Nutrients and the Ocean Conveyor

Nutrients and the Ocean Conveyor

1. Ocean Circulation

   • Ocean circulation is the large scale movement of waters in the ocean basins. Winds drive surface circulation, and the cooling and sinking of waters in the polar regions drive deep circulation. Surface circulation carries the warm upper waters poleward from the tropics.
     • Upwelling of deep water due to Ekman pumping (Wind stress will drive a relatively shallow upper ocean flow and transport water to the left/right) leads to equatorial divergence and upwelling to drive circulation as well.
   • Density (mass/volume; Kg m-3) of seawater controller by temperature and salinity
     • Density Structure of oceans:
       • 2 Major layers in a typical stratified ocean
         • Surface (Mixed layer)
         • Deep Ocean
         • Pycnocline: region of sharp change in density - depth range varies
           • Different Densities based on temperature and salinity form the Thermohaline circulation - cycle 1000 years (he movement of seawater in a pattern of flow dependent on variations in temperature, which give rise to changes in salt content and hence in density.)
             • The Ocean Conveyor
               • The global ocean conveyor belt is a constantly moving system of deep-ocean circulation driven by temperature and salinity.
                 • Allows us to determine age of deep water (old in Pacific)

2. Particle Cycling in the Ocean

   • Photosynthesis by phytoplankton in surface ocean produces Particulate Organic Matter (POM) from CO2 and organic nutrients
     • Respiration (remineralisation) on POM releases nutrients and CO2 back to seawater
       • Drives the Ocean Biological Pump (ocean's biologically driven sequestration of carbon from the atmosphere to the deep sea.) (Google images for cycle)
         • 90% of organic matter is recycled in surface ocean, majority exported POC (Particulate Organic Carbon) is remineralised on the way to seafloor. <1% POC export reaches seafloor.

3. Nutrients

   • Nutrient release upon remineralisation increases along the conveyor in deep ocean with additions from biological pump. (Low in Pacific due to less remineralisation)
     • Distribution of Nutrients determined by:
       • Ocean Circulation
       • Production and destruction of organic matter

4. Nitrate

   • 4000m in global oceans
     • NO3- increases along conveyor: N.Atlantic - S.Atlantic - Indian - Pacific
       • Similar pattern with PO4 3-and inverse with O2

5. Redfield Ratio

   • C:N:P = 106:15:1
     • Almost constant proportion of major nutrients in seawater
   • Atomic ratio of carbon, nitrogen and phosphorus found in phytoplankton and throughout the deep oceans.
   • Redfield ratio in organic matter despite variations amongst species on average N:P - 15-16
   • Remineralization Stoichiometry (elationship between the relative quantities of substances taking part in a reaction or forming a compound)
     • Respiration on organic matter releases inorganic nutrients e.g. nitrates, phosphate
       • Aerobic (with oxygen) CHONP + O2 = 106CO2 +16 HNO3 + H3PO4 + 78H2O
         • Below 400m: C organic:N:P:O2 (117+/- 14)(16+/- 1):(1)(-170+/-10)

6. Box models: assumptions of major inputs and outputs at steady state of ocean

7. Summary: Ocean circulation: deep water formation and ocean conveyor. Nutrient concentrations increases along conveyor and deep ocean, But additions from biological pump. Redfield ratio 106C:15N:1P.