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Objectives

Response of Diatom Biomass and Silica Production to Iron Fertilization during SOFeX Mark A. Brzezinski, Mark S. Demarest and Janice A. Jones Marine Science Institute, University of California, Santa Barbara CA. Objectives. Assess the effect of Fe fertilization on:

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Objectives

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  1. Response of Diatom Biomass and Silica Production to Iron Fertilization during SOFeXMark A. Brzezinski, Mark S. Demarest and Janice A. JonesMarine Science Institute, University of California, Santa Barbara CA.

  2. Objectives Assess the effect of Fe fertilization on: • diatom biomass and silicic acid depletion • silica production and specific Si uptake rates Assess the role of Si limitation in moderating the response to Fe

  3. Location of Experiments: North Patch (55oS, Subantarctic) - high nitrate low silicic acid waters to the north of the Polar Front South Patch (66oS, Antarctic,) - high nitrate high silicic acid waters to the south of the Southern ACC Front

  4. Changes in North Patch bSiO2 and Si(OH)4 25 m The average [Si(OH)4] in the upper 25 m of the patch decreased by ca. 2 mM and remained unchanged outside. Biogenic silica increased by ca. 1 mmol Si l-1 in the upper 25 m of the patch and remained essentially constant outside.

  5. Fe Increased Silica Production in the North Patch 25 m Integrated silica production increased by 480% inside the patch with no change outside the patch. Specific rates of silica production increased by 300% inside the patch compared to a 48% increase outside.

  6. Si Limitation in North Patch Adding Si(OH)4 increased silica production rates both inside and outside the patch.

  7. North Patch Summary • Fe stimulated increases in siliceous biomass and enhanced silicic acid depletion. • Fe stimulated a >400% increase in silica production rates. • Chronic Si limitation of silica production likely reduced the rate of response of diatoms to Fe.

  8. South Patch Day 14 - 15 [Si(OH)4] Chl FL Chlorophyll fluorescence inversely correlated with Si(OH)4 depletion

  9. South Patch Day 14 - 15 [Si(OH)4] Chl FL [bSiO2] High biogenic silica concentrations are found both inside and outside of the patch.

  10. Radial Transect Day 16-17 [bSiO2] [SF6] [Si(OH)4]

  11. A Patchy South Patch • High diatom biomass occurred both inside and outside the patch. • High chlorophyll fluorescence inversely correlated with regions of maximum Si(OH)4 depletion. High SF6 and high chl fluorescence were the main criteria for locating stations. MAY MISS REGIONS OF MAXIMUM Si(OH)4 DEPLETION AND DIATOM BIOMASS!

  12. Changes in South Patch bSiO2 and Si(OH)4 25 m The average [Si(OH)4] in the upper 25 m of the patch decreased by 3 mM with no apparent trend outside. Biogenic silica initially increased, but then declined by ca. 1.5 mmol Si l-1 in the upper 25 m both inside and outside the patch.

  13. Changes in South Patch bSiO2 and Si(OH)4 25 m Transect and Grids indicate maximum depletion of 4 mM Si(OH)4 in the upper 25 m. Transects and Grids indicate maximum [bSiO2] of 4.2 mmol Si l-1 in the upper 25 m.

  14. Fe Increases Silica Production in the South Patch 25 m Integrated silica production increased by 320% inside the patch with no change or a decline outside the patch. Specific rates of silica production increase both inside and outside the patch, but higher rates are achieved in the patch.

  15. No Si Limitation in South Patch Adding Si(OH)4 does not increase silica production rates inside or outside the south patch. System not Si limited.

  16. Fe Increases the Export of Biogenic Silica in the South Patch Flux of biogenic silica increases 370% faster inside compared to outside Thorium-234, Buesseler & Brzezinski Flux increases 46% inside Flux increases 24% outside

  17. Conclusions: • South Patch • Si(OH)4 drawdown of 4 mM to 59.8 mM • bSiO2 increased from ca. 2 to 4 mM • Silica production rates increased 320% for r and 400% for Vb • Si did not limit silica production inside or outside patch • Fe enhanced the export of biogenic silica

  18. Conclusions: • South Patch • Si(OH)4 drawdown of 4 mM to 59.8 mM • bSiO2 increased from ca. 2 to 4 mM • Silica production rates increased 320% for r and 400% for Vb • Si did not limit silica production inside or outside patch • Fe enhanced the export of biogenic silica North Patch • Si(OH)4 depletion of 2.5 mM to ca. 0.5 mM • bSiO2 increase of > 400% reaching ca. 1 mmol Si l-1 • Silica production rates increase 400% for r and 300% for Vb • Silica production Si-limited both inside and outside patch

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