Polish Minister Anna Moskwa said experts had discovered an algal bloom in water samples, after a high mortality that intrigued scientists.
Rare microorganisms known as golden algae may be causing a large fish mortality in the Oder that has intrigued scientists for days, Poland’s environment minister suggested.
“After research, the Olsztyn Institute for Inland Fisheries has discovered rare microorganisms, called golden algae, in water samples from the Oder,” Anna Moskwa said Thursday.
The algal bloom can make it look like the toxins killed the fish and clams but weren’t destructive to humans, he said.
German municipalities banned swimming and fishing in the Oder after dead fish and mollusks were discovered floating on the surface of the water and washed to the banks of the 520-mile (840 km) river, which stretches from the Czech Republic to the Baltic. Sea along the border between Germany and Poland.
Since last Friday, Polish firefighters have recovered more than a hundred tons of dead fish in the Oder, and Germany’s Environment Minister Steffi Lemke has reported some 35 tons of dead fish in their territories.
Scientists have struggled to find an explanation for the death. The first reports about the appearance of degrees of mercury in the waters of the Oder turned out to be misleading.
Water samples were sent to laboratories in the Czech Republic, the Netherlands and Britain in the hope of determining the cause.
Golden algae (Prymnesium parvum) is found in brackish waters where rivers meet the sea, but is known to spread inland when water salinity is high.
“This species of algae requires high levels of salinity that do not occur naturally in the affected stretch of the Oder,” said Jan Köhler of the Leibniz Institute for Freshwater Ecology and Inland Fisheries in Berlin.
The Polish government has investigated whether peak concentrations of pollutants and salinity may be linked to climate change, with exceptionally high temperatures across Europe in recent weeks leading to increased evaporation of water from inland waters.
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Mineral fertilizers seeping into the river from adjacent farmland can also cause large algal blooms, the scientists said.
A chemical spill and other speculation under investigation, with Poland awarding a eulogy of 1 million złoty or €210,000 (£180,000) to anyone who can “help locate the culprits of this environmental disaster”.