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Single point nitrate observations in the mixed layer in the Central Irminger Sea

Single point nitrate observations in the mixed layer in the Central Irminger Sea. Some Basics. Dissolved inorganic Nitrate is a major plant nutrients in the ocean Most Nitrate observations from ship based observations

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Single point nitrate observations in the mixed layer in the Central Irminger Sea

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  1. Single point nitrate observations in the mixed layer in the Central Irminger Sea

  2. Some Basics • Dissolved inorganic Nitrate is a major plant nutrients in the ocean • Most Nitrate observations from ship based observations • Over the annual cycle surface Nitrate may show linear relation with other surface parameters: • Temperature (e.g. Kamykowski and Zentara 1986), • Temperature & Chl-a. (e.g. Goes et al. 1999, 2000) • Temperature & mixed layer depth (Steinhoff et al. 2010) → Surface Nitrate is reconstructed as a linear combination of parameters available in high space and time resolution

  3. End winter summer T/N in the subpolar North Atlantic • T/N for the Irminger Sea from ship data (Henson et al. 2003) • No nitrate depletion

  4. Nitrate timeseries CIS • Nirate (~40m depth): EnvirotechLLC NAS nutrient analyser (model 2 & 3X) • Accompanying sensors: • 14 CTD (0-1400m depth) • Fluorometer (single depth) • Data QC via reference ship sampling (Ni, Chl-a) • Up to 9 month of data (each deployment) • Overall 2.5 yrs data (<2h time resolution)

  5. Mixed layer timeseries:Temperature & Nitrate T (°C) in mixed layer Nitrate in mixed layer

  6. Mixed layer T/N • Data cover September to End March • Linear during intense upper layer cooling phase • Winter (Jan.-Mar.): even large heat flux triggers only little cooling – but Nirate decreases – sign for winter productivity

  7. A 1-d Nitrate mixed layer budget: • End of summer : ~ 7 µmol kg-1 • Peak winter: ~ 17 µmol kg-1(similar to deep water value)

  8. A 1-d Nitrate mixed layer budget: • End of summer : ~ 7 µmol kg-1 • Peak winter: ~ 17 µmol kg-1(similar to deep water value) 17 µmol kg-1 Mixed layer depth (MLD) Reference layer depth (RLD)

  9. A 1-d Nitrate mixed layer budget: Mixed layer nitrate from a “Mole” perspective: • Total # Moles = # Moles in ML + # Moles in layer between ML and RLD + Source/Sink (biogeochemistry) Mixed layer depth (MLD) Reference layer depth (RLD)

  10. A 1-d Nitrate mixed layer budget: Mixed layer nitrate from a “Mole” perspective: Moles NO3, surface to RLD = (MLD)*NO3, ML + (RLD-MLD) * NO3, interior + S Mixed layer depth (MLD) Reference layer depth (RLD)

  11. A 1-d Nitrate mixed layer budget: Mixed layer nitrate from a “Mole” perspective: Moles NO3, surface to RLD = (MLD)*NO3, ML + (RLD-MLD) * NO3, interior + S Assume S=0 and #Moles surface to RLD=constant the Mixed layer depth can be evaluated from NO3, ML observations (NO3, interior=17µmol kg-1): MLDNO3 = (Constant - RLD * NO3, interior) / (NO3, ML - NO3, interior)

  12. Mixed layer cycle Net heat flux at surface Mixed layer Mixed layer

  13. Net heat flux at surface Mixed layer cycle Nitrate inventory Mixed layer

  14. Net heat flux at surface Mixed layer cycle NO3 “Defizit” Nitrate inventory Mixed layer

  15. Utilizing the mixed layer to derive Nirate& Chlorophyll-a inventories • ML from Microcat with NO3 in mixed layer

  16. Utilizing the mixed layer to derive Nirate& Chlorophyll-a inventories • ML from Microcat with NO3 in mixed layer

  17. Utilizing the mixed layer to derive Nirate& Chlorophyll-a inventories Increase in Chl-a inventory during winter (December to April) → by 60 mg/m2 Increase in N inventory up to “saturation” (deep water value) → N uptake of phytokplankton during winter is masked by parallel entrainment of N rich waters...

  18. Nitrate/Chlorophyll utilization ratio

  19. Interannual variability(2003/2004-blue; 2006/2007-red)

  20. Interannual variability(2003/2004-blue; 2006/2007-red) In 2006/07 the heat loss was 25% higher compared to 2003/2004

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