The death of fish in the Oder, which also forms the German-Polish border with its lower reaches to the Baltic Sea, continues to pose a mystery.
The only thing that is clear so far is that the first alarm signals since the end of July reached the Polish, German and Brandenburg governments with a delay of about ten days.
In the meantime, fish kills have also been reported from two other regions of Poland whose rivers do not flow into the Oder, including the river Ner in central Poland.
Political correspondent for Poland, Ukraine, Estonia, Latvia and Lithuania based in Warsaw.
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According to the Polish Ministry of the Interior, around 2,000 police officers, more than 300 firefighters and additional soldiers were on duty on the Oder itself at the weekend to monitor the river and salvage dead fish.
Since the beginning of the fish kill, tons of dead fish have been taken out of the river in Poland alone.
Poland's police are offering a reward of one million złoty (about 210,000 euros) for information leading to the identification of the culprits.
The authorities in both countries recommended that the population refrain from swimming, fishing and taking water from the Oder.
Federal Environment Minister Steffi Lemke (Greens) was "very affected" by the environmental disaster after a visit to Frankfurt an der Oder on Saturday.
She announced a joint expert assessment and an exchange of analysis results from both countries.
Lemke criticized the lack of cooperation between the authorities at the beginning.
"The German-Polish cooperation clearly didn't work at this point," criticized the minister.
Otherwise Germany would have received information about this earlier.
She has already spoken to her Polish colleague Anna Moskwa and will continue to do so.
According to the Federal Environment Ministry, a meeting was planned for Sunday evening in the Polish city of Szczecin, near the border, at which, in addition to Lemke and Moskwa, Poland's Infrastructure Minister Andrzej Adamczyk and the Environment Ministers of Brandenburg and Mecklenburg-Western Pomerania, Axel Vogel (Greens) and Till Backhaus (SPD), should attend.
Mercury excluded as a cause
The pollution only reached the German border on Tuesday or Wednesday last week.
Environment Minister Vogel said on Friday that mercury pollution had been found in the Oder;
however, the cause of the fish kill may have been a combination of factors such as heat, low water levels and toxins.
There are currently historical low water levels.
This could mean that every substance in the water is present in a higher concentration.
At the same time, however, the Polish environmental supervisory authority has ruled out mercury as the cause with what it claims to be “very precise” investigations over the past few days and on several sections of the river.
Minister Moskwa said at the weekend that the laboratory values showed a high salt content of the Oder water, on this point German and Polish studies coincided.
A connection with the fish kill is conceivable, as is "the activation of other toxic substances in the water or on the bottom of the river" by the salt.
Further laboratory tests should soon provide more information.
Poland's Prime Minister Mateusz Morawiecki, whose hometown of Wroclaw is on the Oder, reacted late but violently to the authorities' delayed action.
The poisoning of the river had become the most important topic of public debate within a few hours.
The extent of the pollution is "very large, large enough to say that the Oder will take years to return to its natural state," said Morawiecki.
However, the biologist Piotr Skubała from the University of Silesia also considers a period of "decades" to be possible.
The prime minister on Friday dismissed the head of Poland's water protection agency, Przemysław Daca, and Michał Mistrzak, the chief inspector of the environmental protection agency.
He accused the latter of "acting too slowly".