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W.R. Joubert , M. Bender, N. Cassar , A. Tagliabue, S.J. Thomalla , S. Swart, P.M.S. Monteiro

The Sensitivity of the Seasonal Cycle of Phytoplankton Productivity to sub-Seasonal Mixed Layer Dynamics in the Sub-Antarctic Region. W.R. Joubert , M. Bender, N. Cassar , A. Tagliabue, S.J. Thomalla , S. Swart, P.M.S. Monteiro. winter. Rationale. summer.

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W.R. Joubert , M. Bender, N. Cassar , A. Tagliabue, S.J. Thomalla , S. Swart, P.M.S. Monteiro

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  1. The Sensitivity of the Seasonal Cycle of Phytoplankton Productivity to sub-Seasonal Mixed Layer Dynamics in the Sub-Antarctic Region W.R. Joubert, M. Bender, N. Cassar, A. Tagliabue, S.J. Thomalla, S. Swart, P.M.S. Monteiro

  2. winter Rationale summer • Southern Ocean drivers of HNLC character is Fe and light (and Si) limitation. • Both light and Fe are largely controlled by seasonal mixed layer dynamics [Boyd, 2002]. • What is the changing influence of the drivers (Fe and light) on in situ NCP variability in the Southern Ocean?

  3. Seasonal variability of phytoplankton biomass Different regions have varied seasonal cycle reproducibility. Regions are segregated into high and low chl-a biomass. The seasonal mixed layer is important in modulating the seasonal biomass, but it is complex. Thomalla et al., 2011.

  4. Chlorophyll response intraseasonal MLD variability Different regions have a different response to intraseasonal MLD variability. Fauchereau et al., 2011.

  5. Productivity variability with MLD ΔO2/Ar ratios (NCP) elevated and variable in shallow mixed layers (< 45m) In situ observations between 2008 - 2010 ΔO2/Ar ratios (NCP) low in deep mixed layers (> 45m) What drives the elevated and variable productivity when mixed layers are shallow?

  6. Productivity and water column irradiance Strong positive relationship between chlorophyll normalised NCP and mean water column irradiance. 3 factors implicit: increased iron leads to increased chl-a, higher chl to higher PP higher PP leads to higher NCP Light surprisingly strong influence on chlorophyll normalised NCP, is it the sole driver of variability of NCP?

  7. Meridional PAR and MLD PAR (MODIS) decrease with increasing latitude. MLD (EN3) increases with increasing latitude. Their gradients alone cannot explain the variability in productivity in the SAZ.

  8. Dissolved Fe concentrations High dFe in STZ dFe in SAZ is not inconsistent with a potential Fe source below the mixed layer, but this hypothesis needs to be tested Tagliabue et al., 2012

  9. Meridional gradient in PP However, highest and most variable ΔO2/Ar ratios (NCP) in the SAZ, corresponding with chl-a concentrations PAR or Fe alone would predict highest NCP in STZ and lowest in PFZ

  10. Meridional gradient in PP However, highest and most variable ΔO2/Ar ratios (NCP) in the SAZ, corresponding with chl-a concentrations PAR or Fe alone would predict highest NCP in STZ and lowest in PFZ

  11. Productivity and MLD variability STZ Light SAZ PFZ Shallow MLD Z MLD shallow but nutrients limit productivity MLD alternates between mixing (supplying nutrients) and quiescent periods (stimulating productivity) MLD deep and variable but light limits productivity

  12. In summary Two main points: 1) Non-linear relationship between light and PP. Shallow mixed layers have elevated PP, while PP remains low in deep mixed layers 2) In shallow mixed layers, PP is not only elevated but also highly variable. We propose variability in MLD drives the variability in PP.

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