Anna Moskwa, the minister for climate and environment, said the algea were rare and known to produce toxines that were fatal to fish and molluscs. She added that recent studies of Odra water showed a bloom of golden algae in the river.
She said the toxines were harmless to humans.
According to Moskwa, the algae could have bloomed in the Odra due to droughts and low water levels in the river.
Over 100 tonnes of dead fish have been found since late July in the Odra, Poland's second longest river, in one of the worst environmental disasters to hit Poland in recent years.
Just what killed the fish, and how and where it entered the river remains unclear. (PAP)