179
tion inputs are about 200 Tg N y
−1
, comparable to terrestrial natural and agricultural
rates and industrial synthesis. Several analyses conclude that the N cycle is roughly
in balance with regard to inputs and removal, but that this balance is only achieved
over the time scales of ocean circulation of a few thousand years. How the relative
balance of N 2 fixation and denitrification changes in the future is an open question.
Most modeled, integrative geochemical-based measures include all sources of
input, but field measurements have largely focused on surface populations predominated by cyanobacteria. The relative quantitative contribution of sub-surface N 2
fixation remains to be established.
10.9 Marine N 2 Fixation, Global Change and the Future
Burgeoning human populations and directly related ongoing climate change is
resulting in dynamic shifts in many factors in the oceans which affect biology.
These include temperature, CO 2 concentrations and pH as well as the increased
deposition of various chemical species consisting of forms of combined N and Fe
into the surface ocean through atmospheric flux from land. Diazotrophs are sensitive to all these changing factors, responding positively to some and negatively
to others.
How trends in temperature, CO 2 concentrations and atmospheric deposition will
affect the future distribution of diazotrophs and N 2 fixation are being actively probed
in laboratory experiments as well as in global modelling efforts.
Increasing temperatures may directly affect the metabolism of diazotrophs up to
a point of thermal stress, as well as increasing the range of potential habitats available to them. CO 2 appears to stimulate both N 2 fixation and CO 2 fixation in some
marine diazotrophs such as Trichodesmium and Crocosphaera, although this may
be strain specific. Increased inorganic N in surface waters may depress rates of N 2
fixation, thereby shifting the structure of food webs dependent upon these differing
inputs. Interestingly, many of these factors are interactive where variation in one
factor (e.g. growth limitation by a nutrient) may affect the relative sensitivity to
another (e.g. stimulation by increasing CO 2 concentration).
Model simulations from several research groups which include anticipated physical changes in ocean circulation and stratification indicate that N 2 fixation will be
redistributed within each ocean basin and some suggest it may in fact increase in
certain regions. This has direct implications for primary production and depending
on the specific diazotrophs which predominate, will have important remifications
for the structure of associated food webs in those basins.
Marine N 2 fixation may also potentially play a role in mitigating some aspects of
climate change as well as supporting the nutritional needs of the growing human
populations. N 2 fixation in large areas of the oligotrophic ocean is primarily limited
by the availability of Fe. Large scale stimulation of surface ocean N 2 fixation by the
addition of Fe as an inorganic species or as dust has been proposed as one possible
means of increasing C sequestration in these regions as well as promoting primary
10.9 Marine N 2 Fixation, Global Change and the Future
tion inputs are about 200 Tg N y
−1
, comparable to terrestrial natural and agricultural
rates and industrial synthesis. Several analyses conclude that the N cycle is roughly
in balance with regard to inputs and removal, but that this balance is only achieved
over the time scales of ocean circulation of a few thousand years. How the relative
balance of N 2 fixation and denitrification changes in the future is an open question.
Most modeled, integrative geochemical-based measures include all sources of
input, but field measurements have largely focused on surface populations predominated by cyanobacteria. The relative quantitative contribution of sub-surface N 2
fixation remains to be established.
10.9 Marine N 2 Fixation, Global Change and the Future
Burgeoning human populations and directly related ongoing climate change is
resulting in dynamic shifts in many factors in the oceans which affect biology.
These include temperature, CO 2 concentrations and pH as well as the increased
deposition of various chemical species consisting of forms of combined N and Fe
into the surface ocean through atmospheric flux from land. Diazotrophs are sensitive to all these changing factors, responding positively to some and negatively
to others.
How trends in temperature, CO 2 concentrations and atmospheric deposition will
affect the future distribution of diazotrophs and N 2 fixation are being actively probed
in laboratory experiments as well as in global modelling efforts.
Increasing temperatures may directly affect the metabolism of diazotrophs up to
a point of thermal stress, as well as increasing the range of potential habitats available to them. CO 2 appears to stimulate both N 2 fixation and CO 2 fixation in some
marine diazotrophs such as Trichodesmium and Crocosphaera, although this may
be strain specific. Increased inorganic N in surface waters may depress rates of N 2
fixation, thereby shifting the structure of food webs dependent upon these differing
inputs. Interestingly, many of these factors are interactive where variation in one
factor (e.g. growth limitation by a nutrient) may affect the relative sensitivity to
another (e.g. stimulation by increasing CO 2 concentration).
Model simulations from several research groups which include anticipated physical changes in ocean circulation and stratification indicate that N 2 fixation will be
redistributed within each ocean basin and some suggest it may in fact increase in
certain regions. This has direct implications for primary production and depending
on the specific diazotrophs which predominate, will have important remifications
for the structure of associated food webs in those basins.
Marine N 2 fixation may also potentially play a role in mitigating some aspects of
climate change as well as supporting the nutritional needs of the growing human
populations. N 2 fixation in large areas of the oligotrophic ocean is primarily limited
by the availability of Fe. Large scale stimulation of surface ocean N 2 fixation by the
addition of Fe as an inorganic species or as dust has been proposed as one possible
means of increasing C sequestration in these regions as well as promoting primary
10.9 Marine N 2 Fixation, Global Change and the Future