National Geographic News
Alaska's marine animals have an unexpected nutrient in their diets: ancient carbon from glacier melt, a new study says.
Glaciers that naturally melt each summer along the Gulf of Alaska flush out huge amounts of organic material, made up mostly of dead microbes.
Those microbes had feasted on ancient carbon from boggy forests, which lined the Alaska coast between 2,500 to 7,000 years ago and were later trapped under glaciers.
Once released via glacial melt, the dead microbes provide a tasty treat for living microbes, which are at the base of the marine food web, researchers say. (See map.)
Previous studies had shown that carbon from living forests eventually makes its way into fish through the water cycle, so "fish are made out of the forest," said study leader Eran Hood, an environmental scientist at the University of Alaska Southeast in Juneau.
The new study reveals "the same kind of thing—the fish are probably made out of carbon from glaciers," Hood said. "This is a surprising thing we didn't know before."
Glacial melt may even have a hand in maintaining Gulf of Alaska fisheries, some of the most productive in the country, he added.
Giant Water Pump
The Gulf of Alaska drainage basin is a giant water pump: It includes more than 10 percent of the mountain glaciers on Earth, and annual runoff from the region produces the second largest discharge of fresh water into the Pacific Ocean.
Hood and colleagues analyzed organic matter in runoff from 11 coastal watersheds in 2008, during the annual peak of glacial melting.
The team sampled streams running through watersheds with different amounts of glacial coverage.
Watersheds with no glaciers at all would be expected to contain less meltwater, while those dominated by ice would be filled with glacial runoff.
What they found is that streams fed by glacial melt had a surprising amount of easily digestible, or "bioavailable," carbon.
In addition, the more glacially rich the water, the older the carbon—up to 4,000 years old.
Glacial melt is seasonal—ice builds up during the winter and sloughs off in the summer. But rising temperatures have set off a worldwide thaw of many glaciers and ice sheets, which collectively act as the second largest reservoir of Earth's fresh water, Hood said.
If glaciers continue to disintegrate due to climate change, an initial bounty of carbon released into the oceans would be followed by the complete loss of a major source of nutrients, he said.
Marine ecosystems are nourished by many sources other than glaciers, Hood added.
Ocean upwelling, for example, is a natural cycle in which cold water filled with nutrients rises from the seafloor, feeding surface life.
But a sudden influx of fresh water from melting ice could also disrupt the ocean currents that drive upwelling.
No Expiration Date
The new study goes against a long-held belief that older carbon is less palatable to simple organisms, Hood added.
For instance, in most of the world's water bodies, the older the carbon, the less easily microbes can digest it.
"That's the stuff that's been worked over—it's no good," Hood said. "But in our case the older it was, the more the microbes wanted to eat it."
That's mainly because glacial carbon is made of dead microbes that have been essentially preserved in ice.
The dead microbes contain more easily digestible nitrogen and not much lignin, a plant compound that's tough for microbes to break down.
Overall, the contribution of glaciers to the productivity of rivers and oceans is "greatly underappreciated," the study authors write.
"It's good to understand the uniqueness of glacier ecosystems and the important role that they play as a source of water and nutrients," Hood added.
Findings published this week in the journal Nature.
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