More than 100 tonnes (220,000 pounds) of dead fish have been recovered since July from the river which flows through both countries since July, sparking tensions after Berlin accused Warsaw of failing to communicate the disaster and act quickly enough.
“The examinations conducted so far have confirmed the presence of toxic algae (Prymnesium Parvum),” Polish deputy environment minister Jacek Ozdoba wrote on Twitter.
Meanwhile, German environment ministry spokesman Andreas Kübler told reporters “a mass development of toxic brackish water algae could have contributed to the fish deaths”.
However, the formation of such algae is “not a purely natural phenomenon” and “does not occur to this extent… under natural conditions,” Kübler added, referencing the latest lab results from Germany’s Leibniz Institute and the University of Vienna.
The algae likely developed as a result of high salt levels in the water, which “would not normally exist in the Oder and which can only exist through industrial discharge”, he said.
Polish Prime Minister Mateusz Morawiecki has suggested chemical waste may have been responsible but his country’s Climate and Environment Minister Anna Moskwa said last week that “none of the samples tested so far has shown the presence of toxic substances”.
She said the government was also looking into possible natural causes and in particular higher concentrations of pollutants and salinity as a result of lower water levels and high temperatures.
Moskwa on Saturday warned on Twitter against “fake news from Germany” about the discovery of herbicides and pesticides in the water.
“We regret that the Polish side has come to this assessment,” Kübler said on Monday, stressing that the search for the cause of the disaster was still ongoing.
The Oder has over the last years been known as a relatively clean river, and 40 domestic species of fish are found in the waterway.