Carbon‐Based Estimate of Nitrogen Fixation‐Derived Net Community Production in N‐Depleted Ocean Gyres

Abstract

Accurate estimation of net community production (NCP) in the ocean is important for determining the future trend for carbon dioxide concentrations in the atmosphere and thus for understanding the global carbon cycle and climate change. Most methods for measuring NCP rely on analysis of dissolved fixed inorganic nitrogen species (N), which are believed to be limiting factors for NCP. However, in the vast areas of the ocean gyres only low levels of N are available for phytoplankton during much of the year. In this study the NCP was estimated by summing the seasonal reduction in the concentration of dissolved inorganic carbon (CT) in the surface mixed layer, corrected for changes associated with salinity variation, net air‐sea CO2 flux, horizontal C advection, non‐Redfield diffusive C and N fluxes (deviations from the C:N ratio of 7), and anthropogenic nitrogen deposition. The mixed layer reduction in CT was calculated from an annual CT cycle, deduced from comprehensive records of surface pCO2 and total alkalinity, using an established thermodynamic model. This method yielded a value of 0.6 ± 0.2 Pg of C, which represents the NCP that occurred during the warming period (approximately 8 months) in the nitrate‐depleted (<0.2 μmol/kg) ocean. Our estimate is broadly consistent with the global N2 fixation rate estimated using the 15N‐based method and suggests that N2 fixation by microorganisms is a major driver for this NCP.

Global trends in marine nitrate N isotopes from observations and a neural network-based climatology

“…Our second piece of evidence that the export and remineralization of organic matter drives the inter-basin differences in deep nitrate δ 15 N comes from sediment trap measurements. Averaging published sediment trap organic matter δ 15 N from the subtropical and tropical Pacific gives a value of 8.5 ± 2.9 ‰ (Knapp et al, 2016;Robinson et al, 2012), which is significantly higher than measured in traps from the Atlantic (4.5 ± 1.5 ‰) (Freudenthal et al, 2001;Holmes et al, 2002;Lavik, 2000;Thunell et al, 2004). Given observed Southern Ocean nitrate characteristics (Rafter et al, 2013), we estimate an even lower typical sinking organic matter δ 15 N of +1.5 ‰ for this region, which assumes initial nitrate values equal the upper circumpolar deep water and final values from the open Antarctic zone.…”

The impact of the South-East Madagascar Bloom on the oceanic CO&lt;sub&gt;2&lt;/sub&gt; sink

“…At a global scale, the presence of N 2 fixers in the southwestern Indian Ocean has been detected from satellite data (Westberry and Siegel, 2006;Qi et al, 2020), and relatively high N 2 fixation rates in austral summer in this region were also derived from N 2 fixation data using a machine learning approach (Tang and Cassar, 2019;Tang et al, 2019). A largescale distribution of diazotrophy was further estimated from surface C T observations, suggesting the presence of N 2 fixers in the Mozambique Channel and the south-western Indian Ocean (Lee et al, 2002;Ko et al, 2018). These authors used regional relationships of N-C T versus SST to reconstruct the N -C T field from which they estimated the net carbon production (NCP) in nitrate depleted waters, a proxy for carbon production by N 2 -fixing microorganisms.…”

New insights into the distributions of nitrogen fixation and diazotrophs revealed by high-resolution sensing and sampling methods

“…Nitrogen (N 2 ) fixation, conducted by a group of specialized microorganisms called diazotrophs, provides the largest 28 external nitrogen input into the ocean [1,2]. Alleviating 29 nitrogen limitation over a large portion of the global surface 30 oceans, N 2 fixation supports new production and net ocea-31 nic carbon uptake [3,4]. Over geological time scales, N 2 32 fixation is believed to compensate for nitrogen removal 33 from denitrification and anammox [5].…”