Scientists have discovered a vast pea-soup-green bloom of tiny plant-like marine organisms under Arctic Ocean ice. The bloom represents an enormous, and until now, unknown reservoir of food for marine life in frigid waters at the top of the world.

These waters, in sum, appear to be far more biologically productive than previously believed.

"This wasn't just any phytoplankton bloom," says Kevin Arrigo, a Stanford University marine scientist and lead author of the study. "It was literally the most intense phytoplankton bloom I've ever seen in my 25 years of doing this type of research" in oceans around the world.

The scientists sampled only a relatively small section of ice above the Arctic basin's continental shelves last summer. But the findings suggest that, where the mix of nutrients and sunlight are right, other areas around the basin could be highly productive as well, the researchers say.

The discovery could help explain why previous groups observing open water concluded that the region wasn't hot spot of biological productivity. And it could explain how the ocean has been absorbing larger quantities of carbon dioxide (CO2) from the atmosphere than data could verify, the researchers suggest.

A report on the results from a research cruise aboard the US Coast Guard Ice Breaker Healy last June and July appears in the June 8 issue of the journal Science. The two-year project, known as ICESCAPE, was funded by NASA.

In general, much of what's known about activity at the bottom of the marine food chain in the Arctic has been learned by studying what's happening in open water. That is partly because researchers tend to visit the Arctic in the summer, when sea ice retreats and exposes more open ocean. Moreover, satellites that can measure phytoplankton levels also can't detect what's happening under the ice.

But science, too, suggested that open water was the place to look. Phytoplankton need light, and historically, summer sea ice was thick and – at least early in the melt season – topped with a thick layer of snow. Less than 1 percent of the sunlight hitting the surface made it to the ocean surface underneath, says Don Perovich, a geophysicist at the US Army Corps of Engineers' Cold Regions Research and Engineering Laboratory in Hanover, N.H.

But when the ice breaker turned from the open water of the Chukchi Sea, north of Alaska, and headed into the ice, something unexpected happened.

"I was sure that phytoplankton abundance would drop like a rock," Dr. Arrigo recalls.

Instead, the numbers started to climb until they peaked some 26 miles in from the ice edge. There, the phytoplankton abundance was four time higher than in the open ocean. The layer was about as thick as a five-year-old is tall, Arrigo said, and the waters were as green as pea soup.

The right nutrients had been there all along. What was missing was sufficient light, Dr. Perovich says.

Since satellites first began keeping track of the ice in 1979, the extent of summer ice has declined by about 45 percent due to global warming, wind patterns, and pollution that increase melting. These days, much of the sea ice heading into the melt season tends to be no more than about six feet thick, with little or no snow cover. As the ice melts, ponds of meltwater readily form on the surface and act as skylights, Dr. Perovich says.

Now, 43 percent of sunlight reaches the ocean surface, he adds. Plenty of food and light 24/7 is the perfect recipe for megablooms, he says.

The quantities of plankton are "truly exceptional," says Walker Smith, a marine biologist at the College of William and Mary in Williamsburg, Va., who was not part of the team conducting the research.

If these blooms are widespread under the ice along continental shelves, the primary productivity in these regions could be up to 10 times greater than open-water productivity, the team estimates.

Indeed, the find helps explain why phytoplankton is less abundant in open water: The blooms snag nearly all of the nutrients moving into the basin from the Pacific via the Bering Strait before the ice melts significantly, Arrigo says.

Researchers had interpreted the relative dearth of open-water plankton as a sign of low primary productivity in the Arctic Ocean, but "the real action was going on under the ice," he says. "And where we thought the bloom was beginning when the ice melted, actually the bloom was ending."

In addition, researchers have noted that the Arctic ocean is becoming an enormous sink for atmospheric CO2 as the waters open up in the summer. Yet the open waters in the Chukchi Sea don't show the levels of dissolved CO2 they should if that's the case. Now, it looks as though the answer lies with the under-ice phytoplankton blooms, because they consume the CO2 via photosynthesis, just as land plants do.

More work needs to be done to determine the basinwide extent of the blooms and their timing. Yet the steady retreat of summer sea ice and the increasingly early onset of the melt period raises some troubling prospects, the researchers add.

If the bloom comes earlier, it might occur before the marine creatures who come to feed on it have arrived there to eat. The biggest effect could be on the fish the feed on plankton throughout the water column, rather than on bottom feeders such as whales and walruses, Arrigo says.

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Despite the concerns, the thrill of discovery remains an undercurrent as the researchers talk about their results.

"This is what you live for as a scientist," uncovering something "beyond unexpected," Perovich says. "This is a new Arctic Ocean, full of surprises."