Death bloom of plankton a warning on warming
11-20) 20:27 PST -- Vanishing Arctic sea ice brought on by climate change is causing the crucially important microscopic marine plants called phytoplankton to bloom explosively and die away as never before, a phenomenon that is likely to create havoc among migratory creatures that rely on the ocean for food, Stanford scientists have found.
View Larger ImageA few organisms may benefit from this disruption of the Arctic's fragile ecology, but a variety of animals, from gray whales to seabirds, will suffer, said Stanford biological oceanographer Kevin R. Arrigo.
"It's all a question of timing." Arrigo said. "If migratory animals reach the Arctic and find the phytoplankton's gone, they'll have missed the boat."
Phytoplankton throughout the world's oceans is the crucial nutrient at the base of the food web on which all marine life depends; when it's plentiful, life thrives and when it's gone, marine life is impossible.
Arrigo and his colleagues gathered 10 years of observations from six NASA satellites to study changes in the evidence of chlorophyll - a key to measuring the annual abundance and disappearance of phytoplankton blooms - at the surface of the oceans. The satellite network has also recorded the yearly appearance and disappearance of vast expanses of sea ice and the increasing areas of open ocean all around the Arctic, an indication of how climate change is taking hold in the northern reaches of the globe.
A report of their findings is to appear in the current issue of the journal Geophysical Research Letters.
Unwelcome changes
The annual deep freeze that has covered much of the northern seas with ice around the polar regions was once a regular event, but what has been normal for millennia in the High Arctic is no longer the case. As global climate change has warmed the world's oceans, warmer water has moved into the frigid Arctic, causing changes in the once-regular appearance and disappearance of sea ice over vast areas.
The result is a shift in when explosive blooms of phytoplankton appear and disappear, Arrigo's team has found.
"It's a complex system," Arrigo said in an interview, "but as the changes in ice cover throw the timing of phytoplankton abundance off, then the birds and animals whose brains have long been programmed to migrate north at specific times of the year will have missed the boat if there's no nourishment for them when they get there."
Every spring and summer, phytoplankton in the Arctic blooms richly in explosive pulses, nourished by nitrogen and phosphorous in the seawater, and when those chemicals are consumed, the blooms end, Arrigo said.
Lower sea ice
The summer of 2007 experienced "by far the lowest sea ice cover ever recorded," Arrigo and his colleagues said. The ice cover was an unprecedented 23 percent lower than the previous low recorded only two years earlier, according to a recent report from the National Snow and Ice Data Center at the University of Colorado in Boulder.
As a result of all that open water, "primary production" of phytoplankton in the open ocean of the Arctic reached a peak of more than 10 million tons last year, compared with only 700,000 tons in 2006, Arrigo found.
Most of the explosive increase in plant production was due to the longer growing season made possible by the increasing extent of ice-free open ocean - particularly in the shallower waters of the continental shelves that surround the entire north polar region.
But plankton is short-lived, and when its chemical nutrients run out and the plants disappear, the marine life that depends on it is threatened.
"Continued reductions in Arctic sea ice and the associated increase in primary production (of phytoplankton) are almost certain to impact marine ecosystems ... and could precipitate profound ecological shifts," Arrigo wrote in his team's report.
Some fish and other creatures in the far north that serve as prey for animals higher in the food chain may benefit from increases in phytoplankton, but many migratory animals like gray whales and all the seabirds that shuttle to the Arctic at fixed times are bound to lose out if the timing of the phytoplankton cycle changes, Arrigo said.
His colleagues in this report are Gert van Dijken, the project's technical expert, and Sudeshna Pabi, a geophysics graduate student at Stanford.
E-mail David Perlman at dperlman@sfchronicle.com
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