Several recent studies, in systems as different
from one another as the Sargasso Sea, Equatorial Pacific and Southern Ocean,
have shown that diatoms are responsible for much of the new and export
production in surface waters. Those observations, combined with the diatoms'
absolute growth requirement for Si, suggest that the availability of dissolved
Si may regulate new and export production in much of the sea. Models of
carbon and nitrogen cycling in the upper ocean must therefore incorporate
Si control of diatom productivity and organic-matter export if their goal
is to predict the biological response of the oceans to natural and anthropogenic
forcing. That task is currently impossible for most of the ocean due to
the scarcity of data regarding factors regulating Si cycling (e.g. silica
production rates, silica dissolution rates and Si limitation of diatom
productivity).
It is now clear that the marine Si cycle is strongly bimodal in character.
The Southern Ocean lies at one extreme, where a relatively high fraction
(on the order of 10%) of the silica produced by diatoms in the surface
waters is preserved in the sediments. At the opposite extreme are the mid-ocean
gyres, where annual rates of silica production in surface waters are surprisingly
close to those in the Southern Ocean but almost none of the opal produced
accumulates in the sediments. The mechanisms that produce and regulate
this bimodal Si cycle must be understood, as they play a major role in
controlling the availability of dissolved Si in surface waters. This availability
in turn regulates diatom productivity and the ability of diatoms to contribute
to new and export production. Thus we must achieve a more realistic understanding
of the linkages between the Si cycle and the cycles of carbon and nitrogen,
both to determine when Si is - and is not - a major regulator of organic
carbon export and to model carbon export accurately when it is strongly
influenced by Si availability.
We propose to combine the synthesis of several large data sets obtained
during the U.S. and French JGOFS programs with a new generation of physical/biogeochemical
models which explicitly include Si regulation of diatom productivity, to
make the first data-based determination of the factors controlling the
cycling of Si in the upper 200 - 500 m of the ocean. We propose further
to investigate how changes in the character of the Si cycle affect the
ability of diatoms to contribute to carbon and nitrogen export from surface
waters. A team of U.S. and French investigators (Dave Nelson, Mark Brzezinski,
Paul Tréguer and Philippe Pondaven) will synthesize the information
on Si cycling and Si regulation of diatom productivity from extensive JGOFS
data sets obtained during the U.S. Bermuda Atlantic Times Series (BATS)
program in the Sargasso Sea, the U.S. Antarctic Environment Southern Ocean
Process Study (AESOPS) in the Pacific sector of the Southern Ocean and
the French ANTARES and KERFIX programs in the Indian sector of the Southern
Ocean. BATS, ANTARES and AESOPS are the only three projects yet conducted,
anywhere in the open sea, where studies of Si cycling and Si limitation
have been carried out in coordination with studies of primary production
and nitrogen cycling, with seasonal coverage. It is a great advantage that
these projects also investigated the two end members of the bimodal marine
Si cycle.
While those field programs were underway, new physical/biogeochemical
models were developed which explicitly include Si cycling and Si limitation
terms regulating diatom growth and productivity. One of us (Pondaven) has
been instrumental in developing those models, and we now propose to apply
them to the BATS, ANTARES and AESOPS study areas and the large data sets
on Si, C and N cycling obtained there. Through a combination of data synthesis
and numerical modeling we will: 1) identify those processes that are the
strongest determinants of the character of the Si cycle in the upper ocean,
and 2) assess how the resulting differences in the Si cycle affect the
ability of diatoms to contribute to new and export production.
The results will establish a foundation for the next generation of global
biogeochemical models of marine carbon cycling, which must explicitly incorporate
Si regulation of carbon and nitrogen export in systems where diatom productivity
is limited by Si.
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