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Bloom Plankton Hitches Rides on Eddies

EddyJust as crocus and daffodil blossoms signal the start of a warmer season on land, a similar “greening” event — a massive bloom of microscopic plants, or phytoplankton — unfolds each spring in the North Atlantic Ocean from Bermuda to the Arctic.

Fertilized by nutrients that have built up during the winter, the cool waters of the North Atlantic come alive during the spring and summer with a vivid display of color that stretches across hundreds and hundreds of miles.

North Atlantic Bloom turns ocean into sea of plankton

In what’s known as the North Atlantic Bloom, millions of phytoplankton use sunlight and carbon dioxide (CO2) to grow and reproduce at the ocean’s surface.

During photosynthesis, phytoplankton remove carbon dioxide from seawater and release oxygen as a by-product. That allows the oceans to absorb additional carbon dioxide from the atmosphere. If there were fewer phytoplankton, atmospheric carbon dioxide would increase.

Flowers ultimately wither and fade, but what eventually happens to these tiny plants produced in the sea? When phytoplankton die, the carbon dioxide in their cells sinks to the deep ocean.

Plankton integral part of oceanic “biological pump”

This so-called biological pump makes the North Atlantic Ocean efficient at soaking up CO2  from the air.

“Much of this ‘particulate organic carbon,’ especially the larger, heavier particles, sinks,” says scientist Melissa Omand of the University of Rhode Island, co-author of a paper about the North Atlantic Bloom published March 26 in the journal Science.

“But we wanted to find out what’’s happening to the smaller, nonsinking phytoplankton cells from the bloom. Understanding the dynamics of the bloom and what happens to the carbon produced by it is important, especially for being able to predict how the oceans will affect atmospheric CO2 and ultimately climate.”

University of Maine Darling Marine Center researchers Mary Jane Perry, Ivona Cetinić and Nathan Briggs were part of the team with Omand, Amala Mahadevan of Woods Hole Oceanographic Institution and Eric D’Asaro and Craig Lee of the University of Washington that did just that.

They discovered the significant role that swirling currents, or eddies, play in pushing nonsinking carbon to ocean depths.

“It’s been a challenge to estimate carbon export from the ocean’s surface waters to its depths based on measurements of properties such as phytoplankton carbon. This paper describes a mechanism for doing that,” says David Garrison, program director in NSF’s Division of Ocean Sciences. The NSF funded the research.

Tracking a bloom: Floats, gliders and other instruments

During fieldwork from the research vessels Bjarni Saemundsson and Knorr, the scientists used a float to follow a patch of seawater off Iceland. They observed the progression of the bloom by making measurements from multiple platforms.

Autonomous gliders outfitted with sensors gathered data including temperature, salinity, as well as information about the chemistry and biology of the bloom — oxygen, nitrate, chlorophyll and the optical signatures of the particulate matter.

At the onset of the bloom and for the next month, four teardrop-shaped seagliders gathered 774 profiles to depths of up to 1,000 meters (3,281 feet).

Analysis of the profiles showed that about 10 percent had unusually high concentrations of phytoplankton bloom properties, even in deep water, as well as high oxygen concentrations usually found at the surface.

“These profiles were showing what we initially described as ‘bumps’ at depths much deeper than phytoplankton can grow,” says Omand.

Staircases to the deep: ocean eddies

Using information collected at sea by Perry, D’Asaro and Lee, Mahadevan modeled ocean currents and eddies (whirlpools within currents), and their effects on the spring bloom.

“What we were seeing was surface water, rich with phytoplankton carbon, being transported downward by currents on the edges of eddies. Eddies hadn’t been thought of as a major way organic matter is moved into the deeper ocean. But this type of eddy-driven ‘subduction’ could account for a significant downward movement of phytoplankton from the bloom,” says Mahadevan.

Perry, interim director of the DMC, says the discovery reminds her of a favorite quote from French chemist and microbiologist Louis Pasteur: “Where observation is concerned, chance favors only the prepared mind.”

“I feel that this project is a wonderful example of the chance discovery of an important process in the ocean carbon cycle,” she says. “It all started when I was chief scientist on the R/V Knorr during the North Atlantic bloom expedition, spending hours and hours staring at profiles of temperature and phytoplankton.

“Initially it was very puzzling — how could high surface concentrations of phytoplankton and oxygen make it down intact to 300 and 400 meters? But the combination of many measurements from autonomous gliders and simulations from models lead to the unexpected finding that ocean eddies or whirlpools are important forces in transporting phytoplankton and their associated carbon to great depths.”

In related work published in 2012 in Science, the researchers found that eddies act as early triggers of the North Atlantic Bloom by keeping phytoplankton in shallower water where they can be exposed to sunlight to fuel photosynthesis and growth.

Next, the scientists will seek to quantify the transport of organic matter from the ocean’s surface to its depths in regions beyond the North Atlantic and at other times of year, and relate that to phytoplankton productivity.

Learning more about eddies and their link with plankton blooms will allow for more accurate global models of the ocean’s carbon cycle, the researchers say, and improve the models’ predictive capabilities.

“The processes described in this paper are demonstrating, once again, how important the ocean is for removal of atmospheric carbon and controlling Earth’s climate,” says Cetinić.

Contact: Beth Staples, 207.581.3777

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