Model Uses Ocean Food Web to Assess Global Carbon Export

Researchers have used the lowest members of the ocean food web in a novel model for assessing the ocean's role in the global carbon cycle.

Oceanographer David Siegel, the director of the Earth Research Institute at University of California, Santa Barbara, and his colleagues analyzed the role of excrement from phytoplankton and zooplankton - tiny animals at the bottom of the food chain - in global carbon export. They report their findings in the journal Global Biogeochemical Cycles.

The researchers' model is based around the net primary production (NPP) of organic matter by phytoplankton, which release aqueous carbon dioxide (CO2).

A natural phenomenon in the ocean known as the biological pump exports organic carbon from from the upper-ocean through sinking particulate matter, largely in the form of zooplankton excrement and algae.

"Understanding the biological pump is critical," Siegel said. "We need to understand where carbon goes, how much of it goes into the organic matter, how that affects the air-sea exchanges of CO2 and what happens to fossil fuel we have emitted from our tailpipes."

Once this organic matter leaves the well-lit upper ocean, known as the euthropic zone, and sinks to the depths, carbon can be sequestered there for centuries.

"What we've done here is create the first step toward monitoring the strength and efficiency of the biological pump using satellite observations," said Siegel. "The approach is unique in that previous ways have been empirical without considering the dynamics of the ocean food web."

This method of carbon cycle assessment is more consistent with how the ocean itself functions, he said.

Siegel's model is based around oceanic carbon flux, or the movement of carbon through the ocean. The researchers contend that oceans are a central component to the global carbon cycle through their storage, transport and transformation of carbon constituents.

"Quantifying this carbon flux is critical for predicting the atmosphere's response to changing climates," Siegel said. "By analyzing the scattering signals that we got from satellite measurements of the ocean's color, we were able to develop techniques to calculate how much of the biomass occurs in very large or very small particles."

Using their model, the researchers predict a global carbon flux of 6 petagrams (Pg) per year. (A petagram is also known as a gigaton. One petagram is equal to one quadrillion grams.)

Six petagrams is a huge amount of carbon, roughly the same as the annual global emission for fossil fuel combustion, the researchers said.

"It matters how big and small the plankton are, and it matters what the energy flows are in the food web," Siegel said. "This is so simple. It's really who eats whom but also having an idea of the biomasses and productivity of each. So we worked out these advanced ways of determining NPP, phytoplankton biomass and the size structure to formulate mass budgets, all derived from satellite data."