Phytoplankton explosion beneath Chukchi Sea ice intrigues scientists
Scientists have discovered an immense bloom of healthy phytoplankton teeming in the semi-darkness beneath a layer of sea ice in the Chukchi Sea, suggesting that portions of the Arctic Ocean north of Alaska may be 10 times more productive than anyone may have guessed.
This surprising diatomic riot — hidden from satellite monitoring under ice two to four feet thick — totally outclassed the spring bloom observed in nearby sea exposed directly to the sun.
“Depth-integrated phytoplankton biomass beneath the ice was extremely high, about four-fold greater than in open water,” wrote Stanford University biologist Kevin Arrigo and an international team of 29 co-authors (including oceanographer Jeremy Mathis from the University of Alaska Fairbanks) in a brief report published online Thursday in the journal Science. “This massive under-ice bloom extended for (more than 60 miles) into the ice pack.”
The startling findings, made last July during a cruise of the NASA-sponsored ICESCAPE research project, overturn biological expectations about what might be driving the frigid ecosystem that surrounds the disintegrating ice pack every spring.
Jumpstarting food web
The Arctic marine world relies on the spring bloom — where warming water and 24-hour solar radiation jolts the ocean’s microscopic life into an explosion of growth that jumpstarts the food web for the year. Scientists have always assumed that this seasonal event would be visible out in the open. Not much action would occur under the ice.
Recent monitoring of the Arctic’s spring bloom has always relied on satellites.
“Consequently, current estimates of pan-Arctic primary productivity assume that the growth and biomass of phytoplankton, free-floating single-celled photosynthetic organisms at the base of the marine food web, are negligible in waters beneath ice because of insufficient light,” the scientists explained.
Instead, the scientists found the densest biomass of phytoplankton occurring just beneath the ice and extending down as far 90 feet. Portions of the bloom went even deeper. The species of diatoms were not from the surface, nor where they the result of algal growth on the ice itself.
The decline of the extent and thickness of the ice over the past few decades might be a factor, they wrote, transforming what was once an opague multi-year cap into a translucent window into the depths.
“The light required by the under-ice bloom had to penetrate the fully consolidated ice pack to reach the upper ocean. Light transmission through ice was enhanced by a recent increase in the fraction of first-year ice, which is much thinner … than the historically dominant multi-year ice pack,” the scientists explained. “Although the under-ice light field was less intense than in ice-free waters, it was sufficient to support the blooms of under-ice phytoplankton, which grew twice as fast at low light as their open ocean counterparts.”
Widespread blooms
Analysis of the open water areas — with much less phytoplankton and biological activity — found that they had been depleted of nutrients and had other signs that they might have also bloomed before the ice melted away.
“Previous reports hinted at similar blooms in the Barents Sea, Beaufort Sea, and Canadian Arctic Archipelago, suggesting that under-ice blooms are widespread,” the researchers wrote. “If so, current rates of annual net primary production on Arctic continental shelves, based only on open water measurements, may be drastic underestimates, being 10-fold too low in our study area.”
More investigation is needed, the scientists wrote. No one knows the timing or length of these under-ice blooms — or how they relate to the decades-long decline of the summer sea ice extent or the overall marine ecosystem.
Gobbling up phytoplankton
“This is particularly important if we are to understand and predict the biological ... impacts of ongoing and future changes in the Arctic Ocean physical environment,” they wrote.
If ice continues to decline, the scientists speculated that these blooms might even become more common or appear earlier in the year, according to a release discussing the report.
“Many animals fly or swim to the Arctic to gobble up phytoplankton,” the release said. “If these blooms start cropping up earlier, some animals may have difficulty adjusting to an earlier season.”
http://www.alaskadispatch.com/article/phytoplankton-explosion-beneath-chukchi-sea-ice-intrigues-scientists
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