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 My name is …

Rationale winter 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? Both light and iron (drivers of productivity) are driven by the seasonal mixed layer, ie in winter when mixed layers are deep productivity is low, whereas in summer shallow mixed layers stimulate productivity, also on the back of tapping in to an iron source in winter. Summer has intraseasonal component

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. Various regions have varied mean seasonal cycle reproducibility: We think it is mld variability that is driving the intraseasonal Thomalla et al., 2011.

Chlorophyll response intraseasonal MLD variability Different regions have a different response to intraseasonal MLD variability. Correlation between MLD and Chl to explore variable intraseasonal response to MLD variability (particularly in relation to the light and iron limitation) Fauchereau et al., 2011.

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) Take more time to explain 2 points. Non linear relationship between light and NCP elevated and highly variable in high light Light seems to be an important driver for NCP What drives the elevated and variable productivity when mixed layers are shallow?

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 Explain normalised NCP and mean light plot (and its significance) Shows that light is important driver surprisingly in an Fe limited system, but is it the only driver? Light surprisingly strong influence on chlorophyll normalised NCP, is it the sole driver of variability of NCP?

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. What would you expect from productivity?

Dissolved Fe concentrations High dFe in STZ Explain diagram a bit more. In contrast, if iron was the only control. High 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

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

Meridional gradient in PP PAR or Fe alone would predict highest NCP in STZ and lowest in PFZ However, highest and most variable ΔO2/Ar ratios (NCP) in the SAZ, corresponding with chl-a concentrations What about the 45m line?? Explain.

Productivity and MLD variability Light STZ 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

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. Nuanced explanation of Fe and Light driving PP, Light and intraseasonal resupply of Fe