Previously rare blooms of Didymosphenia geminate, or didymo, are the culprits.
The evolved forms the native algae species have now been found in rivers of the United States, Canada, Europe and New Zealand.
Researcher Brad Taylor Dartmouth College says earlier spring melts, runoff of nitrogen from agriculture and even the burning of fossil fuels could be responsible for the recent appearances.
While not harmful to humans, there is evidence that the long filaments of the algae blooms are promoting the growth of a worm that hosts a parasite responsible for "whirling disease” in salmon and trout.
When the parasite penetrates the heads and spines of the fish, it causes them to swim erratically, or whirl. The affliction makes it hard for the fish to feed or avoid predators.
Since the algae is a native species, Taylor says current efforts to eradicate it are futile. He suggests addressing the environmental conditions that are promoting the blooms.
Photo: Mark Hoddle - University of California, Riverside