Jasmine Saros, a lake ecologist at the University of Maine, has been studying Arctic lakes in Kangerlussuaq, Greenland, since 2013, documenting their various stages of ecological health. She and her colleagues have spent each summer collecting samples and data from the pristine blue waters.
But in 2023, they returned to find many once-clear lakes had turned brown. Though gradual changes due to climate change are expected, the sudden color shift was a surprise.
Water samples and a look back at weather patterns from the previous year revealed that nine atmospheric rivers that struck the region in rapid succession the previous fall had flushed copious nutrients into the lakes, altering their color. Whether the change is permanent remains to be seen.
Lake Chemistry
For more than a decade, Saros and her colleagues would paddle out to the middle of each lake on a raft to collect sediment cores and water samples and measure light conditions below the surface. They also tracked dissolved organic carbon levels in the water and observed the microscopic organisms living (and that had once lived) in the lake.
“Every time we went to the lakes, we’d measure hydrogen and oxygen isotopes of the water.” The ratio of heavy and lighter isotopes of these elements in the samples from 2023 revealed that a lot of extra precipitation had fallen on the lakes compared to previous years.
Lake browning often results from high concentrations of dissolved organic carbon, primarily from decaying vegetation. It can also be caused by an increase in iron, typically resulting from natural processes such as weathering of iron-rich soils and rocks into the water, as well as anthropogenic influences such as agricultural runoff and industrial discharges.
Data from the fifth-generation European Centre for Medium-Range Weather Forecasts atmospheric reanalysis (ERA5)—a global weather hindcast—showed that nine atmospheric rivers had dumped precipitation over the area between September and October of 2022.
“Those atmospheric rivers drove not only record precipitation but also record heat.”
Atmospheric rivers are known for transporting moisture, but they can also carry warm air. The ERA data also highlighted that September 2022 was the hottest and wettest September on record in West Greenland since 1940.
By early July 2023, when Saros and her colleagues were back on the lake, dissolved organic carbon levels had risen by 22% compared to the 2013–2023 average. Iron concentrations had increased 1,000%. Other metals, such as aluminum, cobalt, chromium, and copper, had also surged within the lakes.
“Those atmospheric rivers drove not only record precipitation but also record heat,” Saros said. Higher temperatures caused precipitation to fall as rain instead of snow. The heavy rainfall saturated the landscape, thawed permafrost, and released organic material and iron into the lakes, turning them brown, she explained.
The study was published in the Proceedings of the National Academy of Sciences of the United States of America.
Ecosystem Transformation
The browning led to a 50% reduction in light penetration in the lakes. Light is critical for phytoplankton that create their energy through photosynthesis, and losing some of it caused some species to shift to mixotrophic behavior, feeding both on light and organic material.
The researchers also found a decline in microbial diversity, with certain species, such as the picocyanobacterium Cyanobium, becoming more dominant in the brown lakes, signaling a shift in the microbial community.
“A lot of primary production used to happen at the bottom of the lakes because light penetration was deep,” explained Václava Hazuková, a limnologist at the University of Maine and coauthor of the study. “Now that the transparency is lower, we are seeing a shift of the phytoplankton moving upwards.” In shallower waters, they may face higher temperatures and stress, affecting their growth and the entire food web.
Murkier water absorbs more solar radiation than clear water. “You can actually see more heat being trapped in the surface layers of the lake,” Saros said.
“This research is providing us with new insights into ecosystem resiliency and the impacts of climate change on remote northern lakes.”
It is difficult to tell whether these lakes can revert to clear water, according to Saros and Hazuková. “Sunlight can often help in bleaching this brown material,” Hazuková said, but future precipitation will continue to influence the lake ecosystem.
“I think what was really stunning about it was how this population of lakes changed in the exact same way, in a similar magnitude. You don’t often see that,” Hazuková explained. The uniform response of these lakes may suggest that the region is entering a more volatile phase of environmental change, where rapid, large-scale shifts become the norm, though the researchers can’t say for sure whether this is the case.
“Whether this extreme climate event has induced lasting limnological changes, or the lakes return to their previous state soon, this research is providing us with new insights into ecosystem resiliency and the impacts of climate change on remote northern lakes,” said Matthew Bogard, a limnologist at the University of Lethbridge in Canada who was not part of the study.
—Larissa G. Capella (@CapellaLarissa), Science Writer
