Nutrients; Aerobic Carbon Production and Consumption

Transcription

1 Nutrients; Aerobic Carbon Production and Consumption OCN 623 Chemical Oceanography Reading: Libes, Chapters 8 and 9

2 Formation and respiration of organic matter DINutrients POM Primary Producers Autotrophs Mostly photosynthesizers (they use light energy) called phytoplankton phyto = light plankton = small drifting organisms Some chemotrophs (don t need light) live in unusual environments like hydrothermal vents, anoxic environments C, H, O, N, P, S + trace elements Oceanic reservoirs of N, P are small P forms phosphates, others facilitate e- transfer

3 Production and destruction biogeochemistry Redfield-Richards Equation: P CO 2 + N + P + H 2 0 Organic matter + O 2 R We will look first at the so-called inorganic nutrients : N, P and Si They are also called biolimiting elements -- Why? 1. Small reservoir size in oceans 2. Fast turnover time 3. Required for many kinds of biological activity

4 Foreshadowing: Controls on Atmospheric CO 2 Remarkable consistency for glacial/interglacial concentrations of CO 2

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

6 Every (other) breath you take is a by-product of plankton primary production Libes Figure 8.1

7 Chemical Composition of Biological Particulate Material Hard Parts - Shells Name Mineral Size (um) Coccoliths CaCO 3 Calcite 5 Diatoms SiO 2 Opal Silicoflagellates SiO 2 Opal 30 Foraminifera CaCO 3 Calcite ~100 and Aragonite Radiolaria SiO 2 Opal ~100 Pteropods CaCO 3 Aragonite ~1000 Acantharia SrSO 4 Celestite ~100

8 Soft Parts - protoplasm from Redfield, Ketchum and Richards (1963) The Sea Vol. 2 Also for particles caught by sediment traps. Limiting nutrients, blooms, hypoxia Residence time>>mixing time = stable 16:1

9 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 ( O 2 ) respiration (reverse) is written as: 106 CO HNO 3 + H 3 PO H 2 O + trace elements (e.g. Fe, Zn, Mn ) light (h ν) ( C 106 H 263 O 110 N 16 P ) O 2 or (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) Algal Protoplasm The actual chemical species assimilated during this reaction are: HCO - 3 NO - 3 PO 3-4 NO - 2 NH + 4

10 1. 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 (CH 2 O), that N has an oxidation state of -III and that H and P do not change oxidation states. 2. Photosynthesis is endothermic. This means is 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. Why is organic matter an electron donor?

11 Z-scheme for photosynthetic electron transport Falkowski and Raven (1997) ADP ATP Ferredoxin Energy Scale ADP ATP to Krebs cycle and carbohydrate formation photooxidation of water energy from sun converted to C-C, energy rich, chemical bonds

12 3. Stoichiometry breakdown of oxygen production CO 2 + H 2 O (CH 2 O) + O 2 C : O 2 1 : 1 H + + NO H 2 O (NH 3 ) + 2O 2 N : O 2 1 : 2 4. Total oxygen production: 106 C + 16 N x 2 = 138 O 2 5. If ammonia is available it is preferentially taken up by phytoplankton. If NH 3 is used as the N source then less O 2 is produced during photosynthesis 106 CO NH 3 + H 3 PO H 2 O + trace elements light (hν) (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) O 2 The relationship between O 2 and NO 3 /NH 4 is 2:1 (as shown in point #3) 16 HNO H 2 O = 16 NH O 2

13 Inorganic Nutrients 1. Physical Speciation (operational definitions!) A. Dissolved -- pass thru a specified filter B. Particulate -- retained by a specified filter C. Colloidal -- pass thru conventional filters, but are not dissolved

14 2. Chemical Speciation A. Phosphorus i. Dissolved Inorganic Phosphorus (DIP) a. ph-dependent speciation of Orthophosphate: H 3 PO 4 H 2 PO - 4 HPO 2-4 (most important at sw ph) PO 4 3- b. Polyphosphate linked phosphate polymers Dissolved Organic Phosphorus (DOP) e.g., Phospholipids, ATP, ADP

