W.R. Joubert , M. Bender, N. Cassar , A. Tagliabue, S.J. Thomalla , S. Swart, P.M.S. Monteiro
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Transcript of 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
Rationale
• 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?
winter summer
Thomalla et al., 2011.
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.
Chlorophyll response intraseasonal MLD variability
Fauchereau et al., 2011.
• Different regions have a different response to intraseasonal MLD variability.
Productivity variability with MLD
ΔO2/Ar ratios (NCP) elevated and variable in shallow mixed layers (< 45m)
ΔO2/Ar ratios (NCP) low in deep mixed layers (> 45m)
What drives the elevated and variable productivity when mixed layers are shallow?
In situ observations between 2008 - 2010
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?
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.
Dissolved Fe concentrations
Tagliabue et al., 2012
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
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
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
SAZSTZ PFZShallow MLD
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
Light
Productivity and MLD variability
Z
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.