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.