Chapter 9: Respiration - Chemical Tracers James W. Murray(4/19/01) Univ. WashingtonRespiration corresponds to the RKR reaction run backwards. This means that thedissolved oxygen concentration is a tracer for respiration. In the euphotic zone there isusually excess oxygen above atmospheric saturation because there is net biologicalproduction. Production of O2 by primary production is greater than consumption of O2 byrespiration. Below the euphotic zone there is only consumption by respiration so theoxygen goes down as respiration proceeds. In some parts of the open ocean with restrictedcirculation all of the dissolved oxygen is consumed by respiration. Examples of theselocations are the oxygen minimum zones in the eastern tropical north and south Pacificand Arabian Sea. At these locations respiration proceeds using nitrate to combust theorganic matter. In some enclosed basins like the Black Sea, Cariaco Trench, Saanich Inletand Framvaren Fjord respiration also uses up all the nitrate and sulfate is reduced tohydrogen sulfide (H2S)Aerobic respirationOxygen is consumed and nutrients are released.(CH2O)106(NH3)16(H3PO4) + 138 O2Algal Protoplasm ↓bacteria 106 CO2 + 16 HNO3 + H3PO4 + 122 H2O + trace elementsThe oxidation of the NH3 in organic matter to NO3 is referred to as nitrification.Apparent Oxygen UtilizationWe can calculate the extent of respiration by calculating a parameter called theApparent Oxygen Utilization or AOU. AOU is defined as:AOU = O2' - O2where: O2' = value of O2 the water would have if it was in equilibrium with theatmosphere at the temperature and salinity of the water. This is called saturation. Thisimplies that all waters are in equilibrium with the atmosphere (100% saturated) when theysink to become the deep ocean water. O2 is the dissolved oxygen actually measured in thesame water sample.The distribution of AOU throughout the ocean at 4000m is shown in the attached Figure9-1 (from Broecker and Peng). The lowest values (50 µmol kg-1) are in the NorthAtlantic. The highest values (>190 µmol kg-1) are in the oldest water in the North Pacific.North-south sections of AOU in the western Atlantic and western Pacific Oceans areshown in Figures 9-2a,b. Some key intermediate and deep density surfaces (σo = 27.0,27.5 and σ4 = 45.2) are indicated.Fig 9-1 Distribution of AOU at 4000m in the world's oceans. Lowest values are in theAtlantic and the largest (>190 µmol kg-1) are in the Pacific. (from Broecker and Peng,1982).Fig 9-2 a,b Sections of AOU through the Atlantic and PacificRegenerated NutrientsOnce you've calculated the AOU in a water sample you can calculate the CO2, HNO3 andH3PO4 released by respiration.1 mol O2 consumed = 106/138 mol CO2 + 16/138 mol HNO3 + 1/138 mol H3PO4 = 0.77 CO2 + 0.12 HNO3 + 0.0072 H3PO4Preformed NutrientsPreformed nutrients are those initially present in seawater at the time of downwelling.Hence, preformed nutrient = total nutrient - regenerated nutrient.An example from Park (1967) of the relationships between AOU:PO4:NO3:Si is shown inthe Fig. 9-3. The location of these profiles is at 44°N; 127°W (off the coast of Oregon).The nutrients of oxidative origin (Pox) were calculated from AOU and subtracted fromtotal nutrients to get the preformed nutrient concentrations.Preformed nutrients are characteristic of waters originating in different regions and hencecan be used as water mass tracers (e.g. Broecker, 1974; Broecker et al, 1985). Broecker(1974) originally proposed that O2 and NO3 and O2 and PO4 data can be combined insuch a way that the alteration by respiration is cancelled. From the original RKRrespiration equation given above, roughly 1/9 mole of N is released as NO3 and 1/135mole of P is released as PO4 for each mole of O2 consumed. Thus the sum of 9NO3 + O2was defined as "NO" and 135PO4 + O2 as "PO" . Both NO and PO should be nearlyconservative tracers. An example of NO3, O2 and "NO" versus salinity for a singlestation in the western basin of the South Atlantic is shown in Fig. 9-4. You can see thatO2 and NO3 show curvature with salinity but "NO" varies linearly, or conservatively,between two end members that represent the core of the AAIW (at 1150m) and the top ofthe NADW (at 2270m).Revised Stoichiometric RatiosIsopycnal surfaces are surfaces of constant density. Potential temperature andsalinity are perfectly correlated on isopycnal surfaces. In the ocean most mixing andtransport occur on isopycnal surfaces. The winter surface outcrops of various isopycnalsurfaces in the North Atlantic Ocean are shown in Fig. 9-5. The densest waters thatoutcrop at the surface have a density of σo = 27.6. The north-south distribution of densitysurfaces in the 0 - 1500m depth range of the western side of the Atlantic, Indian andPacific Oceans are shown in Fig. 9-6. The density surfaces shoal and outcrop at highlatitude, then sink to maximum depths at mid-latitudes. They tend to shoal again near theequator where there is upwelling. To a first approximation the distributions of tritium(3H), which is a tracer for water, which has an atmospheric source, follow these densitysurfaces.Takahashi et al (1985) first argued that the correct approach for determiningstoichiometric regeneration ratios was to utilize data along isopycnal surfaces. Thechange in NO3 and PO4 concentrations along isopycnal surfaces away from the surfaceoutcrop can be estimated from:∆P = [PO4] - [PO4]° = RPO4/O2 x AOU∆N = [NO3] - [NO3]° = RNO3/O2 x AOUThey found that the stoichiometric ratios (on σo = 27.0-27.20) varied for differentlocations in the Atlantic and Indian oceans (Table 9-1). They argued that the widely usedRKR values of P:N:C:-O2 of 1:16:106:138 should be replaced by 1:16:122+22:172 forthe Atlantic and Indian Ocean.This approach was improved upon by Anderson and Sarmiento (1994) whocalculated the stoichiometric ratios on 20 neutral surfaces in the South Atlantic, Indianand Pacific Basins between 400 to 4000m. Neutral surfaces are almost the same asisopycnal surfaces but are considered to be a more precise estimate of the horizons uponwhich water masses preferentially move and mix (McDougall, 1987). The -O2/P, Corg/Pand N/P utilization ratios are shown in Fig. 9-7a,b,c. The P:N:C:-O2 ratios ofremineralization below 400m are estimated with uncertainties as 1:16+1:117+14:170+10.These values are very consistent with those of Takahashi et al (1985). In ocean regionswhere there is denitrification the coefficient