ASLO2005 Follows

							Macro-Nutrient Transport Pathways and
   Interactions with the Iron Cycle.
  Mick Follows, Stephanie Dutkiewicz, Payal Parekh: MIT
             Taka Ito: University of Washington
            Ric Williams: University of Liverpool



                                            Export and
                                            remineralization of
                                            sinking, organic
                                            particles moves
                                            nutrients to denser
                                            isopyncals:
 Return of macro-nutrients to euphotic zone requires diapycnal transfer




Atlantic basin
 Return of macro-nutrients to euphotic zone requires diapycnal transfer




Pacific basin
   Upwelling in Southern Ocean major return pathway...




Atlantic basin
Mode and intermediate waters formed equatorwards of ACC
- pathway for macro-nutrients to northern basins
(Sarmiento et al., 2003)

        surface nitrate              surface silica
Mode waters   Hanawa and Talley (2001)
Residual mean flow transports nutrients northwards to
sites of mode and intermediate water formation




  Ψres = ΨEkman + Ψeddy

Marshall (1997)
Residual mean flow transports nutrients northwards to
sites of mode and intermediate water formation




  Ψres = ΨEkman + Ψeddy

Marshall (1997)
●balance between            iron limited    macro-nutrient
northward residual mean                           limited
advection and export sets
                                           dust
subducted nutrient
concentration

●export sensitive to iron
availability

                                           subduction
                              upwelling
                              +NO3
                              -Fe




                               NO3 (uM)
Macro-nutrient supply to the subtropical gyres.




WOCE A20 North Atlantic
Lateral Ekman transfer into N. Atlantic subtropical gyres
Williams and Follows (1998)




                              Convergence of horizontal Ekman nitrate
                              flux in N. Atlantic (10-3 mol N m-2 yr-1)
 Role of eddies?
 Residual mean flow
North Atlantic subtropical gyre: schematic nutrient budget

Ekman transfer significant source to subtropical bowl


(~ mol N m-2 yr-1)              organic export (Jenkins, 1988)
Connecting southern and northern hemispheres:
“nutrient stream” centred at σθ = 27.0 supplies nutrients
 to northern gyres                (Pelegri and Csanady, 1991)

                           σθ = 27.0
  PO4* (“conservative”) (μM)                   NO3 (μM)
Nutrient stream outcrops close to intergyre
boundary in winter
         March NO3 and σθ (10m)




         World Ocean Atlas
Illustration in a global biogeochemical model:

regulation of macro-nutrient pathways by aeolian iron
source                        (Dutkiewicz et al., 2005)


 Modeled Surface Chl (mg m -3)
                                 ●Explicit, coupled phosphorus,
                                 silica and iron cycles
                                 ●Two phytoplankton classes:
                                 Diatoms and "other"
                                 phytoplankton
                                 ●   Single grazer
                                 ●Prescribed aeolian iron
                                 source
               Aeolian Iron Source (mmol Fe m-2 yr-1)




Luo et al (2003)


      Sensitivity studies with uniform “high” and “low” aeolian iron flux...
Sensitivity of primary production to aeolian iron source

  difference in primary producitivity
  (high – low) aeolian iron supply                (g C m -2 y-1)




                                                   More dust,
                                                   higher
                                                   productivity




                                                   More dust,
                                                   lower
                                                   productivity
Pacific basin reflects regulation of intergyre exchange
Atlantic productivity reflects southern ocean surface macro-
nutrient utilization




                                             Atlantic
                       Summary

● Lateral transfer in surface ocean is significant route
for diapycnal return of macro-nutrients to light
isopycnals.
(Southern Ocean, inter-gyre boundaries)


●Advection by residual mean flow (Ekman + eddy) is
key physical process


●Balance between residual mean advection and iron
stress regulates the lateral fluxes of macro-nutrients.
Two regimes:

- iron limited upwelling regions
- macro-nutrient limited subtropical gyres + Atlantic
    Export and remineralization of sinking, organic particles moves
    nutrients to denser isopyncals




Atlantic basin
Pelegri and Csanady (1991): “nutrient stream”




                                      v NO3 at 36N




core of stream at σθ ~ 27.0
Numerical model



Eulerian mean
(Ekman) upwelling
(200m)




Residual mean
upwelling (200m)