Vertical Nitrate Fluxes in the Oligotrophic Ocean

Abstract

The vertical flux of nitrate across the thermocline in the upper ocean imposes a rigorous constraint on the rate of export of organic carbon from the surface layer of the sea. This export is the primary means by which the oceans can serve as a sink for atmospheric carbon dioxide. For the oligotrophic open ocean regions, which make up more than 75% of the world's ocean, the rate of export is currently uncertain by an order of magnitude. For most of the year, the vertical flux of nitrate is that due to vertical turbulent transport of deep water rich in nitrate into the relatively impoverished surface layer. Direct measurements of rates of turbulent kinetic energy dissipation, coupled with highly resolved vertical profiles of nitrate and density in the oligotrophic eastern Atlantic showed that the rate of transport, averaged over 2 weeks, was 0.14 (0.002 to 0.89, 95% confidence interval) millimole of nitrate per square meter per day and was statistically no different from the integrated rate of nitrate uptake as measured by incorporation of (15)N-labeled nitrate. The stoichiometrically equivalent loss of carbon from the upper ocean, which is the relevant quantity for the carbon dioxide and climate question, is then fixed at 0.90 (0.01 to 5.70) millimole of carbon per square meter per day. These rates are much lower than recent estimates based on in situ changes in oxygen over annual scales; they are consistent with a biologically unproductive oligotrophic ocean.

Nitrate uptake and diffusive nitrate supply in the Central Atlantic

“…Herbland and Voituriez 1979). Data on nitrogen uptake rates in the gyre areas of the North Atlantic are few (Table 2), but the results we obtained in the northern end of our transect were similar to those reported in a station nearby by Lewis et al (1986). The uptake rates reported by Glibert et al (1988) in the Sargasso Sea were, however, even lower than those we report here for the gyre areas.…”

“…Compilation of nitrate and ammonium uptake rates and the corresponding f-ratios observed in the tropical ocean. Glibert et al 1998Glibert et al 1998Metzler et al 1997Lewis et al 1986LeBouteiller 1986 This study ¶ 0.001 h Ϫ1 (overall mean Ϯ SE ϭ 0.0013 Ϯ 0.0002 h Ϫ1 ), three times slower than that for ammonium uptake (overall mean Ϯ SE ϭ 0.0038 Ϯ 0.0002 h Ϫ1 ). The dominance of ammonium over nitrate uptake is indicative of a system dominated by recycled production, as described for other similarly oligotrophic regions of the tropical ocean (Eppley 1981;Murray et al 1989;Dugdale et al 1992;Peña et al 1992;Harrison et al 1996).…”

“…In the open ocean, under equilibrium conditions the nitrogen needed to support primary production is believed to be mostly supplied from the nutrient-rich deeper waters via advective fluxes of nitrate during upwelling or vertical mixing or diffusive fluxes driven by the nitrogen gradient across the thermocline (Lewis et al 1986). Advective nitrate fluxes in the open ocean tend to be higher at high latitudes, where convective mixing favors high levels of primary production.…”

“…Studies in the Atlantic Ocean are largely restricted to the North Atlantic (Lewis et al 1986;Glibert et al 1988;Eckman 1994;Harrison et al 1996), and there is a remarkable paucity of information on the Central and South Atlantic (Le Bouteiller 1986;Metzler et al 1997). These studies have provided some evidence of imbalances between the diffusive nitrogen supply and new production (McGillicuddy et al 1998;Oschlies and Garçon 1998).…”

The supply of nutrients due to vertical turbulent mixing: A study at the Porcupine Abyssal Plain study site in the northeast Atlantic

“…Winter convective mixing (Williams et al, 2000), mesoscale upwelling (Pollard & Regier, 1992;Allen et al, 2005) and small-scale turbulent mixing (Lewis et al, 1986;Carr et al, 1995;Law et al, 2001;Law et al, 2003) are three that have received perhaps the most attention. To understand the controls on phytoplankton growth in a region, it is necessary to quantify the contributing flux associated with each pathway.…”