Geoscience Reference
In-Depth Information
atmosphere. In the case of N
2
, the formula for its transformation is the
following:
NO
3
-
+ 1,25CH
2
O + H
+
→ 0,5N
2
+ 1,25CO
2
+ 1,75H
2
O [1.9]
The compounds produced by denitrification, which contain
nitrogen, are transported in dissolved form by oceanic circulation and
molecular diffusion up to the surface of the ocean, where they are then
in excess and cause a balancing flux into the atmosphere. The quantity
of oceanic nitrate is thus diminished.
When the atomic ratio N/P in the interior ocean is greater than 16
(Redfield ratio), photosynthesis is not limited by the nitrate and, all
things being equal, gives rise to a greater productivity and an
increased exportation of organic matter into the interior ocean. The
consequences are a development of anoxic environments, an increase
in denitrification and, finally, a loss of nitrate from the ocean. This
decreases the N/P ratio, thus readjusting the proportion of
nitrate/phosphate to biological demand.
1.5.3.
The regulation of the N/P ratio
Measurements made over a number of years in the diverse water
masses of the current global ocean (e.g. [FOR 12]) show a statistical
nitrate/phosphate ratio whose slope is close to 16, which is also the
N/P ratio of the biological demand for phytoplanktonic productivity in
the ocean.
For a long time, oceanographers wondered whether this similarity
was due to the fact that the phytoplankton had adapted to the ocean's
chemistry or whether, conversely, the chemistry resulted from the
long-term action of the phytoplankton. Monitoring, over the course of
the last 20 years, of the phenomena described above [ALT 95,
AND 07, MAR 06] now enables us to confirm that both participants,
the life forms and the biochemistry, permanently adjust to each other.
This adjustment reacts especially to disturbances linked to climate
change on timescales of the order of a few thousand years.
But this adjustment also reacts to more long-term evolutions. For
example, the influx of phosphate and the ocean thermohaline
