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Gross Primary Production (GPP) - triple oxygen

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Gross primary production (triple oxygen isotopes)
Approach: triple oxygen isotope mass spectrometry
Context: in situ
Spatial scale: mixed layer, regional
Temporal scale: days to months (integrated)
Units: mmol O2 m-2 d-1; convertible to mmol C m-2 d-1
Community captured: bulk
Co-measurements: gas transfer velocity (k), temperature, salinity, wind speed history; O2/Ar for NCP and NCP/GOP ratio

Method Overview

The triple oxygen isotope (TOI) method exploits the mass-independent fractionation (MIF) of oxygen isotopes. Photosynthesis produces O2 with an isotopic composition reflecting the water source, which has a mass-independent signature relative to atmospheric O2. The 17O excess (Δ17O = δ17O − 0.516 × δ18O) in dissolved O2 therefore records the balance between photosynthetic O2 production (which drives Δ17O away from atmospheric equilibrium) and air-sea gas exchange (which drives Δ17O back toward the atmospheric value). Gross primary production (GPP) can be calculated from this steady-state balance when gas exchange parameterisations are known[1].

Water samples (300–500 mL) are collected from the mixed layer and dissolved gases are extracted by vacuum-line methods; Δ17O is measured by dual-inlet or continuous-flow IRMS.

Scale of measurement

Each water sample integrates GPP over the mixed layer depth and over the gas exchange timescale (days to months depending on wind speed). Because the signal builds up slowly, the method provides a longer time average than bottle incubations. Spatial resolution is limited by sampling frequency.

Data generated

The method yields GPP in mmol O2 m-2 d-1. When combined with simultaneous O2/Ar measurements (NCP), the NCP:GPP ratio can be derived, indicating the gross carbon use efficiency of the community.

Units & currency

Units are mmol O2 m-2 d-1. The currency is oxygen; carbon conversion requires the photosynthetic quotient (PQ).

Sample size

Typical samples are 300–500 mL for IRMS analysis.

Repositories & databases

Limitations

Calculation of GPP from Δ17O requires model-based assumptions: steady-state conditions, no horizontal or vertical advection of O2 (or these must be corrected for), and known isotopic end-member ratios for atmospheric O2 and photosynthetically produced O2. In dynamically active regions (fronts, upwelling zones), non-steady state dynamics can violate these assumptions. Gas transfer parameterizations contribute significant uncertainty, as in the O2/Ar method.

Example Applications & Protocols

Classic examples

  • Luz & Barkan (2005) The isotopic ratios 17O/16O and 18O/16O in molecular oxygen and their significance in biogeochemistry [1]

Recent applications

  • Reuer et al. (2007) New estimates of Southern Ocean biological production rates from O2/Ar ratios and the triple isotope composition of O2 [2]
  • Prokopenko et al. (2011) Exact evaluation of gross photosynthetic production from the oxygen triple-isotope composition of O2 [3]

Common calculations/conversions

  • GPP is derived from the steady-state Δ17O budget: GPP = k × ρ × [O2]sat × (Δ17Omeas − Δ17Oeq) / (Δ17Ophot − Δ17Oeq).
  • GPP (mmol C m-2 d-1) = GPPO2 / PQ.

References

  1. ↑ 1.0 1.1 Luz, B., & Barkan, E. (2005). The isotopic ratios 17O/16O and 18O/16O in molecular oxygen and their significance in biogeochemistry. Geochimica et Cosmochimica Acta, 69(5), 1099–1110. https://doi.org/10.1016/j.gca.2004.09.001
  2. ↑ Reuer, M. K., Barnett, B. A., Bender, M. L., Falkowski, P. G., & Hendricks, M. B. (2007). New estimates of Southern Ocean biological production rates from O2/Ar ratios and the triple isotope composition of O2. Deep-Sea Research Part I, 54(6), 951–974. https://doi.org/10.1016/j.dsr.2007.02.007
  3. ↑ Prokopenko, M. G., Pauluis, O. M., Granger, J., & Yeung, L. Y. (2011). Exact evaluation of gross photosynthetic production from the oxygen triple-isotope composition of O2: implications for the net-to-gross primary production ratios. Geophysical Research Letters, 38, L14603. https://doi.org/10.1029/2011GL047652
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