15 Ruttenberg

16 Ruttenberg

17 Seasonal P variations Sedimentation of particulate phosphorus (org P dep) Release of DIP from seds Canfield Fig

18 Dissolved Inorganic Nitrogen (DIN) Dissolved Organic Nitrogen (DON) B. Nitrogen Redox-dependent speciation of dissolved forms: Species Oxid State NO 3 - (nitrate) +V NO 2 - (nitrite) +III N 2 O (nitrous oxide) +I N 2 (dinitrogen) 0 NH 4 + -III Organic-N -III (e.g., Urea H 2 N-CO-NH 2 ) NH 4 + (ammonium ion) NH 3 (ammonia )

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20 Aquatic microbial N cycling A) nitrogen fixation B) NOx assimilation C) ammonification D) NH4+ assimilation E) NH4+ oxidation F) NO2- oxidation G) NO3- ammonification H) Denitrification I) anammox a, burial b, downward diffusion c, upward diffusion d, NH4+ adsorption e, NH4+ desorption Canfield Fig

21 C. Silica Soluble forms: H 2 SiO 3 (95% of total dissolved silica over a broad ph range) HSiO - 3 (5% of total dissolved silica) SiO 2-3 (<<1% of total dissolved sillica)

22 Nutrient Regeneration and AOU (dissolved species) Detrital POM + lateral water mass movement + aerobic respiration = O 2 consumption AOU = Normal Atmospheric Equilibrium Conc [O 2 ] in situ

23 Libes Figure 8.2

24 RESPIRATION Modified from Sarmiento & Gruber 2006

25 Food Web Structure Different N Sources New Production - NO 3 - as N source (from diffusion/upwelling from below and from the atmosphere via nitrogen fixation and nitrification) Regenerated Production - NH 4 + and urea as N source New/Net/Export Flux The f-ratio: f = NO 3 uptake / NO 3 + NH 4 uptake (defined by Dugdale and Goering, 1969) If we write P = gross production and R = respiration then we can also approximate f as: f = P - R P also called the ratio of net to gross production

26 Libes Figure 9.1

27 Libes Figure 9.1

28 Libes Figure 9.2

29 Nutrient Vertical Profiles

30 Mid-Ocean Nutrient Profiles - Phosphorus Main processes controlling vertical distribution of nutrients: [P] several µmol L -1 High consumption of inorganic nutrients; high production of organic nutrients Depth Slow release of inorganic nutrients due to decomposition of falling particles; slow utilization of organic nutrients 2000 m DOP DIP

31 Mid-Ocean Nutrient Profiles - Nitrogen [N] tens of µmol L -1 Depth NH 4 + NO 2 - Low-[O 2 ] loss of NO 3 - (denitrification) 2000 m DON NO - O 2 3 Denitrification (nitrate reduction): 2NO CH 2 O + 8H + + 6e - N 2 + CO 2 + 5H 2 O

32 Nitrite - An Indicator of Suboxia Typically, nitrate and nitrite are measured together (reported as their sum). However, nitrite maxima can be observed: Subsurface maximum (presumably due to suboxic zone in/on particles O 2 -minimum zone maximum NH 4 + profiles look similar (two maxima)

33 Oxygen Nutrient Diagrams Redfield-Richards Equation in Action NW Pacific Slope -120 µm O 2 µm PO 4 Slope -12 µm O 2 µm NO 3 Redfield: AOU/ΔP = 138/1 = 138 AOU/ΔN = 138/16 = 9 Actually, NO NO 2-. For simplicity, ignore NH 4 +

34 Why Are Nutrient Concs Different in Each Ocean? Look at Ocean Net Flow at 4000 m

35 Dissolved Oxygen at 4000 m

36 Dissolved Nitrate at 4000 m

37 Measurement & Use of AOU Equator [Preformed] N [Measured] For biogeochemically regenerated elements in seawater, the Redfield- Richards Equation indicates: [Measured] = [Preformed] + [Oxidative] [Oxidative] Change in conc due to organic matter oxidation

38 [Measured] = [Preformed] + [Oxidative] [P] [O 2 ] Depth Depth AOU - [O 2, oxidative ] AOU P preformed P oxid P measured O 2, measured O 2,preformed

39 (mol/l) (Solve for ΔC)

40 Use appropriate local Redfield (C:P) ratio

41 AOU and Denitrification Denitrification (nitrate reduction): 2NO CH 2 O + 8H + + 6e - N 2 + CO 2 + 5H 2 O

42 Particle Composition Spatial differences Temporal differences (highest C:N due to lack of nutrients) C : N