A river on the German-Polish border has become a mass grave as over 110 tonnes of dead fish that floated to the top have been removed this week by authorities and volunteers following the environmental disaster.  

Polish Environment Minister Anna Moskwa has announced that lab tests showed toxic algae in water samples from the Oder River.

The discovery of toxic algae could explain the mass deaths of fish, but raises another question about what caused the algae to be present in large numbers.

"After further investigations, the Institute of Inland Fisheries in Olsztyn has found rare microorganisms, so-called golden algae, in water samples from the Oder river," Environment Minister Anna Moskwa said.

The massive die-off affected a stretch of the river more than 300 miles long. Authorities are struggling to find answers.

More than 500 firefighters are taking part in the efforts to clean up the mess on the Polish side of the river.

Poland's government says no toxic substances have been found in water samples from the Oder, and Environment Minister Anna Moskwa said there was no evidence of pesticides in samples from the dead fish.

"We are focused on, on the one hand, stopping what is happening, and on the other hand, finding the reason for this situation," said climate ministry spokesman Aleksander Brzozka.

Researchers in Germany and Poland's climate ministry have pointed to a large overgrowth of toxic algae as a possible cause for the mass die-off.

"The most likely hypothesis is that it was a combination of various natural factors," said Brzozka.

Local people told Reuters that firefighters and territorial defence forces deployed by the government to remove tonnes of dead fish were not prepared for what awaited them in the river.

The stench around the waters was so bad that most of them vomited during their work, according to residents of the village.