Scientist unlocks mystery of nutrient surge in Lake McDonald

In 2018, the water quality in Glacier National Park’s Lake McDonald had scientists concerned.

The amount of dissolved nitrogen and phosphorous in the lake had zoomed up to levels unseen before.

Lake McDonald is one of the first things people see when they enter the park and it’s pristine. Was Glacier’s most coveted lake suddenly becoming eutrophic? Eutrophic lakes are high in nutrients and are susceptible to algae blooms.

In essence, they’re polluted.

Armed with full grant funding from the Glacier National Park Conservancy, University of Montana doctoral student and limnologist Brooke Bannerman began investigating the lake. She took a host of different water samples, often working deep into the night and into the next morning to fully analyze them.

Lake McDonald has been surveyed before over the past 50 years, dating as far back to Environmental Protection Agency studies in 1975, but nutrient levels had never been this high.

Once Bannerman’s work began last year, she began eliminating possible causes of the pollution. One is septic tanks from homes that are on the lake’s shores. 

Previous study had found they really weren’t contributing to nutrients in the lake and Bannerman found the same to be true this time around. She also looked at other sources of pollution, like airborne dust. 

She also wondered if glacier melt might be a culprit. They weren’t contributing, she found.

Bannerman also suspected one other factor — wildfire. The lake in 2017 and again in 2018 saw large wildfires along its flanks. The Sprague Creek Fire burned the northeasterly side in 2017 and the Howe Ridge Fire burned the northwesterly side in 2018.

Both were contributing factors, she found.

The good news is the lake has rebounded since then — dissolved nitrogen and phosphorous levels have plummeted by 67% and 92% respectively.

“I’m thrilled to report it remains oligotrophic,” she said during a recent talk hosted by the Conservancy. Oligotrophic means it’s low in nutrients — as it should be.

Still, she suggested the Park Service employ a regular lake monitoring program. She’s also sampled several other lakes in Glacier and as climate change continues to alter the park, and the fire regime, it’s important to keep an eye on the park’s lakes, she contends.