Nitrate uptake along repeat meridional transects of the Atlantic Ocean

Measurements of nitrate (NO3−) uptake made using 15N tracers in the surface oligotrophic Atlantic Ocean on three meridional transects are compared. Cruises occurred over the course of 1 year (AMT12 May–June 2003, AMT13 September–October 2003, AMT14 April–May 2004). Significant differences (p < 0.001) in mean daily integrated rates of NO3− uptake are identified between the two boreal spring cruises (AMT12 and AMT14) over the entire tropical and subtropical ocean (~ 40°S–40°N) which is caused by widespread increases in ambient NO3− concentrations in the surface ocean. The increase in NO3− concentration is small, typically between 5–10 nmol L− 1 but promotes a significant uptake response. Carbon fixation rates meanwhile show small increases during AMT14 relative to AMT12 in the equatorial region but not in gyral latitudes suggesting that the increased NO3− did not generally increase production rates but may instead have invoked a shift in the preferred form of nitrogen used by photosynthetic organisms. A greater proportion of the observed production might therefore have been available for export during 2004 compared to 2003. Observed intercruise differences in the separation distance between the base of the mixed layer and the nitracline suggest that this may be an important physical consideration which allows for greater nutrient transfer during periods of smaller separation, as was the case during 2004, and that this represents a potential control on export production.