Ocean Science

Fixed Up

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Science  02 Apr 2004:
Vol. 304, Issue 5667, pp. 18
DOI: 10.1126/science.304.5667.18a

Nitrogen-fixing bacteria supply a large fraction of the nitrate input to the upper layer of the ocean, thereby facilitating biological production in surface waters, but quantifying the global flux of this fixed nitrogen has proven to be difficult. It should be possible to distinguish this avenue of nitrogen input, which occurs in warm stratified water, from the other important route of nitrate delivery—the upwelling of cold nutrient-rich water from below—on the basis of surface water properties.

Coles et al. analyze satellite data for a summer phytoplankton bloom in the western tropical North Atlantic and find that ocean color indicative of high values of chlorophyll occurs where anomalies of sea surface height show the presence of warm pools of water. Using a model that incorporates the diazotrophic cyanobacteria Trichodesmium, they show that this summertime bloom can be attributed to nitrogen fixation, thereby validating a technique that has the potential to distinguish production fueled by fixation from that caused by upwelling, and hence to quantify the amount of nitrate produced by nitrogen-fixing bacteria on a global scale. This is of particular interest because fixed nitrogen, unlike upwelled nitrogen that comes from the remineralization of dead organisms, can fuel new biological production and thereby remove CO2 from the atmosphere. — HJS

Geophys. Res. Lett. 31, 10.1029/2003GL019018 (2004).

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