Everybody knows that inside the great oceanic gyre in the North Pacific, nutrient levels are so low it's effectively a lifeless desert.
(Photo: NASA image of a plankton bloom off Australia.)
And as commonly happens, what everybody knows may be based on a kernel of fact but is in other ways wrong.
It turns out certain forms of life are not only capable of surviving there, but they they regularly thrive.
A group of University of Hawai'i and Oregon State researchers recently published “Summer phytoplankton blooms in the oligotrophic North Pacific Subtropical Gyre: Historical perspective and recent observations. The authors include John Dore, Matthew Church, Roger Lukas and Dave Karl, all of the Department of Oceanography in the University of Hawai'i's School of Earth Science and Technology, and Ricardo Letelier of Oregon State's College of Oceanic and Atmospheric Sciences.
Their paper was published in January 2008 in the Elsevier journal “Progress in Oceanography.”
Phytoplankton is tiny drifting plant matter. Oligotrophic refers to areas that don't seem to have much nutrient material to sustain life. Many of the world's oceans have great gyres, which are deep regions around which the currents and winds flow, and which are often extremely low in nutrients.
The North Pacific Subtropical Gyre dominates the middle of the north Pacific, and is surrounded by the great clockwise current that runs north along Japan, east along Alaska, south along California and then west above the equator. It extends well to the east and west of the Hawaiian Archipelago.
The waters within the gyre tend to be very clear because there is so little stuff living in it. But a few years ago, scientists began noticing that occasionally, and generally in summer, they'd see evidence of a spike in life—plankton blooms.
Eventually, they were even able to get images of these blooms from satellites. They showed up as great drifting clouds in the ocean.
That's a huge burst of life. But how were these plants getting fed?
Dore and his colleagues, after sampling the ocean within such blooms, found that the forms of plankton that dominated were ones that could “fix nitrogen,” meaning they didn't need nitrogen in the water for fertilizer. They could convert nitrogen from forms plants can't use into forms they can.
Most appeared to be a blue-green algae called Trichodesmium and diatoms that contain nitrogen-fixing blue-green algae, which are also called cyanobacteria.
While early indications were that these phytoplankton blooms might be rare, Dore said researchers now believe they might be every-summer events.
“I believe that the bloom is an annual phenomenon, but its coverage and exact timing are variable. Its surface expression, which can be viewed from satellite ocean color sensors, can appear in one area, then diminish, only to intensify later in the summer/fall season in another area,” Dore said in an email.
At some point, the plants in these blooms die off, and much of their material probably drifts down into the deep ocean. There, it may form a sudden burst in food resources for the creatures of the dark depths.
“The seafloor biota tend to lead a feast-or-famine existence and the demise of this bloom often results in a big feast for them. Their life strategies may well be 'tuned' to this seasonal pulse of fresh organic matter,” Dore said.
© 2007 Jan W. TenBruggencate
