Nitrogen fixation in the eastern Atlantic reaches similar levels in the Southern and Northern Hemisphere

Abstract

Euphotic layer dinitrogen (N2) fixation and primary production (PP) were measured in the eastern Atlantic Ocean (38°N–21°S) using 15N2 and 13C bicarbonate tracer incubations. This region is influenced by Saharan dust deposition and waters with low nitrogen to phosphorus (N/P) ratios originating from the Subantarctic and the Benguela upwelling system. Depth‐integrated rates of N2 fixation in the north (0°N–38°N) ranged from 59 to 370 µmol N m−2 d−1, with the maximal value at 19°N under the influence of the northwest African upwelling. Diazotrophic activity in the south (0°S–21°S), though slightly lower, was surprisingly close to observations in the north, with values ranging from 47 to 119 µmol N m−2 d−1. Our North Atlantic N2 fixation rates correlate well with dust deposition, while those in the South Atlantic correlate strongly with excess phosphate relative to nitrate. There, the necessary iron is assumed to be supplied from the Benguela upwelling system. When converting N2 fixation to carbon uptake using a Redfield ratio (6.6), we find that N2 fixation may support up to 9% of PP in the subtropical North Atlantic (20°N–38°N), 5% in the tropical North Atlantic (0°N–20°N), and 1% of PP in the South Atlantic (0°S–21°S). Combining our data with published data sets, we estimate an annual N input of 27.6 ± 10 Tg N yr−1 over the open Atlantic Ocean, 11% of which enters the region between 20°N and 50°N, 71% between 20°N and 10°S, and 18% between 10°N and 45°S.

Assessing Marine Nitrogen Cycle Rates and Process Sensitivities With a Global 3‐D Inverse Model

“…The use of World Ocean Atlas fields for this fixed N input function (examined closely by Martin et al, ) vastly simplifies the computational requirements of the model and allows us to focus on other aspects of the N cycle with this model. Our model parameterization yields a global N 2 fixation rate of 131 Tg N/year, which is similar in magnitude to estimates from both regional and global modeling and observational studies (Table S4; Deutsch et al, ; Luo et al, ; Fonseca‐Batista et al, ; Marconi et al, ). A fixed δ 15 N value (−1‰) was also assumed for newly fixed N (Carpenter et al, ; Hoering & Ford, ).…”

N<sub>2</sub> fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)

“…The high contribution measured in the MA region is an order of magnitude higher than that reported in previous studies performed in the Pacific Ocean (Moutin et al, 2008;Raimbault and Garcia, 2008;Shiozaki et al, 2014), the Atlantic Ocean (Fonseca-Batista et al, 2017;Rijkenberg et al, 2011) and the Mediterranean Sea (Moutin and Prieur, 2012), where it never exceeds 5 %, and also slightly higher than the contribution reported from a mesocosm experiment in the New Caledonia lagoon during a UCYN bloom (10.8 ± 5.0 %; Berthelot et al, 2015). As there was low supply of NO − 3 through vertical diffusion (< 8 %) and atmospheric deposition (< 1.5 %), N 2 fixation sustains nearly all new production during the austral summer in the WTSP.…”

Carbon Budgets of Mesozooplankton Copepod Communities in the Eastern Atlantic Ocean—Regional and Vertical Patterns Between 24°N and 21°S

“…Detailed hydrographic data sets down to 200 and 2,000 m are presented by Bode et al () and Bode et al (), respectively, as well as chl a distributions (Bode et al, ). Fonseca‐Batista et al () published the PP data. Station 5 (23.7°N) was located in the subtropical North Atlantic (STNA) with high sea surface salinity (SSS) of 36.6 and high sea surface temperature (SST) of 25°C.…”

“…The particulate organic carbon (POC) flux to each sampling layer was calculated using PP values for each station applying the equation published by Suess (): where C flux is the POC flux (mg C m −2 day −1 ) at depth Z , C prod is the PP (mg C m −2 day −1 ), and Z is the water depth (≥50 m). PP equals the average depth‐integrated carbon uptake rate at each station (see Figure 5a in Fonseca‐Batista et al, ).…”