150
cruises from the GEOSECS and WOCE programs, N 2 fixation was modeled in the
Atlantic Ocean by two approaches, (1) examining the accumulation of excess DIN
(N*) in waters diverging in the sub-euphotic zone waters coupled with estimates of
transport volumes associated with the upper limb of the Atlantic Meridional
Overturning Circulation and (2) the accumulation of excess DIN (N*) in deep
Atlantic waters (Moore et al. 2009). Values of 15.4 ± 4.2 and 21 ± 5.6 Tg N y
−1
,
respectively (Table 8.1) were derived. The first analysis indicated the bulk of N*
accumulation occurred north of the equator, and the authors concluded that Fe
rather than PO 4
−3
has a stronger influence as the primary factor controlling N 2 fixation in this basin. They also concluded that the high rates of N 2 fixation, particularly
in the North Atlantic, suggest a spatial uncoupling of these inputs with removal
processes can only be balanced on the time-scales of ocean circulation. Similarly,
combined estimates of NO 3
−
fluxes in deep North Atlantic and differences in isotopic signatures gave an estimate of 30 Tg the N y
−1
, with 90% of this occurring in
the tropics between 11°S and 30°N (Marconi et al. 2017). More recent derived
estimates for N 2 fixation in the Atlantic basin, using 2 very distinct modeling
approaches are remarkably similar (each about 30 Tg y
−1
) (Table 8.1) (Wang
et al. 2019).
The global distribution of N 2 fixation in modeling efforts are largely consistent
with observational data and with the distributions of diazotrophs (see Chap. 7),
although some spatial anomalies remain (e.g. higher rates in the S. Atlantic than
field observations would suggest). Despite the issues with the assumptions of each
approach (Gruber 2019), such models based on the distributions of nitrogenous
nutrients and their isotopes continue to be the best current way to integrate observational data and to make estimates at basin to global scales.
In general, global estimates have trended upwards over time, as the information
and databases of the distribution of diazotrophs, estimates of their rates and of geochemical proxies have greatly expanded. Early estimates in the 10s or low 100s of
Tg N y
−1
have been largely supplanted with global pelagic estimates around the
200 Tg N y
−1
range, placing oceanic N 2 fixation on a par with natural terrestrial,
managed agricultural and synthetic fertilizer inputs of N (Table 8.1).
The relative proportion of N 2 fixation among major basins may be affected by the
differential between diazotrophs compared to non-diazotrophic plankton with
regard to essential nutrients (Weber and Deutsch 2014). In their analysis, where Fe
requirements were similar for the two groups, N 2 fixation in the Pacific dominated
the global budget. As the relative Fe requirement of diazotrophs was raised, the
Atlantic (with its greater iron supply) became more important globally.
Heterotrophic N 2 Fixation Inputs
One of the earliest credible reports of measurable N 2 fixation in sub-euphotic zone
waters was by Voss et al. (2004) from a transect in the tropical North Atlantic.
Numerous observations have now followed in diverse pelagic marine systems. As
8 N 2 Fixation in Ocean Basins
cruises from the GEOSECS and WOCE programs, N 2 fixation was modeled in the
Atlantic Ocean by two approaches, (1) examining the accumulation of excess DIN
(N*) in waters diverging in the sub-euphotic zone waters coupled with estimates of
transport volumes associated with the upper limb of the Atlantic Meridional
Overturning Circulation and (2) the accumulation of excess DIN (N*) in deep
Atlantic waters (Moore et al. 2009). Values of 15.4 ± 4.2 and 21 ± 5.6 Tg N y
−1
,
respectively (Table 8.1) were derived. The first analysis indicated the bulk of N*
accumulation occurred north of the equator, and the authors concluded that Fe
rather than PO 4
−3
has a stronger influence as the primary factor controlling N 2 fixation in this basin. They also concluded that the high rates of N 2 fixation, particularly
in the North Atlantic, suggest a spatial uncoupling of these inputs with removal
processes can only be balanced on the time-scales of ocean circulation. Similarly,
combined estimates of NO 3
−
fluxes in deep North Atlantic and differences in isotopic signatures gave an estimate of 30 Tg the N y
−1
, with 90% of this occurring in
the tropics between 11°S and 30°N (Marconi et al. 2017). More recent derived
estimates for N 2 fixation in the Atlantic basin, using 2 very distinct modeling
approaches are remarkably similar (each about 30 Tg y
−1
) (Table 8.1) (Wang
et al. 2019).
The global distribution of N 2 fixation in modeling efforts are largely consistent
with observational data and with the distributions of diazotrophs (see Chap. 7),
although some spatial anomalies remain (e.g. higher rates in the S. Atlantic than
field observations would suggest). Despite the issues with the assumptions of each
approach (Gruber 2019), such models based on the distributions of nitrogenous
nutrients and their isotopes continue to be the best current way to integrate observational data and to make estimates at basin to global scales.
In general, global estimates have trended upwards over time, as the information
and databases of the distribution of diazotrophs, estimates of their rates and of geochemical proxies have greatly expanded. Early estimates in the 10s or low 100s of
Tg N y
−1
have been largely supplanted with global pelagic estimates around the
200 Tg N y
−1
range, placing oceanic N 2 fixation on a par with natural terrestrial,
managed agricultural and synthetic fertilizer inputs of N (Table 8.1).
The relative proportion of N 2 fixation among major basins may be affected by the
differential between diazotrophs compared to non-diazotrophic plankton with
regard to essential nutrients (Weber and Deutsch 2014). In their analysis, where Fe
requirements were similar for the two groups, N 2 fixation in the Pacific dominated
the global budget. As the relative Fe requirement of diazotrophs was raised, the
Atlantic (with its greater iron supply) became more important globally.
Heterotrophic N 2 Fixation Inputs
One of the earliest credible reports of measurable N 2 fixation in sub-euphotic zone
waters was by Voss et al. (2004) from a transect in the tropical North Atlantic.
Numerous observations have now followed in diverse pelagic marine systems. As
8 N 2 Fixation in Ocean Basins