Surprising find: Phytoplankton mega-bloom happening under the Arctic ice
Phytoplankton - microscopic plant-like organisms at the bottom of the marine food chain - need light and nutrients to grow, just like any other plant. And until now, it was assumed that they didn't grow much underneath the northern ice.
As NASA's ICESCAPE (Impacts of Climate on EcoSystems and Chemistry of the Arctic Pacific Environment) team moved from open water toward the ice-covered landscape taking samples, Don Perovich, a cryosphere scientist working on the mission, said he expected the findings to be "boring compared to the exciting stuff we thought would be in open water."
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But what they found was a thriving plankton bloom bigger than the likes of any in even the most productive ecosystems in the world.
"This is like finding the Amazon rainforest in the middle of the Mojave Desert," said Paula Bontempi, NASA's ocean biology and biogeochemistry program manager in Washington. "We embarked on ICESCAPE to validate our satellite ocean-observing data in an area of the Earth that is very difficult to get to," Bontempi said. "We wound up making a discovery that hopefully will help researchers and resource managers better understand the Arctic."
The sub-freezing temperatures of the Arctic aren't conducive to lots of growth, and most of the year, the waters are dark beneath the four-foot-thick ice, with just 1 percent of the sunlight making its way to the icy waters below.
Ken Arrigo, the ICESCAPE mission lead, said they had always assumed phytoplankton would need to wait for the ice to melt in order to have enough light to bloom. But the Arctic is changing, and so are the habits of some of its smallest inhabitants.
"In years since 1979, we've seen a remarkable and even astounding decrease in ice," said Walker Smith, a marine scientist on the team. He says ice coverage in Arctic summers is down by about 45 percent since that time. "As a result there is a huge amount of Arctic Ocean that does not have snow and ice over it and therefore photosynthesis is no longer negligible."
The thinning of Arctic ice and the melting of the usual 8-inch snow cover in the region allows for light to reach the waters beneath the surface of the ice. In the summers, as June turns to July and the sun stays out 24 hours a day, ponds of melted snow and ice act like windows that let in about 50 percent of light to the water-dwelling plankton below. And with water rich with nutrients like nitrogen and phosphorous needed to grow, the under-ice environment turned out to be a perfect place for phytoplankton to proliferate.
Arrigo described the plankton-laden waters as "pea soup-like" and full of extremely active organisms. The phytoplankton they observed doubled in number more than once a day, producing food for the ecosystem at a rate of 10 times the blooms in open water. These growth rates are among the highest ever measured for polar waters.
The blooms extended from the sea-ice edge to 72 miles into the ice pack, and ocean current data showed that they developed under the ice instead of drifting in from open water, where high concentrations of the microscopic plants can be found.
Fast-growing phytoplankton consume large amounts of carbon dioxide. The study, published today in the journal Science, concludes that scientists will have to reassess the amount of carbon dioxide entering the Arctic Ocean through biological activity if the under-ice blooms turn out to be common.
Data suggests this may have been happening for some time, and the ICESCAPE team says it could even accelerate in the future.
"At this point we don't know whether these rich phytoplankton blooms have been happening in the Arctic for a long time and we just haven't observed them before," Arrigo said. "These blooms could become more widespread in the future, however, if the Arctic sea ice cover continues to thin."
Warming temperatures and earlier ice melts could shift the timing of the blooms, which would affect migratory animals like whales and birds. Phytoplankton is the main source of food for small ocean animals that are then eaten by larger fish and ocean animals. The changing timing of the bloom could disrupt that chain, leaving animals that time their life cycles on the peak of the bloom without as large a food supply.
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