Macro-Nutrient Transport Pathways and Interactions with the Iron Cycle. Export and remineralization of sinking, organic particles moves nutrients to denser isopyncals: Mick Follows, Stephanie Dutkiewicz, Payal Parekh: MIT Taka Ito: University of Washington Ric Williams: University of Liverpool

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

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

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

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 Marshall (1997) Ψ res = Ψ Ekman + Ψ eddy

Residual mean flow transports nutrients northwards to sites of mode and intermediate water formation Marshall (1997) Ψ res = Ψ Ekman + Ψ eddy

NO3 (uM) upwelling +NO 3 -Fe dust iron limited macro-nutrient limited subduction ● balance between northward residual mean advection and export sets subducted nutrient concentration ● export sensitive to iron availability

Macro-nutrient supply to the subtropical gyres. WOCE A20 North Atlantic

Convergence of horizontal Ekman nitrate flux in N. Atlantic (10 -3 mol N m -2 yr -1 ) Lateral Ekman transfer into N. Atlantic subtropical gyres Williams and Follows (1998) 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 σ θ = 27.0 supplies nutrients to northern gyres (Pelegri and Csanady, 1991) σ θ = 27.0 PO 4 * (“conservative”) (μM) NO 3 (μM)

Nutrient stream outcrops close to intergyre boundary in winter March NO 3 and σ θ (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...

difference in primary producitivity (high – low) aeolian iron supply (g C m -2 y -1 ) Sensitivity of primary production to aeolian iron source 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” core of stream at σ θ ~ 27.0 v NO3 at 36N

Numerical model Eulerian mean (Ekman) upwelling (200m) Residual mean upwelling (200m)