Geoscience Reference
In-Depth Information
decades and whether they can rapidly respond to the current
anthropogenic changes. In this respect, if N
2
fixation stopped in the
ocean (which is not considered possible), all the available nitrogenous
marine nutrients would be used up within 2,000 years (i.e. a very short
period of time geologically speaking), which would put an end to
marine biological production.
In the ocean, oceanographers distinguish between new nitrogen
and nitrogen regenerated in the part of the water column where
photosynthesis creates new organic matter (i.e. in surface waters).
New N is the nitrogen that enters surface waters from external
sources, by vertical mixing (mainly NO
3
-
) and by other processes such
as N
2
fixation. Regenerated N is the nitrogen that is recycled in surface
waters (mainly NH
4
+
). The synthesis of organic matter by
phytoplankton is called primary production (section 4.1.2); new
nitrogen gives rise to new primary production, and regenerated
nitrogen to regenerated primary production. The rate at which
organic matter can be exported from a pelagic marine ecosystem, for
example toward deep waters or by fishing, cannot exceed its new
production otherwise the ecosystem would disappear. In an
oceanic area where all nitrogen would come from recycling
within surface waters (i.e. an area where new production would be
zero), no organic matter could be exported from the surface layer,
for example by fishing, without destroying the ecosystem.
Compared with economics, regenerated production would correspond
to capital and new production, to interest. Hence, the sustainable
exploitation of marine renewable resources requires that the
maximum amount of biomass that is taken per unit time is not
greater than new production. In other words, sustainability requires
that the renewal of the exploited biomass per unit time be equal
to or greater than regenerated production. This is a necessary but
insufficient condition because the equilibrium of an ecosystem
may still be disturbed despite a rate of removal of biomass that is
smaller than new production, for example when there is overfishing of
species or groups of species that play a key role in structuring the food
web.
