Si Cycling and Si Control of New Production and Organic Carbon Export in the Southern Ocean and the Sargasso Sea: A JGOFS Synthesis

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Several recent studies in very different systems, such 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 silica (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 prodctivity 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. Si production rates, Si dissolution rates and Si limitation of diatom productivity).The PIs will synthesize several large data sets obtained during the U.S. and French JGOFS programs and use a newly developed physical/biogeochemical model, which explicitly inclues silica 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. They will 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. Through a combination of data synthesis and numerical modeling the PIs will: 1) identify those processes that are the strongest determinants of the character of the Si cycle in the upper ocean, and2) 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 regualtion of carbon and nitrogen export in systems where diatom productivity is limited by Si.