Only once the ice begins to melt each summer does life begin to bloom in the nutrient-rich waters of the Arctic Ocean—or so scientists have thought. But life may not wait for the ice to retreat, after all. Scientists visiting the continental shelf of the Chukchi Sea between Siberia and Alaska last July discovered a bright green bloom of phytoplankton lurking under ice more than a meter thick.
Such under-ice blooms—invisible to satellites—may be on the rise as a result of global climate change, pushing the Arctic's most productive season to earlier in the year. The finding also suggests that researchers may be dramatically underestimating biological productivity in the Arctic, which is connected to the region's uptake of carbon dioxide.
To thrive, phytoplankton—the photosynthesizing organisms in the Arctic food web that support zooplankton, fish, and migratory species from Arctic terns to gray whales—need nutrients and light. Parts of the Arctic Ocean, including the Chukchi Sea and the Bering Sea, are rich in nutrients, so light was thought to be the limiting factor, says Kevin Arrigo, a biological oceanographer at Stanford University in Palo Alto, California. "The assumption in the Arctic is that nothing gets going until the ice melts … there's just not enough light under the ice."
But last year, Arrigo and his team noted a proliferation of pools of water, known as melt ponds, on the surface of the Chukchi Sea ice, which were also a few meters thinner than in past years. Thick ice reflects incoming sunlight, but the melt ponds can transmit more than 50% of the light to the waters below, promoting under-ice blooms, he says. After sampling along two 250-kilometer-long segments of sea ice running from the open water into the ice pack, the team mapped a massive green under-ice bloom that extended as much as 30 meters deep and more than 100 kilometers across.
Similar blooms likely occur in many places along the Arctic's nutrient-rich continental shelf, Arrigo says. How long these under-ice explosions of life have been going on is uncertain, he adds, because it is not year clear how closely tied the blooms are to the thinning sea ice and proliferating melt ponds caused by global climate change. But the size of the observed bloom could mean that current estimates of the rates of so-called primary production—the production of organic molecules that results from phytoplankton consuming nutrients and photosynthesizing—on the Arctic continental shelf may be as much as 10 times too low, the team reports online today in Science. And that would significantly affect scientists' understanding of the changing biogeochemistry of the Arctic, including how much carbon the region can take up.
The magnitude of the phytoplankton bloom is one of the startling findings of the study, says Walker Smith, a biological oceanographer at the Virginia Institute of Marine Science in Gloucester Point, who was not connected with the study. "There are very few places anywhere in the ocean that such a bloom is observed," Smith says. "It's very surprising, both in location and magnitude."
The implications for the food web are unclear. Changes in bloom timing might not be good news for migratory animals, such as gray whales traveling from the Gulf of California to feed in the nutrient-rich waters of the Arctic shelf. "The blooms are happening possibly weeks before the ice begins to retreat," Arrigo says. Timing, he notes, is a really important issue in the Arctic, with its short seasons. "We have no idea how this might be changing those [migratory] patterns."
Indeed, it's hard at this point to extrapolate the effect to the entire food web, Smith says. "It's entirely possible that it would be a disaster, and entirely possible they could adapt to this change fairly easily—most likely it's in between. The fauna tends to be smarter than we give them [sic] credit for, in terms of adaptations and acclimations which they undergo in response to changing environment."