State agencies continue to investigate ‘rock snot’ in North Shore rivers and streams

An unwelcomed freshwater algae called ‘rock snot’ is once again in the news. This time, fisheries biologists and other researchers are trying to learn how the algae impacts what various types of fish are eating in North Shore rivers and streams.

In 2018, WTIP and the Minnesota Department of Natural Resources advised anglers, hikers and other users of the Poplar River to be aware that a species of freshwater algae called didymo, also known as “rock snot,” was identified in upstream waters of the Poplar River near Lutsen. News that the algae was in the river meant it moved beyond the Lake Superior splash zone where it was previously documented. In fact, this type of algae is present in low densities in Lake Superior, but was not previously documented in the upper watersheds at any point in history.

According to the DNR, the freshwater algae live in low nutrient, low-temperature environments that are common in North Shore streams and Lake Superior. Under the right conditions, this type of algae can form dense mats of brown slime that smother streambeds, according to the DNR. It is also quite unpleasant by physical appearance, though it does not present a danger to humans. That being the case, in the summer of 2020, it was being confused for toilet paper and human waste floating in Lake Superior.

WTIP’s Joe Friedrichs spoke with Heidi Rantala, a freshwater ecologist for the DNR based in Duluth, about the 2023 ‘rock snot’ reports and findings concerning what trout are eating in select rivers and streams along the North Shore. Audio below.

Joe Friedrichs June 25, 2023