Cycling of Silicon and the Significance of Silicon Limitation to Cell Flocculation in Coastal Diatom Blooms

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Dr. Brzezinski proposes to study the cycling of silicon and silicic acid as a limiting nutrient during the development and demise of diatom blooms of Southern California. These and other diatom blooms frequently flocculate and sink en masse without the intervention of grazers, unknown. Flocculation often occurs coincident with the depletion of silicic acid from the euphotic zone suggesting that silicon limitation can trigger these events. They will combine the fine scale sampling capabilities of divers using SCUBA with sophisticated tracer methods to test the hypothesis that silicon limitation is a prerequisite for the mass flocculation of diatom blooms. Recent evidence from a Gulf Stream warm-core ring suggests that the depletion of silicic acid from the euphotic zone is also accompanied by a shift in the source of silicic acid supporting silica production. Before depletion, silica production is supported mainly by "new" silicic acid entrained from deeper waters. Upon nutrient depletion, there is a shift to a heavy reliance on "regenerated" silicic acid derived from the local dissolution of siliceous particles. They propose to examine whether this transition occurs in more productive waters and to establish its relationship to the vertical flux of silicon and carbon.