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Huge phytoplankton bloom found under Arctic ice

Photo by Sam Laney, Woods Hole Oceanographic Institution

Researchers have been shocked to find a record-breaking phytoplankton bloom hidden under Arctic ice. “It’s much bigger [in concentration] than any natural open water bloom in the most productive ecosystems in the world,” says Kevin Arrigo of Stanford University in California. “The growth rates were astonishingly high — these cells were doubling more than once every day.”

“I would have told you a year ago that this couldn’t happen in the Arctic,” says Arrigo. Now, he notes that some 25% of the Arctic Ocean has conditions conducive to such blooms. The finding implies that the Arctic is much more productive than previously thought, and might help to explain why Arctic waters have proven such a good carbon dioxide sink, the researchers say.

As Arctic ice melts earlier in the summer thanks to climate change, these blooms could grow in extent or happen earlier in the year. The implications of that are unknown, but it could be bad news for fish that feed on open-water phytoplankton, or animals that time their summer trips to the Arctic to match what has traditionally been the peak of phytoplankton blooms. “There’s going to be winners and losers,” says Arrigo.

Researchers have long assumed that phytoplankton blooms in the Arctic start in summer, in open waters after the ice melts. In charting this, as part of a mission to help ‘ground-truth’ NASA satellite measures of such blooms, a team of researchers was making measures in the Chukchi Sea as part of the ICESCAPE mission of summer 2011. But when they looked under the thin ice, they were shocked to find a ‘pea soup’ of phytoplankton about 100 km on a side, extending up to 70 metres deep in places, they report today in the journal Science. The natural concentration of phytoplankton there is greater than anywhere else Arrigo is aware of — not including blooms caused by fertilizer runoff in places like the Gulf of Mexico or the Baltic Sea.

In hindsight the finding makes sense. Shallow Arctic waters are known to be rich in nutrients such as nitrogen. And sea ice is known to be getting thinner — in the mid-1980s, about 75% of Arctic spring ice was thick ‘multi-year’, which is often about 3 metres thick; but by 2001 that had plummeted to 45%. Ice that forms and melts in a single year is often just a metre thick. That thinner ice, and the melt ponds that form on top of it in spring, act like a skylight to let light into the waters below, while still blocking out harmful ultraviolet rays. The result is perfect growing conditions for phytoplankton. “If I were a phytoplankton that’s where I would want to grow,” says Arrigo.

The researchers guess that the blooms seen previously in open Arctic waters were not the beginnings of phytoplankton season, as previously thought, but the tail, dying end of it.


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