Nature 447, 999–1002 (2007)
The method used to estimate how marine phytoplankton transports carbon to the deep sea has been challenged by a new study. Because it is difficult to directly measure the sinking of carbon, for over 20 years scientists have used an indirect measure called the f-ratio to estimate the strength of the so-called biological pump that brings carbon to depth. f-ratio measurements assume that what sinks down to the deep ocean — carbon-containing organic matter — equals what wells up, that is, nitrate.
Now, Andrew Yool of the National Oceanography Centre, UK and colleagues have calculated rates of nitrification — the process by which nitrate is formed — around the world by integrating open-ocean measurements in a global ecosystem model. Rather than being exclusively formed in deep water and welling upwards, as previously thought, the researchers found that nitrate is formed at all depths. About half of the nitrate phytoplankton consume is created in surface waters.
If less nitrate is welling up from depth then it follows that less carbon is reaching the deep ocean through the biological pump. The study suggests that the f-ratio cannot be used to reliably estimate carbon export production and storage in the deep sea.