Swirling Eddies Trigger Phytoplankton Bloom in North Atlantic

By Chelsea Whyte on July 6, 2012 8:04 PM EDT

phytoplankton bloom in north atlantic
In May 2010, peacock-hued swirls of blue and green in the North Atlantic contained tiny organisms, phytoplankton, that grow explosively in the North Atlantic—from Iceland to the shores of France—in the spring and summer. (Photo: NASA/MODIS Rapid Response Team)

In the spring and summer, the North Atlantic explodes with color as microscopic plants called phytoplankton bloom green and white atop the water. Scientists have long attributed this burst of activity to calmer seas and longer days, which provide the sunlight the plankton need to grow.

"When conditions are right, diatom blooms spread across hundreds of miles of ocean," researcher Craig Lee of the University of Washington told UPI, "bringing life-sustaining food to sometimes barren waters."

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But now scientists have found a new trigger for the flourishing phytoplankton. Eddies that form when the heavier, colder water from the north slips under the lighter, warmer water from the south churn up the water and help the plankton float to the surface.

"Our results show that the bloom starts through eddies, even before the sun begins to warm the ocean," said lead author of the study Amala Mahadevan, an oceanographer at the Woods Hole Oceanographic Institution in Massachusetts.

"That timing makes a significant difference if you think about the animals that eat the phytoplankton," said Eric D'Asaro, the corresponding author on the paper.

Phytoplankton are at the bottom of the food chain, so the timing of their spring bloom affects the other animals that rely on the tiny plants for food, and the animals that rely on those smaller fish for sustenance.

Because many small sea animals spend the winter in a deep sleep under the ocean and emerge only when food is available, "if they get the timing wrong, they'll starve," said Craig Lee, an oceanographer who worked on the study.

The scientists studying this phenomenon believe that climate change may be affecting ocean circulation patterns that produce the eddies.

To determine just when the phytoplankton blooms are occurring, the team used specially designed submersible 'robots' that dove down to 3,280 feet below the water's surface and then floated back up, measuring the temperature, salinity and speed of the water, and gathering information about the chemistry and biology of the bloom itself.

The data the team received from these dives confirmed that the bloom was indeed happening even though the conditions were still winter-like during the dives of April 2008.

Plugging this data into 3-D models that looked at what would have happened had the eddies not be at work, the team found that the bloom would not have started as early as it did.

"What we're learning about eddies is that they're a critical part of life in the ocean," said study co-author Mary Jane Perry. "They shape ocean ecosystems in countless ways."

The changes in the annual bloom could mean that ecosystems will have to adapt to the new timeline, and they may also have an effect on the distribution of carbon dioxide.

Phytoplankton blooms absorb about one-third of the carbon dioxide humans put into the air each year through the burning of fossil fuels, reports The Christian Science Monitor. The North Atlantic is critical to this process; it's responsible for more than 20 percent of the ocean's uptake of carbon dioxide.

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