Iron Limitation of Silica Production in the Southern Ocean: A Companion Proposal to the SOFeX Experiment

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The results of iron enrichment experiments in the high nitrate, low chlorophyll (HNLC) waters of the Equatorial Pacific, the Subarctic Pacific and most recently in the Southern Ocean demonstrate that phytoplankton growth and biomass in those areas is limited by low concentrations of available iron. A key result common to almost all shipboard Fe enrichment experiments from all Fe-limited regions is that diatoms are stimulated to a greater degree than are other phytoplankton taxa. The same preferential stimulation of diatoms over other taxa, was observed during IlronEx 11 where an in situ iron fertilization in the equatorial produced a bloom dominated by diatoms. Despite such a consistent strong response of diatoms to iron addition, the effects of Fe limitation on diatom silica production in situ remains un-examined.Dr. Brzezinski will examine the effect of in situ iron fertilization on silica production and the kinetics of Si(OH)4 use in the Polar Front region of the Southern Ocean in conjunction with the SOFeX projects led by Moss Landing Marine Laboratory. The Southern Ocean is the largest HNLC region on Earth. Of all the HNLC areas, diatoms are likely to be important in the Southern Ocean as over half of modem global accumulation of siliceous sediment occurs there forming the Great Sedimentary Opal Belt that girdles most of the Southern Ocean.Data from the recent AESOPS program indicate that Fe and Si both regulate diatom biomass and silica production in the Polar Front. In addition, this laboratory has produced the first data from two separate HNLC regions that demonstrate a strong response of diatom silica production to added Fe and Zn. Given the strong response of diatom growth to iron enrichment observed in several HNLC regions in the past, our new direct observations that additions of Fe enhance silica production in the Polar Front and the dominance of diatoms in the phytoplankton of the Southern Ocean, they anticipate that stimulation of diatom silica production will be one of the most pronounced biogeochemical responses to Fe fertilization during SOFeX. That result will have strong implications for the mechanisms controlling silica production rates and the contribution of diatoms to carbon flux in the very region that dominates the silica cycle of the modem ocean.