Dataset: Si Cycling and Si Control

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.