Roseanne McDonald

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Measuring aquatic carbon fluxes

I spent the day at Loch Katrine with a second-year student who demonstrated a method of measuring aquatic carbon flux, which will be heavily used throughout my PhD. There are many forms of carbon in freshwaters: organic from soil, leaves and oil; inorganic from weathering, in-stream respiration of organic matter, and soil carbon dioxide inputs. One way of measuring dissolved greenhouse gases is via the Headspace method (Hope et al., 1995):

  1. Fill a syringe under water
  2. Release some volume of the water to create a ‘headspace’
  3. Shake the syringe under water to equilibrate the gas
  4. Analyse the headspace gas concentration by gas chromotography

Dissolved gases can also be measured from indirect estimate of partial pressure from pH and alkalinity measurements (Neal et al., 1998), continuous monitoring via non-dispersive infrared sensors (Dinsmore and Billett, 2008), and floating chambers connected to an infrared gas analyser (Hardie et al., 2005).

References

  • Dinsmore, K. J. and Billett, M. F. (2008). Continuous measurement and modelling of CO2 losses from a peatland stream during storm flow events. Water Resources Research 44, W12417.
  • Hardie, S. M. L. et al., (2005). Carbon dioxide capture using a zeolite molecular sieve sampling system for isotopic studies (13C and 14C) of respiration. Radiocarbon 47(3), 441-51.
  • Hope, D. et al., (1995). A method for measuring free CO2 in upland streamwater using headspace analysis. Journal of Hydrology 166, 1-14.
  • Neal, C. et al,. (1998). An assessment of excess carbon dioxide partial pressures in natural waters based on pH and alkalinity measurements. The Science of the Total Environment 210-211, 173-185.