Canada and the U.S. are being urged to crack down on sources of phosphorus runoff blamed for a rash of harmful algae blooms on Lake Erie.
The International Joint Commission says urgent steps are needed to curb runaway algae — which produce harmful toxins and contribute to oxygen-deprived "dead zones" where fish cannot survive.
The issue prompted both nations to reach their first agreement to improve Great Lakes water quality more than 40 years ago, when some considered Erie ecologically dead.
Tougher standards for municipal and industrial waste treatment produced improvements by reducing the flow into the lake of phosphorus on which algae feeds.
The report's Canadian co-author, Glenn Benoy, says algae blooms had almost disappeared but are back again.
In 2011, the largest mass on record formed in the lake's western basin, eventually reaching more than 160 kilometres from Toledo to Cleveland, Ohio.
Benoy says there is evidence an algae bloom is starting to spread now, but he doesn't know how severe it will be as blooms tend to peak in the fall.