Geoscience Reference
In-Depth Information
N
2
(a major component of the atmosphere). The process of biological
fixation of nitrogen is also called “diazotrophy”.
The use of nitrates by oceanic photosynthesis has a better energetic
yield than that of N
2
. Moreover, nitrate is the dominant form of
nitrogen dissolved in the ocean. Therefore, where the photosynthetic
production is not limited by other nutritive elements locally, it
preferentially uses nitrate.
The limit on this use lies in the fact that the photosynthetic
products are not entirely recycled in the ocean surface (a fine mixed
layer, some tens of meters thick, of which the upper, or euphotic part,
is where oceanic photosynthesis occurs), but it leads to an exporting of
organic matter, including nitrogen and phosphorus, into the ocean.
Numerous transitory phenomena, where non-photosynthetic species
(heterotrophs) are also involved, may or may not favor the yield of
this exportation. The turbulent mixing is the main process by which
nitrate or phosphate is reinjected from the reservoir of the internal
ocean, which constitutes the greatest part of the oceanic volume. This
mixing takes place under the mediation of certain physical phenomena
such as variations in the thickness of the layer of mixture, turbulence
affecting the base of this layer [FOR 12], known as thermocline, as
well as oceanic resurgences, or
upwellings
.
The use of nitrate by oceanic photosynthesis is therefore limited by
the atomic ratio N/P in the interior ocean, where the exported organic
matter is usually recycled, a small proportion being integrated
(fossilized) in sediments. When this ratio is less than the Redfield ratio
(16), the replenishing of the nutrient content of the mixed layer
spreads the deficit in nitrate up to the surface. The nitrate of the
euphotic zone is thereby rapidly exhausted, which favors the
photosynthetic species capable of carrying out the fixation of N
2
(this
concerns only a few species, which are sometimes represented by a
very large number of individuals). Thus, this time the replenishing
comes from the atmosphere. The exportation of organic matter and its
degradation in the interior ocean nevertheless continue, which leads to
the recycling of nitrogen, initially coming from the atmosphere as
gaseous N
2
, in the form of dissolved nitrate. The ocean thus
