161
9.2 Large-Scale Shifts in N 2 Fixation in the Global Ocean
Upper ocean primary production and N 2 fixation will change in the face of increasing CO 2 concentrations and associated effects. Trichodesmium, which is generally
found at surface seawater temperatures at or above 20 °C (Carpenter 1983), is likely
to expand its geographic range as ocean surface temperatures increase into higher
latitudes in the future ocean (Boatman et al. 2017; Breitbarth et al. 2007; Fu et al.
2014; Hutchins et al. 2013). Specific strains of Crocosphaera and Trichodesmium
adapted to higher CO 2 concentrations may also be able to expand their contributions
to N 2 fixation globally as a result of such adaptations and potential niche differentiation (Hutchins et al. 2013).
Forward-looking predictions of N 2 fixation have been made in several biogeochemical models. As noted above and in Chap. 7, increased atmospheric deposition
of nutrients may affect the distribution of diazotrophs globally either positively or
negatively (Jickells et al. 2017; Krishnamurthy et al. 2009). Cases in which possible
effects of increased Fe and combined N alone and in combination over the next
550 years yielded changes of +15 Tg N y
−1
, −7 Tg N y
−1
and + 5 Tg N y
−1
for the
three scenarios, respectively, relative to the control case (Krishnamurthy et al. 2009)
(Table 8.1). Diazotroph biogeography at the end of the twenty-first century was
considered in an ocean in which physical dynamics are altered by warming with a
constant Fe (pre-industrial) input resulting in a redistribution of diazotrophic biomass which expand into the equatorial and South Pacific with an overall increase in
ocean area inhabited by diazotrophs of 17% along with a 17% increase in N inputs
(Dutkiewicz et al. 2014). When Fe inputs were doubled, the area expanded to 38%
of the surface ocean with a 28% increase in N inputs. The elevated optimal growth
temperature under Fe-limited conditions by Trichodesmium erythraeum is predicted
to allow up to 22% greater N 2 fixation rates in severely Fe-limited regions of the
Ocean (Jiang et al. 2018).
A relatively recent model which simulates N cycle processes over a 6000 year
future scenario with weakening global ocean circulation indicates that N 2 fixation
lags but compensates for increasing denitrification in the world’s oceans, with each
process approximately doubling from about 140 Tg N y
−1
over the next
1000–2000 years (Oschlies et al. 2019). Wrightson and Tagliabue (2020) compared
the response of N 2 fixation across an ensemble of nine earth systems models with
divergent results. While most models agreed that N 2 fixation would decrease in the
coming decades to 2100, and that this decrease would be most pronounced in the
Atlantic and Indian Oceans, regional variation in the Pacific may offset this. The
models also indicate that this variation of N 2 fixation will directly influence primary
productivity.
These possible shifts in diazotrophic communities and distributions would have
direct consequences on the structure of associated food webs. This will also depend
on the size spectra of specific diazotrophs that proliferate (Berthelot et al. 2016;
Caffin et al. 2018).
9.2 Large-Scale Shifts in N 2 Fixation in the Global Ocean
9.2 Large-Scale Shifts in N 2 Fixation in the Global Ocean
Upper ocean primary production and N 2 fixation will change in the face of increasing CO 2 concentrations and associated effects. Trichodesmium, which is generally
found at surface seawater temperatures at or above 20 °C (Carpenter 1983), is likely
to expand its geographic range as ocean surface temperatures increase into higher
latitudes in the future ocean (Boatman et al. 2017; Breitbarth et al. 2007; Fu et al.
2014; Hutchins et al. 2013). Specific strains of Crocosphaera and Trichodesmium
adapted to higher CO 2 concentrations may also be able to expand their contributions
to N 2 fixation globally as a result of such adaptations and potential niche differentiation (Hutchins et al. 2013).
Forward-looking predictions of N 2 fixation have been made in several biogeochemical models. As noted above and in Chap. 7, increased atmospheric deposition
of nutrients may affect the distribution of diazotrophs globally either positively or
negatively (Jickells et al. 2017; Krishnamurthy et al. 2009). Cases in which possible
effects of increased Fe and combined N alone and in combination over the next
550 years yielded changes of +15 Tg N y
−1
, −7 Tg N y
−1
and + 5 Tg N y
−1
for the
three scenarios, respectively, relative to the control case (Krishnamurthy et al. 2009)
(Table 8.1). Diazotroph biogeography at the end of the twenty-first century was
considered in an ocean in which physical dynamics are altered by warming with a
constant Fe (pre-industrial) input resulting in a redistribution of diazotrophic biomass which expand into the equatorial and South Pacific with an overall increase in
ocean area inhabited by diazotrophs of 17% along with a 17% increase in N inputs
(Dutkiewicz et al. 2014). When Fe inputs were doubled, the area expanded to 38%
of the surface ocean with a 28% increase in N inputs. The elevated optimal growth
temperature under Fe-limited conditions by Trichodesmium erythraeum is predicted
to allow up to 22% greater N 2 fixation rates in severely Fe-limited regions of the
Ocean (Jiang et al. 2018).
A relatively recent model which simulates N cycle processes over a 6000 year
future scenario with weakening global ocean circulation indicates that N 2 fixation
lags but compensates for increasing denitrification in the world’s oceans, with each
process approximately doubling from about 140 Tg N y
−1
over the next
1000–2000 years (Oschlies et al. 2019). Wrightson and Tagliabue (2020) compared
the response of N 2 fixation across an ensemble of nine earth systems models with
divergent results. While most models agreed that N 2 fixation would decrease in the
coming decades to 2100, and that this decrease would be most pronounced in the
Atlantic and Indian Oceans, regional variation in the Pacific may offset this. The
models also indicate that this variation of N 2 fixation will directly influence primary
productivity.
These possible shifts in diazotrophic communities and distributions would have
direct consequences on the structure of associated food webs. This will also depend
on the size spectra of specific diazotrophs that proliferate (Berthelot et al. 2016;
Caffin et al. 2018).
9.2 Large-Scale Shifts in N 2 Fixation in the Global Ocean