NATURE GEOSCIENCE | VOL 6 | SEPTEMBER 2013 | www.nature.com/naturegeoscience 711 T he coastal upwelling regimes located along the eastern boundaries of the Pacifc and Atlantic Ocean basins are small in size, but make a disproportionate contribution to the productivity and microbial biogeochemistry of the ocean. Tese regimes include the California and Peru/Humboldt current systems in the Pacifc, and the Canary and Benguela current systems in the Atlantic. Teir characteristic equatorward wind patterns lead to the net ofshore transport of surface water, allowing cold, nutrient-laden water to upwell into the illuminated zone where phytoplankton photosynthesis occurs. Rich upwelled supplies of nitrate, phosphate and silicate, and the tremendous blooms of resident phytoplankton they support, render these regimes the ‘new production factories’ of the world ocean 1 (Fig. 1). Te teeming proliferation of phytoplankton biomass that develops during upwelling feeds into short, productive food chains which support a signifcant share of the biological resources that humans harvest from the ocean 2–4 . Natural variability in carbon, oxygen, nitrogen and iron Coastal upwelling systems consistently experience natural ranges in surface seawater carbon dioxide concentrations and pH that are among the most extreme in the ocean. As it upwells, older, deeper water carries high levels of carbon dioxide — the biogeochemical imprint of accumulated microbial respiration of organic matter — to the surface. Tis is especially true in the Pacifc Ocean, where underlying waters have been isolated from the atmosphere for many decades. As a result, carbon dioxide levels in surface sea water in upwelling zones can exceed 1,000 ppm, and pH can drop as low as 7.6–7.7 (ref. 5). In comparison, typical pH values in most of the surface ocean are ~8.1, and equivalent partial pressures of carbon dioxide will not be reached in the atmosphere for a century or more 6 . As this naturally acidifed water ages and warms at the surface, phytoplankton blooms consume much of the inorganic carbon through photosynthesis. Tis leads to the rapid drawdown of carbon dioxide and an increase in pH in the upwelled waters. Phytoplankton can deplete seawater carbon dioxide concentrations in these regions far below current atmospheric levels of ~400 ppm. For instance, the partial pressure of carbon dioxide in surface waters in a Mauritanian upwelling plume was found to fall by 140 ppm in just over a week, Microbial biogeochemistry of coastal upwelling regimes in a changing ocean Douglas G. Capone* and David A. Hutchins Coastal upwelling regimes associated with eastern boundary currents are the most biologically productive ecosystems in the ocean. As a result, they play a disproportionately important role in the microbially mediated cycling of marine nutrients. These systems are characterized by strong natural variations in carbon dioxide concentrations, pH, nutrient levels and sea surface temperatures on both seasonal and interannual timescales. Despite this natural variability, changes resulting from human activities are starting to emerge. Carbon dioxide derived from fossil fuel combustion is adding to the acidity of upwelled low-pH waters. Low-oxygen waters associated with coastal upwelling systems are growing in their extent and intensity as a result of a rise in upper ocean temperatures and productivity. And nutrient inputs to the coastal ocean continue to grow. Coastal upwelling systems may prove more resilient to changes resulting from human activities than other ocean ecosystems because of their ability to function under extremely variable conditions. Nevertheless, shifts in primary production, fish yields, nitrogen gain and loss, and the flux of climate-relevant gases could result from the perturbation of these highly productive and dynamic ecosystems. with photosynthetic carbon fxation responsible for ~96% of this drawdown 7 . Similarly, carbon dioxide uptake in both the southern California current and northern and central Canary current was shown to increase in line with upwelling intensity in a comparative modelling study. In other sectors of these two upwelling regimes the response seems more muted, however, owing to stronger nutrient limitation, slower growth and shorter water residence times 8 . Such large gradients in carbon dioxide concentrations between freshly upwelled water and older upwelled water result in a complex regional mosaic of surface-water carbon chemistry. In the central California upwelling system, pH can range between 7.85 and 8.15 as a result of this variability, according to model simulations 9 . Tis is because the pH of sea water and the concentrations of inorganic dissolved carbon species — including carbon dioxide, bicarbonate and carbonate — are inextricably linked through the carbonate bufer system, such that higher carbon dioxide levels mean lower pH and carbonate levels, and vice versa. Te carbon captured by marine phytoplankton eventually makes its way into the detrital organic matter pool. Te sinking fux of particulate organic carbon out of the surface ocean sequesters carbon dioxide away from the atmosphere in deeper waters and sediments (Fig. 2a). High rates of bacterial respiration are supported by this dense rain of organic material. Combined with poorly ventilated source waters (particularly in the Pacifc), this leads to the depletion of oxygen in the underlying water column and seafoor sediments. Indeed, the oxygen minimum zone centred on the Peruvian upwelling system in the eastern tropical south Pacifc represents one of the largest reservoirs of suboxic water in the world ocean 10 . Oxygen minimum zones also occur in association with the Atlantic upwelling systems 11,12 , although they are relatively small compared with those in the eastern Pacifc, and little of the very low oxygen water is entrained in the upwelling 12 . Tese suboxic zones serve as globally signifcant sites of marine nitrate loss 11,13,14 . Microbial denitrifcation (the anaerobic conversion of nitrate to dinitrogen gas) and anammox (the microbial oxidation of ammonium using nitrite under anaerobic conditions) reduce biologically available oxidized nitrogen species (nitrate and nitrite, respectively) back to dinitrogen gas. Denitrifcation and anammox Marine and Environmental Biology Section, Department of Biological Sciences, University of Southern California, Los Angeles, California 90089, USA. *e-mail: capone@usc.edu INSIGHT | REVIEW ARTICLES PUBLISHED ONLINE: 29 AUGUST 2013 | DOI: 10.1038/NGEO1916 © 2013 Macmillan Publishers Limited. All rights reserved