Global mass-balance considerations, vertical nutrient profiles, seasonal nutrient cycles and preliminary silica flux data all indicate the large subtropical mid-ocean gyres may account for the majority of the global oceanic production of biogenic silica. Diatoms are reported to bloom episodically in summer in response to mixing events. However, the available evidence suggests that most of the annual silica production occurs during extended periods of low primary productivity and low diatom abundance in summer and autumn. While virtually all of this silica is redissolved within the water column, the most conservative estimates of the export of silica out of the upper 150m indicate that it is considerable in global terms. This project represents a seasonal study of the production, redissolution and vertical transport of biogenic silica in the Sargasso Sea in conjunction with the U.S. JGOFS time-series observations southeast of Bermuda. The study will address two major objectives: (1) to evaluate the role of the oligotrophic mid-ocean gyres in the global oceanic silica cycle, and (2) to assess the importance of the siliceous phytoplankton in new production and the vertical export of organic matter in these areas. This study will obtain year-round data for three years on the vertical flux of particulate silica at the JGOFS time-series site and will measure silica production and dissolution rates in the upper ocean at key times of year. These measurements will be joined with the planned JGOFS time-series core measurements of dissolved silicic acid and particulate silica to achieve the first full seasonal study of the cycling of silica in the ocean.
