Fisherman Henry Schneider stopped work for several months after a toxic algae bloom hit the Oder river last August, decimating his catch.

Schneider, 43, whose family has made its living from the river for over a century, finally took up his activities again in May.

But fears of a repeat are growing, as global warming adds to the toxic mix of pollution putting the delicate ecosystem under pressure.

"It was unbelievable that we had fish left at all," Schneider, who lives on the German bank of the river, says of the poisoning.

The disaster wiped out more than half the fish in the river, which traces a long stretch of the Polish-German border.

And while the Oder has had some time to recover, the organism responsible for the disaster, known as Prymnesium Parvum, has not been eradicated.

"In a milliliter of Oder water we still see several hundred cells," says scientist Martin Pusch from the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) in Berlin.

He warns that the risk of another deadly bloom is high.

"It is just waiting for the right conditions to explode."

Polish and German authorities on either side of the river say they are determined to stop that happening.

But the two have clashed over responsibility for the disaster and the best way to keep the algae in check.