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Mass deaths: Workers and soldiers collect dead fish on the Oder near Widuchowa in Poland with the help of a flexible dam
Photo: Marcin Bielecki / dpa
Now it is clear what was the fate of the animals in the Oder: A species of algae that is native to brackish water, which consists of salt and fresh water.
The assumptions about the suspected cause of the mass death of fish, mussels and snails have thus been confirmed, the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) said on Friday in Berlin.
The algae species Prymnesium parvum do not occur in large numbers under natural conditions on the affected river sections.
To do this, they need a salt content that can only be caused by industrial discharges.
"Die Zeit" first reported on the new results.
It has been known since Wednesday that the alga itself was found in large quantities in water samples.
The proof of the poison is new.
"We were able to detect a subspecies of the algal toxin, the so-called 'prymnesine', without any doubt and actually in significant quantities in samples from the Oder from different locations," explained the scientist Elisabeth Varga from the University of Vienna.
Varga had analyzed the samples at the local mass spectrometry center.
The poison is strongly bound to the algae itself.
According to the results, a "very high concentration of poison" and "a direct connection with the death of fish and molluscs can be assumed".
According to IGB algae expert Jan Köhler, Prynmesium parvum currently accounts for 36 percent of the total algae mass in the Oder, even after being diluted by the inflow of the Warthe tributary.
"As far as I know, such a mass development has never been observed in our waters." It was probably made possible by salt discharges, plenty of nutrients, high water temperatures and long residence times in barrages and in the developed river.
The genetic assignment of the algae strain is still to be clarified.
More than 50 different strains of Prymnesium parvum with different environmental requirements and toxin production have been described.
Elisabeth Varga also saw a need for research into how toxic the prymnesins were.
In addition, it has not been finally clarified to what extent animals other than fish and molluscs could be affected.
Possible effects on the human organism are also unclear.
However, the IGB warned that the increased salt concentration occurs more often in the Oder due to industrial pollution in the upper reaches.
Therefore, a mass poisoning like this could be repeated if the summers are still too hot and dry.
a.k