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June 24, 2010

Scientists discover spikes of nutrients fueling mid-ocean plankton

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Food’s great. Service is spotty.

What’s true of some restaurants also is true of ocean waters near Hawaii. There, nutrient-rich waters from deep in the ocean are carried up into the nutrient-poor surface waters, but only for a few days each month and only during part of the year. Although sporadic, the input of nutrients allows plankton to thrive in waters near the surface that are nearly barren of nutrients.

Waters like those near Hawaii are found throughout the subtropical ocean, the area between 40 degrees N to 40 degrees S of the equator. That 5,000-mile-wide belt of ocean circling the globe has surface waters that, although nutrient poor, somehow support an abundance of microscopic algae, also referred to as phytoplankton.

Needing nutrients, sunlight and carbon dioxide to grow — just like land plants — this generally unseen profusion of life is getting more attention: Plankton growing in the mid-ocean use so much carbon dioxide during photosynthesis that they account for a fifth of the CO2 taken out of the atmosphere by plants and algae worldwide.

Understanding how these plankton get the nutrients they need has been a puzzle for almost three decades, says Steve Riser, UW professor of oceanography, and co-author of a letter in the June 24 issue of Nature. Take for example nitrates, one necessary nutrient. The average amount of nitrates in surface waters in the mid-ocean is about 80 percent too low to account for the observed amount of plankton growth.

“These are microscopic plants, but they are just as important to the balance of carbon dioxide in the atmosphere as trees and other land plants, so understanding how they get what they need to grow is a key question,” Riser says.

He and colleagues Ken Johnson of the Monterey Bay Aquarium Research Institute and David Karl of the University of Hawaii relied on measurements made by a profiling float built by the UW. The float measured conditions from more than a half-mile deep in the ocean to the ocean surface for 2 1/2 years, transmitting data every five days via satellite. NOAA funds the UW to build 100 to 110 similar floats per year, of which some 650 work around the globe making long-term ocean measurements. Some floats, including the one used for this work, have been outfitted with oxygen sensors and sensors developed at the Monterey Bay Aquarium Research Institute to measure nitrite.

From January through October, during the annual cycle of growth and decay, the scientists found the oxygen released as plankton grew corresponded closely to the amount of nitrates disappearing from deeper in the water column.

“When the water column from the surface to 250 m [meters] is considered as a whole, there is near equivalence between nutrient supply and demand,” the Nature letter says.

The only problem is the oxygen was increasing at about 330 feet (100 meters) from the surface and nitrates were disappearing from waters 820 feet (250 meters) from the surface, a separation of about 500 feet (150 meters).

“This was a big surprise,” Riser says. “We’d expected that nutrients would be removed from the same layer where oxygen was being generated.”

Instead, steady week-in-week-out monitoring by the float revealed spikes of nutrients being injected from below, over short periods of less than 10 days, roughly once a month during the growing season.

Just why the diner in the deep is providing takeout on such a sporadic basis is not known. Maybe eddies moving through the area, deep in the ocean, draw nutrients upward. Or perhaps the kinds of plankton that migrate at night deep into the ocean and return to the sunlit waters in the day are carrying loads of nutrients.

“To find out more, the UW-MBARI consortium will be deploying as many as 40 more of these floats throughout the world in the next two years, which should help us to learn a great deal more about the relation between ocean circulation and the cycling of nutrients and oxygen,” Riser says.

In addition to building the float used in the experiment, Riser and his UW group integrated Johnson’s nitrate sensor into the float, and were responsible for deploying the float and receiving data. Besides NOAA, the UW receives funding from the National Science Foundation, Office of Naval Research and the National Oceanographic Partnership Program.