry statement. 

It's not the first time the bleach has been used to kill the organism: In 2017 UK trials also found it to be successful. However, according to Christian Wolter, a researcher at Germany's Leibniz Institute of Freshwater Ecology and Inland Fisheries, it is the first time it has been used in a river rather than stagnant water. 

The environmental impacts of using the chemical are not yet fully clear, explained Jacek Engel, president of Greenmind Foundation, a Polish environmental NGO. Engel added that his colleagues are conducting research on how hydrogen peroxide has impacted other parts of the ecosystem, such as plankton, tiny organisms that are a crucial food source in bodies of water.  

The local impact it may have on a specific part of the river needs to be weighed up against the potential far wider impact of allowing the golden algae to spread, explained Wolter. "Because, of course, this will not kill only prymnesium parvum. It kills everything ... but still, they kill the organisms at one particular site to save the whole Oder system."

Wolter added that Polish authorities have set up several monitoring points to measure how quickly the hydrogen peroxide disappears and its effects on invertebrates, algae and fish.

To avoid harming other organisms, getting the right concentration of the chemical is key, explained Elisabeth Varga, a scientist at the University of Veterinary Medicine in Vienna.  "You have to find a concentration which is high enough to damage the micro algae and prevent the growth of the micro algae, but which is not too high so that all the other wildlife is affected."

According to UK trials of the treatment, hydrogen peroxide breaks down after a short period and quickly becomes harmless. The tests also showed that "fish and macroinvertebrates were unharmed by the treatment."

Is using hydrogen peroxide to kill algae blooms an effective response? 

"The hydrogen peroxide, it's a short-term measure that they're using," said Julios Kontchou, an environmental toxicologist with Greenpeace Germany. "But it's not the solution for the problem that we have in the Oder River."

The Polish climate ministry emphasized that the use of hydrogen peroxide was a crisis measure.

The key solution is for authorities to significantly reduce the current wastewater threshold values for what coal mines and other companies are discharging into the river, said Kontchou, as this significantly increases the salinity of the water.

Using hydrogen peroxide is an example of fighting against the symptoms but not the original problem, said Wolter. "We know prymnesium parvum is a brackish water algae, and the primary reason that it is in the River Oder is the salt content."

A number of Oder tributaries, which receive sewage from several hard coal mines, have water salinity higher than the Baltic Sea, according to a Greenpeace Poland study published last year. The group called the result "terrifying" because the Oder is a freshwater river, while the Baltic is a saltwater sea.

"I would say the situation is similar to what we observed last year and at some points even worse than what we observed back in 2022," said Kontchou, referring to salt levels in the river. 

The Polish deputy climate minister has announced plans to build desalination plants to improve the quality of the water in the Oder.

However, the German environmental group BUND and others in Poland have argued that protecting the Oder must go beyond measures to monitor and stop salt discharges, and extend to keeping it as wild as possible instead of treating it as a water way for barges and tourist traffic.

There are plenty of strong EU regulations concerning rivers, said Wolter. 

"The problem is the implementation. And this is not only Poland, there are other member states ... European wide 60% of rivers are not in a good ecological state." Wolter said, adding that most German rivers fail to meet the targets of the EU Water Framework Directive, established to raise the quality of European water bodies.

While the International Commission on the Protection of the Oder against Pollution — a joint agreement between Czech Republic, Germany and Poland — has become more involved in monitoring the river since the 2022 catastrophe, it could be empowered to act further, said Wolter.  

Edited by: Jennifer Collins