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Lecture 12 Primary Production – Nutrient Stoichiometry

Lecture 12 Primary Production – Nutrient Stoichiometry. Topics Stoichiometry Biolimiting Elements. Broecker two-box model ( Broecker , 1971). B = VmCd + VrCr - VmCs. B. (1-f)B. fB. see Fig. 2 of Broecker (1971). v is in m y -1 then flux is mol m -2 y - 1

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Lecture 12 Primary Production – Nutrient Stoichiometry

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  1. Lecture 12 Primary Production – Nutrient Stoichiometry Topics Stoichiometry Biolimiting Elements

  2. Broecker two-box model (Broecker, 1971) B = VmCd + VrCr - VmCs B (1-f)B fB see Fig. 2 of Broecker (1971) v is in m y-1 then flux is mol m-2y-1 Flux = VmixCsurf = m yr-1 mol m-3

  3. Mass balance for surface box VsdCs/dCt= VrCr + VmCd – VmixCs – B At steady state: B = VrCr + VmixCd– VmixCsand fB= VrivCriv

  4. Broecker (1971) defines some parameters for the 2-box model g = B / input = (VmixCD + VrCr – VmixCs) / VmixCd + VrCr f = VrCr / B = VrCr / (VmixCd + VrCr - VmixCs) f x g In this model Vr = 10 cm y-1 Vmix = 200 cm y-1 so Vmix / Vr = 20 fraction of input to surface removed as B because fB = VrCr fraction of element removed to sediment per visit to the surface Here are some values: g f f x g N 0.95 0.01 0.01 P 0.95 0.01 0.01 C 0.20 0.02 0.004 Si 1.0 0.01 0.01 Ba 0.75 0.12 0.09 Ca 0.01 0.12 0.001 Q. Explain these values and why they vary the way they do. See Broecker (1971) Table 3

  5. Why is this important for chemical oceanography? What controls ocean C, N, P? g ≈ 1.0 Mass Balance for whole ocean: C/ t = VRCR – f B CS = 0; CD = CD VU = VD = VMIX Negative Feedback Control: if VMIX↑ VUCD↑ B ↑ f B ↑ (assumes f will be constant!) assume VRCR  then CD↓ (because total ocean balance VUCD↓ has changed; sink > source) B ↓ The nutrient concentration of the deep ocean will adjust so that the fraction of B preserved in the sediments equals river input! CS CD if VMIX = m y-1 and C = mol m-3 flux = mol m-2 y-1

  6. Example: Perturbation analysis – Mass Balance Control Double Upwelling Rate Double rate of ocean mixing sequence of events Paleo record VrCr = fB at the beginning and at the end! The deep concentration (Cd) is cut in half

  7. Example: Perturbation Analysis 1. Double River Input 2. CaCO3 burial increases and Carbonate Compensation Depth (CCD) deepens

  8. Controls on Atmospheric CO2 Remarkable consistency for glacial/interglacial concentrations of CO2. A main Control on atm CO2 is the B flux! We need to understand B…

  9. How do we get from the marine food web to a global assessment of CO2 flux??? With great difficulty!

  10. Chemical Composition of Biological Particulate Material Hard Parts - Shells NameMineralSize (mm) Plants Coccoliths CaCO3 Calcite 5 Diatoms SiO2 Opal 10-15 Silicoflagellates SiO2 Opal 30 Animals Foraminifera CaCO3 Calcite ~100 and Aragonite Radiolaria SiO2 Opal ~100 Pteropods CaCO3 Aragonite ~1000 Acantharia SrSO4 Celestite ~100

  11. Soft Parts - protoplasm from Redfield, Ketchum and Richards (1963) The Sea Vol. 2 Also for particles caught by sediment traps.

  12. The Redfield or "RKR" Equation (A Model) The mean elemental ratio of marine organic particles is given as: P : N : C = 1 : 16 : 106 The average ocean photosynthesis (forward) and aerobic ( O2 ) respiration (reverse) is written as: 106 CO2 + 16 HNO3 + H3PO4 + 122 H2O + trace elements (e.g. Fe) light (h n)  ( C106H263O110N16P ) + 138 O2 or (CH2O)106(NH3)16(H3PO4) Algal Protoplasm The actual chemical species assimilated during this reaction are: HCO3- NO3- PO43- NO2- NH4+

  13. This is an organic oxidation-reduction reaction - during photosynthesis C and N are reduced and O is oxidized. During respiration the reverse occurs. There are no changes in the oxidation state of P. • We assume C has an oxidation state of 0 which is the value of C in formaldehyde (CH2O), that N has an oxidation state of -III and that H and P do not change oxidation states. • 2. Photosynthesis is endothermic. This means it requires energy from an outside source. In this case the energy source is the sun. Essentially plants convert the photo energy from the sun into high energy C - C bonds. This conversion happens in the plants photosystems. • Respiration is exothermic. This means it could occur spontaneously and release energy. In actuality it is always mediated by bacteria which use the reactions to obtain their energy for life. • 3. Stoichiometry breakdown of oxygen production • CO2 + H2O  (CH2O) + O2 C : O2 1 : 1 • H+ + NO3- + H2O  (NH3) + 2O2 N : O2 1 : 2 • 4. Total oxygen production: 106 C + 16 N x 2 = 138 O2 Oxidation State of C?

  14. 5. If ammonia is available it is preferentially taken up by phytoplankton. If NH3 is used as the N source then less O2 is produced during photosynthesis 106 CO2 + 16 NH3 + H3PO4 + 122 H2O + trace elements light (hn)  (CH2O)106(NH3)16(H3PO4) + 106 O2 The relationship between O2 and NO3/NH4 is 2:1 (as shown in point #3) 16 HNO3 + 16 H2O = 16 NH3 + 32 O2

  15. Dissolved seawater data from Atlantic GEOSECS Program (Broecker and Peng, 1982) small deficit in NO3 Remarkable congruence between ratios in the ocean and plankton composition.

  16. Nutrient stoichiometry from US JGOFS EqPac Suspended Matter and sediment trap particles Lines show Redfield slopes Murray et al, unpublished

  17. Why the Redfield Ratios?? From a chemistry point-of-view. Each class of organic compounds has its own unique stoichiometry carbohydrates are C rich but N and P poor (e.g. (CH2O)n) proteins are C and N rich but P poor (e.g. amino acids) nucleotides are rich in both N and P (e.g. 4 bases) lipids are C rich Questions: Why 16:1? Why not 6:1 or 60:1? How does an organism end up with a certain composition? What happens if one constituent is not available in adequate amounts?

  18. Stoichiometry based on organic composition Average Plankton 65% protein 19% lipid 16% carbohydrate Average formula for plankton biomass C106H177O37N17PS0.4 Oxidation consumes 154 moles of O2 106 CO2 + 17 HNO3 + H3PO4 + 122 H2O + trace elements light (h n)  ( C106H177O37N17PS0.4 ) + 154 O2 Hedges et al (2002) Marine Chemistry

  19. R = RKR P = Protein L = Lipid C = Carbohydrate E = Equatorial Pacific A = Arabian Sea 1-3 = Southern Ocean 1a = Anderson et al From Hedges et al (2002)

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