For almost three decades, oceanographers have been puzzled by the ability of microscopic algae to grow in mid-ocean areas where there is very little nitrate, an essential algal nutrient. In this week's issue of Nature, MBARI chemical oceanographer Ken Johnson, along with coauthors Stephen Riser at the University of Washington and David Karl at the University of Hawaii, show that mid-ocean algae obtain nitrate from deep water, as much as 250 meters below the surface. This finding will help scientists predict how open-ocean ecosystems could respond to global warming.
The sea around Hawaii may be clear and blue, but it hides an enduring oceanographic mystery. Surface waters in this and other mid-ocean areas contain almost no nitrate or other plant nutrients. Yet each year, microscopic algae (phytoplankton) flourish in these vast, open-ocean areas. Although miniscule in size, these mid-ocean algae consume about one fifth of all the carbon dioxide taken up by plants and algae worldwide.
To solve this mystery, Johnson and his fellow researchers used a robotic drifter called an Apex float, which automatically moves from the sea surface down to 1,000 meters and then back again, collecting data as it goes. Researchers at the University of Washington outfitted this drifter with an oxygen sensor and a custom version of Johnson's In Situ Ultraviolet Spectrophotometer (ISUS), which measures nitrate concentrations in seawater.
The design and deployment of this custom drifter was funded by grants from the National Science Foundation, the Office of Naval Research, the National Oceanic and Atmospheric Administration, the Gordon and Betty Moore Foundation, and the David and Lucile Packard Foundation.
In December 2007, researchers from University of Hawaii placed the drifter in the ocean northeast of Oahu, where it collected ocean profiles once every five days for almost two years.
From January through October of each year, the instruments on the dr
|Contact: Kim Fulton-Bennett|
Monterey Bay Aquarium Research Institute