AUTHORS Making the Paper

ANDREW YOOL A study challenges the method used to estimate carbon biochemistry transport by phytoplankton to the deep sea.

Twenty-eight years ago, in an attempt to quantify primary production by oceanic genetics phytoplankton at the global scale, Richard Eppley and Bruce Peterson developed a technique to separate nitrogen nutrients engineering that are recycled quickly at the sea surface from those that sink into the deep ocean. Writing in Nature in 1979, they coined the astronomy f-ratio to distinguish this sinking material, reasoning that the transport of nutrients to the deep ocean should, at equilibrium, balance their upwelling supply. More molecular biology signifi cantly, because phytoplankton growth couples nitrogen and carbon, their “We were very aware that we didn’t have any work framed the ‘biological pump’, the fi eld experience between us, but in trying to mechanism by which biology enhances the make sense of published measurements we ocean’s storage of carbon dioxide. However, were joined by Camila Fernández and Darren in formulating the f-ratio, they included Clark, two experts in the fi eld,” says Yool. a caveat about the role of a then poorly With a collection of global measurements, quantifi ed process known as nitrifi cation, a Yool’s team created an ecosystem model that caveat which Andrew Yool and his research included nitrifi cation and an accountancy team have now re-examined (see Nature trick to separate nitrate transported from the 447, 999–1002 (2007) and the associated deep ocean from that produced at the surface. highlight in this issue). “Th e nitrifi cation data covered several orders Yo ol’s fi rst run-in with the f-ratio of magnitude, but fortunately for us the model came while he was working towards his behaved almost the same everywhere,” he PhD in the mid-1990s. “At that time I was says. Unfortunately for the f-ratio, however, working with a basic model of the plankton it revealed that most nitrate was supplied ecosystem, and the f-ratio was just this near the surface, thus implying that f-ratio strange diagnostic I had no use for but measurements have largely over-estimated the which other people got excited about,” sinking fl ux. says Yool. Aft er completing his PhD, and “Our results suggest that the f-ratio pharmacology switching to three-dimensional ocean isn’t as good a proxy as we’d like, and that models, the f-ratio was quietly forgotten if we want to quantify the sinking fl ux about. However, when a poster appeared to the deep ocean, we’ll need to do it in chemistry outside his offi ce describing possible fl aws more time-consuming and diffi cult ways,” in the f-ratio at a time-series station near says Yool. Although the biological pump Bermuda, Yool was hooked again. Written is believed to be thus far unaff ected by by co-author Adrian Martin with Phillipe increasing atmospheric carbon dioxide, earth sciences Pondaven, the research looked at the climate change may aff ect nutrient supplies supply of nutrients to the ocean’s surface in to the ocean’s phytoplankton, and could subtropical oligotrophic waters, a topic that alter the balance of species that drives the physics was still puzzling researchers worldwide, pump. “Hopefully our work will persuade and found that something wasn’t adding up. oceanographers to develop more reliable Instead of the supply of nitrate coming ‘odometers’ for the biological pump, but oceanography mostly from upwelling of deep water, we’d also like it to focus people more on nitrifi cation of surface ammonium — itself nitrifi cation. Th ere’s still a lot — too much a recycled nutrient — seemed to be playing — that’s not known about it.” an oft en dominant role. Realizing that they could pool Martin’s Bermuda work with Andrew Yool is a Post-doctoral research fellow www.nature.com Yool’s three-dimensional models, they in the Ocean Modelling and Forecasting (OMF) decided to broaden the canvas to examine the group at the UK National Oceanography importance of nitrifi cation at the global scale. Centre, Southampton.

46 nature reports climate change | VOL 3 | AUGUST 2007 | www.nature.com/reports/climatechange