Scientific Method / Science & Exploration

Plankton go deep, ride currents to fetch nutrients

Plankton may be riding currents to bring deep-water nutrients and surface …

Plankton may take a more active role than we thought when it comes to removing inorganic carbon from the ocean's surface layers, according to new data collected off the coast of Hawaii.

Tracking the nutrients in ocean water revealed that chemicals needed for photosynthesis are transported from as deep as 250m below the ocean's surface. Scientists think that plankton may be grabbing these nutrients and riding currents back to the surface of the ocean, where they use the nutrients for photosynthetic processes.

Researchers noticed some time ago that the concentrations of dissolved inorganic carbon, such as carbon dioxide and carbonic acid, often drop at the ocean's surface during spring and summer. They suspected the carbon was being incorporated by phytoplankton during photosynthesis; however, they couldn't make the relevant equations balance, as there were never enough nutrients (like nitrates) in the water to support all of the photosynthesis going on.

To get a better understanding of the situation, researchers took profiles of water columns near Hawaii over the course of 21 months. They noted that the nutrient concentrations were much higher deeper in the ocean, where it's too dark for photosynthesis to take place.

The scientists realized the nutrients must be transported upwards somehow, and there are many currents that carry deep water up to the surface and vice versa. But they couldn't figure out how the nitrates and carbon didn't end up trapped on opposites sides of a water current merry-go-round—if the nutrients came up on a current from the deep, that deep water should displace the carbon in the surface water. There's no obvious way for phytoplankton to have access to both materials at once.

The authors propose a model where that a small population of phytoplankton sink to the high-nutrient depths, and are able to snag the nutrients and ride up to the surface on currents, where they "detrain" and are able to perform photosynthesis with the light and carbon now available to them. This may be an active process, as the authors suggest that "buoyancy regulation" by the plankton might be involved. The researchers postulate that there are heavier phytoplankton migrations during the spring and summer, with a lower volume during the rest of the year. 

Nature, 2010. DOI: 10.1038/nature09170  (About DOIs).

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