Oxygen starvation leads to new wave of deaths in the Oder River

A new second wave of deaths has been seen in the Oder River for a few days now. “What we are seeing now is different from the mass deaths in recent weeks,” said Piotr Parasiewicz, head of the department of riverine fisheries at the Stanislaw Sakowicz Institute for Inland Fisheries in Olsztyn, northeastern Poland. It is one of the institutes involved in investigating the ecological catastrophe that has unfolded in the Oder River in recent weeks, with large numbers of dead animals.

“All those organisms that died in recent weeks and ended up at the bottom of the Oder are now being digested and broken down by bacteria.” This process requires a lot of oxygen, especially with the current water level low, there is a shortage. As a result, organisms become unable to breathe. “They are suffocating.”

It was very different with the first wave of death that started about three weeks ago. Large numbers of dead fish were suddenly found, from the town of Glogov, and then gradually downstream of the 850-kilometer river, which rises in the Czech Republic and flows into the Baltic Sea through Poland and the Polish-German border. Media reported that the Polish fire brigade had recovered more than 100 tons of dead fish from the river by August 17. “That number has now risen to an estimated 200 tons,” says ecologist Christian Walter of the Leibniz Institute for Freshwater Ecology and Inland Fisheries in Berlin, who is also involved in the study.

High concentrations of salt

On August 17, this institute published its first results. Remarkable were the “abnormally high salt concentrations” that were found, among other things, at a measuring station in Frankfurt an der Oder. In addition, when looking for algae, the species Prymnesium parvum To jump. It usually does not occur in large numbers in the fresh waters of the Oder. Works well in brackish water. But now, these species sometimes accounted for half of the algal biomass in the studied water samples.

Suspicion was soon expressed that there was an industrial discharge of salt – whether it was legal or not. This could have resulted in a massive bloom of this typical golden algae. Golden algae, in turn, produce toxins, including prymnesins, which are lethal to gill-breathing animals. The prymnesines could have caused the mass deaths. However, the institute had a hard time. Because on August 17, these toxins were not detected – only a few laboratories in Europe focus on this group of toxins.

There are a few places animals can escape to in an emergency

Piotr Parasiewicz Polish Fisheries Institute

But two days later, the Walter Institute reported that a researcher from the University of Vienna had found “irrefutable large amounts of prymnesins” in water samples sent to her from Oder.

Another day later, on Saturday 20 August, a new press release followed. This time with satellite images. Oder is colored based on the concentration of chlorophyll (the green leaf, which algae needs for photosynthesis). In the August 4-6 photo, this concentration peaked in the area between the cities of Wroclaw and Glogov. In the photo from August 16-18, the peak of chlorophyll has moved downstream and can be seen over a wide area, almost as far as the city of Szczecin, which is located at the mouth of the Oder.

Satellite images, in which Oder is colored based on chlorophyll concentration. Left July 24-25, middle August 4-6 and right August 16-18.
Image Brockmann Consult

Walter says that although the chlorophyll peak in satellite images cannot be specifically attributed to Prymnesium parvum, but in general, all the data obtained so far fit the picture of a salt peak that led to a massive bloom of salt-loving algae, which was followed by a poisonous peak and a death peak. But caution remains. An investigation is underway, and other causes of death may be revealed.

Parasiewicz says he has now received dead samples from 20 species of fish. “The larger species in particular, the easier it is to see.” More small species are also expected in the coming weeks.

Walter stresses that he has not only fallen victim to fish. Mussels and mollusks such as snails and mosquito larvae were also found dead in large numbers. He estimates that it will take two to three years for fish stocks in the degraded part of the Oder to recover. “It’s longer for mussels, because they don’t move as fast.”

20,000 dead baby sturgeon

The Leibniz Institute for Freshwater Ecology and Inland Fisheries also announced in its first press release that it regrets the deaths of 20,000 young sturgeon in the house. The institute is involved in the reintroduction of sturgeon in the Oder. Therefore, it generates small samples of this type. Walter says his institute uses two of his containers. “They are washed with oder water until the fish get used to it.” He calls the discharge of salt “at the boundary of the offender”.

In Poland, a (legal) investigation into the source of the salt discharge is still ongoing, Parasivic says. But he also wants to stress that the conditions were unfavorable anyway. The water level of the Oder was very low due to the constant drought. A problem that will become more common in the future due to climate change. “If there is less water, soon the concentration of pollution is higher and more harmful.” In addition, many fertilizers are washed into the river from the fields, stimulating the growth of algae. The Oder is the river into which man has greatly intervened, by building dams and straightening banks. “There are a few places animals can run to in an emergency.”

Parasiewicz says it’s time for a “master restoration plan” for Oder. Walter thinks so, too. “The water needs to be kept better so that it doesn’t dry out too quickly in the summer,” he says. There should be more floodplains, and more places for animals to escape to in case of emergency. Perhaps a number of dams could be removed. Walter: “Salt discharge should be restricted, especially in the summer.”

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