Reduced ice cover is stimulating a new phenomenon in the Arctic -- extra blooms of phytoplankton occurring in the fall, just before freeze-up, in addition to the usual springtime blooms, new research finds.
A study, led by scientists at Canada’s Laval University and published online in the journal Geophysical Research letters, describes the autumn blooms that have become more frequent in the past 15 years.
Most regions of the Arctic Ocean are now developing two blooms a year, with microscopic plants proliferating in waters that are staying open longer than in the past, according to the study. “The occurrence of these fall blooms is now possible due to both delayed sea-ice freeze-up and increased exposure of the sea surface to wind stress,” lead author Mathieu Ardyna, a Ph.D student in oceanography, said in an email.
The conclusions are based on examination of satellite imagery and data collected by NASA, the study said. Researchers focused on areas north of 50 degrees latitude where ice is seasonal.
The twice-a-year blooms are happening in waters up to 73 degrees latitude in the western Arctic and even farther north than that in the eastern Arctic, according to the findings. The difference between the eastern and western Arctic patterns might be related to the “vertical stratification,” the changes in water density in the upper water column, Ardyna said in his email.
Are these extra blooms good or bad for the Arctic ecosystem? It is hard to tell, Ardyna said. Fish and marine mammals swimming in the upper layers of the water might benefit from the increased carbon coming into the bottom of the food chain, he said, but the benthic ecosystem at the bottom of the sea and the ecosystem connected to the ice might suffer.
The spring blooms in the Arctic Ocean, long known to occur regularly, remain the primary blooms, according to the paper. Future research will focus on comparing the spring and autumn blooms and investigation of any potential links between them, the mechanisms behind the new fall blooms and the reasons why they occur in some places but not others, Ardyna said.
Above latitude 80, no discernable blooms were detected, according to the study.
Blooms of phytoplankton, otherwise known as microalgae, have grown bigger in general in Arctic waters, other research suggests.
The 2011 discovery of vast amounts of phytoplankton beneath ice, made during a Chukchi Sea expedition led by Stanford scientist Kevin Arrigo, contradicted past assumptions that blooms were possible only in open waters.
Arrigo and his research colleagues concluded that thinning of the ice layer and more widespread melt ponds atop the sea ice is allowing more sunlight to penetrate to the water underneath, likely simulating phytoplankton growth. Findings were described in a study published in 2012 in the journal Science.
Blooms beneath the ice are probably much more widespread than earlier believed and may have been underestimated by a tenfold magnitude, the study said.
Arrigo and his colleagues spent much of this summer back in the Chukchi searching for phytoplankton beneath the ice. Arrigo is chief scientist in an expedition that traveled to the area on the U.S. Coast Guard icebreaking cutter Healy.