Green Algae Carpeting Rocky Mountain's Alpine Lakes: Nutrients and Warming

Sky Pond, one of three alpine lakes in the Loch Vale watershed of Rocky Mountain National Park, has long been classed, like many of the park's high lakes, as oligotrophic: rich in oxygen, poor in nutrients, with very little algae, clear water and sparse aquatic plants. Lakes like it give habitat to trout and high-quality drinking water to communities downstream.

Human footprints in Sky Pond, filled with green algae. NPS photo/I. Oleksy.

What changed

In 2010 researchers began to see lush mats of green algae on the bottoms of The Loch and Sky Pond along the shore, and later found them across both lakes, not just at the edges. To find out why, and what it means for the lakes' productivity, researchers from the U.S. Geological Survey and Colorado State University studied lake sediment cores and ran field and laboratory experiments.

What they found

• History: a timeline of algal species from the sediment cores shows that bottom-dwelling (benthic) green algae are more abundant now than at any time in the last 200 years.
• Nutrients: in field experiments, adding nitrogen and phosphorus, the fertilisers that reach the lakes from the atmosphere, favoured green algae and produced the most algae overall.
• Temperature: laboratory experiments showed warmth also matters for algal growth and lake processes.

Alpine lake communities and processes appear to be shifting in response to climate warming and rising nutrients. Over time that could mean bigger algal blooms, murkier lakes and changes in water quality. The research is described in the paper "Nutrients and warming alter mountain lakes", and was funded in part by the Rocky Mountain Conservancy's Bailey Research Fellowship Program.

The scientists

• Isabella Oleksy, an ecosystem ecologist who studies how nutrients move through watersheds into lakes, and a postdoctoral researcher at the University of Wyoming, stresses the value of long-term records: they separate natural swings from changes caused by air pollution and climate change, and grow more valuable every year.
• Jill Baron began as a National Park Service researcher in 1976 and has been a USGS research ecologist since 1994, studying how atmospheric deposition and climate change affect mountain soils, plants and lakes. She argues that public lands matter both to visitors and to those who simply know they exist, and that managing them on a foundation of science and patient long-term observation is essential to helping mountains adapt to fast-changing conditions.

Sources

• National Park Service, Rocky Mountain National Park, "Nutrients and warming alter mountain lake benthic algal structure and function." https://www.nps.gov/articles/algal-booms-mountain-lakes.htm
• Two portraits of the scientists are not reproduced here.
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