LARGE-SCALE BIOGEOCHEMICAL/PHYSICAL INTERACTIONS IN ELEMENTAL CYCLES

Reference
Gruber, N., and J. L. Sarmiento, Biogeochemical/Physical Interactions in Elemental Cycles,
in THE SEA: Biological-Physical Interactions in the Oceans , edited by A. R. Robinson, J. J. McCarthy, and B. J. Rothschild, John Wiley and Sons, Volume 12, 337-399, 2002.

Introduction
Why is the distribution of most chemicals not uniform within the ocean
(Figure1)? In the absence of sources and sinks the effect
of ocean circulation and mixing would be to smooth the distribution of
chemicals. An important reason for non-uniformity is the increase in
concentration that occurs in regions of evaporation, and the reduction in
concentration that occurs in regions of rainfall or river input at the ocean
atmosphere interface. Evaporation, precipitation, and river input
lead to concentration ranges usually well below 10% as evidenced in
the oceanic variability of salinity. However, as Figure1
shows, many chemicals have concentration ranges much greater than
this. The concentration of inorganic nitrogen, for example, is more
than five times smaller near the surface compared to the deep ocean.
Dissolved inorganic carbon shows a surface depletion that is also
substantially larger than the 10% variation that could be induced by
freshwater fluxes.

The primary mechanism that drives such large concentration gradients
is the use of dissolved inorganic chemicals by living organisms for
formation of organic matter and inorganic solids. Photosynthetic
organisms use the energy from light in the upper part of the ocean to
produce both particulate and dissolved organic matter and a wide range
of inorganic solids as well. These "biogenic" materials are often
transported long distances before being converted back to dissolved
inorganic chemicals by processes such as "remineralization" and
dissolution. The largest such transports are vertical, giving rise to
greatly reduced concentrations in the surface ocean, and enhanced
concentrations in the abyss. A few chemicals, the most important
being oxygen, are affected by these processes in the opposite
direction. They are released during photosynthesis and consumed
during remineralization. The wide range of biological and associated
processes that affect such chemicals are referred to as
"biogeochemical" processes.

The view that therefore emerges with regard to the large scale
distribution of chemicals in the ocean is one where biogeochemical
processes are constantly creating spatial and temporal gradients in
the chemicals, whereas ocean circulation and mixing are in general
attempting to homogenize these gradients. The resulting distribution
of the chemicals in the ocean can thus be understood as the result of
a complex interplay between biogeochemical cycles and ocean physics.
These large scale biogeochemical/physical interactions are the focus
of this chapter. In particular, we are interested in determining the
relative roles of the various ``gradient makers'' in creating the
large-scale distribution of chemicals in the ocean. We will focus our
discussion on the two most important biogeochemical elements, carbon
and nitrogen. We will include the cycling of phosphorus and oxygen
since they allow us to gain much additional information about carbon
and nitrogen.

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