Changes in remote lakes likely caused by human activities: study

Nitrogen levels in remote lakes have increased faster than can be accounted for due to natural causes, says a new study. (PHOTO BY P. LEAVITT)

HILARY ROBERTS
Postmedia News

Human activities such as burning fossil fuels and using commercial fertilizer are affecting remote lakes across the Northern Hemisphere, including British Columbia and Northern Canada, according to a study by researchers from Canada, Sweden, China and the United States.

The study, from the Dec. 16 issue of Science, found that since the Industrial Revolution and the introduction of industrial fertilizer, nitrogen levels in remote lakes have increased faster than can be accounted for due to natural causes, said Peter Leavitt, one of 19 co-authors from over a dozen research institutes and universities.

“It’s interesting that on the eve of the announcement of Canada pulling out of the Kyoto Protocol that we’re now seeing evidence of fossil fuel effects through much of Northern Canada,” Leavitt said.

“If people were worried or disbelieving that burning fossil fuels had an effect on the environment, here’s another piece of the puzzle that suggests that yes, in fact, it does have an effect.”

The researchers looked at 33 lakes, all situated far from where humans could have directly polluted them, Leavitt said.

What’s interesting, Leavitt said, is the similarity between the findings from such a wide variety of lakes, with nitrogen levels rising at around the same time just over 100 years ago in all of them.

Because researchers studied remote lakes unlikely to have received nitrogen from anywhere other than the atmosphere, they were able to rule out direct pollution from human populations, including cities and local agriculture, as possible sources for the elevated nitrogen levels, said Leavitt.

“It’s nothing as simple as water going from an industry into the lake,” said Leavitt, the Canada Research Chair in Environmental Change and Society from the University of Regina.

“So that leaves either climate changes that have changed the biology of the lake or something to do with the atmosphere.”

Climate change can’t be to blame because the lakes all come from different climates, Leavitt said. It’s got warmer around test lakes in the Arctic, but cooler in Baffin Island over the last 100 years, meaning global warming can’t explain the increase in nitrogen.

The only explanation left is the atmosphere, he said, noting the type of nitrogen they’ve found has the chemical fingerprint of nitrogen originating from fossil fuels and fertilizer.

Researchers used aquatic archeology to conduct the study. They lowered plastic tubes into the mud at the bottom of the lakes and pulled them up to get test samples, Leavitt said.

“What lives in the lake, dies in the lake, and what falls in the lake from the atmosphere comes in and it’s floating in the water,” Leavitt said. “It eventually sinks and ends up in the bottom as mud.”

Samples from the top of the tube were the most recent sediments, with things in the mud getting older the further along the tube, and the deeper into the mud, they went, he said.

Researchers sliced the mud up, Leavitt said, and used process similar to carbon dating to determine the age of each section.

They were able to determine the source of the different types of nitrogen in the mud by comparing two different nitrogen isotopes, knowing each isotope can be found in different ratios depending on the source, said Gordon Holtgrieve, the study’s lead author.

“You can kind of think of it like red paint and blue paint, where there’s two kinds of nitrogen,” Holtgrieve said. “Chemically, it’s functionally the same. It does the same type of thing in all chemical reactions. It just has an extra neutron, which means we can measure the amount of one isotope versus another.”

In fossil fuels, there’s less of one nitrogen isotope and more of the other, said Holtgrieve, an ecosystem ecologist from the University of Washington.

“What this means is that by measuring this ratio, we can infer how much of this new source, which is the atmospherically derived human nitrogen, is appearing in these lakes,” he said, adding they looked at mud samples dating back as far as 500 years and found increases occurred on average about 110 years ago.

Both Leavitt and Holtgrieve say the impact of higher nitrogen levels in these remote lakes is uncertain, but it’s likely to affect food webs, starting with the lakes’ microbes.

“The present community is different from every other community that’s existed in some of these sites for 10,000 years,” Leavitt said. “We’re literally entering a new era where the base foundation of food webs, of biology in these lakes, is changed to a position that we’ve never seen before.”

The microbes and algae in the lakes can affect everything else in the ecosystem, Leavitt said. And with the Earth’s ever-growing population, the use of fossil fuels and fertilizer will continue to grow.

“The concern is that when you change the base of the food chain, you change the base of everything,” Leavitt said. “It could be that the change is not particularly harmful, we have to be clear about that, but the question is, how do you feel about risks?”

The study was co-authored by researchers from the University of Regina, the University of Alberta, the University of Washington, the Science Museum of Minnesota, the National Marine Fisheries Service, McGill University, Yunnan Normal University in China, Idaho State University, Lund University in Sweden, the U.S. Fish and Wildlife Service, the Mountain Studies Institute, the University of Maine and Fisheries and Oceans Canada.