Geoscience Reference
In-Depth Information
by nitrogen if the N/P ratio in the ocean is less than 16 or by
phosphorus if the N/P ratio in the ocean is greater than 16.
Phosphorus in the form of dissolved PO
4
3-
is a result of continental
erosion and is carried to the ocean by rivers. It escapes the ocean
through the accumulation of sediments as organic matter or
precipitated solid phosphates. The latter are mainly created by living
organisms or by epigenesis in superficial sediments (dissolution of
carbonate particles in certain conditions, leaving room for a
precipitation of calcium phosphate). The phosphorus used by
photosynthesis is largely recycled in the ocean by the breaking down
of organic matter (respiration). As the variations in inward and
outward flux are weak relative to the oceanic stock of dissolved
phosphorus, the variation in this stock is very slow. It is estimated that
the average residence time of phosphorus in the ocean is in the order
of 80,000 years [FRO 82]. As a result, the control on oceanic
photosynthesis by the availability of phosphorus occurs on the same
timescale [TYR 99].
Unlike phosphorus, the inward and outward fluxes of oceanic
nitrogen do not occur only through continental erosion and
sedimentation, since nitrogen can evaporate quickly into the
atmosphere in the form of N
2
. The average residence time of nitrogen
in the ocean, mainly as nitrate (NO
3
-
), is only 8,000 years [SCH 91].
The stock of oceanic nitrogen can therefore be modified fairly rapidly
in response to fluctuations of the Redfield ratio N/P in the ocean,
which tends to regulate the general photosynthetic productivity in the
ocean. In the following sections, the processes by which the regulation
is carried out are presented.
1.5.1.
When the ocean is deficient in nitrate
The majority of species of phytoplankton find nitrogen in its nitrate
(NO
3
-
) form. Some, a minority, are capable of carrying out photo-
synthesis using nitrogen (N
2
) dissolved in seawater, of which the
partial pressure on the surface tends to balance with that of atmospheric
