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.
"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.