Measuring CO2 in the Cow Bay River Watershed, Nova Scotia.

Introduction

Headwaters of rivers and streams are important components of the carbon cycle, as they are typically supersaturated with carbon dioxide (CO2) relative to the atmosphere. While aquatic processes produce some CO2 in situ, a large portion of the CO2 present in headwater streams is fixed by surrounding forests and returned to the atmosphere as stream gas efflux (Aufdenkampe et al. 2011; Leith et al. 2015). Despite their importance to the terrestrial and aquatic carbon balance, headwater streams remain poorly understood. Generally, as stream order (stream classification method as per Strahler 1957) increases, gas velocities (the rate at which a stream is offgassing) decrease (Butman and Raymond 2011). It is now known that rivers and streams release more CO2 into the atmosphere than previously thought, due to an international effort undertaken by Raymond et al. (2013). Their findings suggest that global carbon cycle estimates have been underestimated in the past. The purpose of this application note was to replicate Butman & Raymond’s (2011) study on a smaller scale, using Eosense’s CO2 Gas Probe (eosGP).

Methods

Two of Eosense’s CO2 gas probes (eosGP) were used to monitor dissolved CO2 concentrations at various points along the Cow Bay River Watershed. The eosGP uses non-dispersive infrared (NDIR) sensing technology to measure CO2 concentration. During this field visit, data from the eosGP units was collected using either the Windows-based eosConnect-GP software (Probe 1) or the beta version of the Eosense wireless smartphone app (Probe 2). In order to avoid waiting for the allotted warm up time (30 mins) at each site , the eosGP units were powered continuously across site changes using small 12V batteries (< 0.5 W power draw per unit). Sites were sampled in a downstream gradient, with our first location being the closest to the headwater, and our last site being where the stream empties out into the Atlantic Ocean. Post-processing of the CO2 concentration data was calibrated using a two point calibration against the Picarro G2508, for maximum accuracy.

Results
Location 1 Description
  • Direct sunlight, featuring a 2 foot waterfall; Probe 2 sampled above the waterfall, while Probe 1 sampled below it
  • Footpath close by
eosGP-probe-1

Location 1, probe 1

Probe 1

  • Average CO2: 1,394.4 ppm
  • Water temperature: 10 C
  • Stream depth: 0.10 m
  • Stream width: 1.17 m
  • Flow rate: 0.33 m/sec
eosGP-probe-2

Location1 , probe 2

Probe 2

  • Average CO2: 1,021.2 ppm
  • Water temperature: 9 C
  • Stream depth: 0.15 m
  • Stream width: 1.52 m
  • Flow rate: 0.01 m/sec
Location 2 Description
  • Featured half-shade from the bridge, where Probe 2 sampled. Probe 1 sampled just off the shore in partial sunlight
  • Footpath close by

Probe 1

Location 2, probe 1

Location 2, probe 1

  • Average CO2: 1,094.4 ppm
  • Water temperature: 10 C
  • Stream depth: 0.15 m
  • Stream width: 4.4 m
  • Flow rate: 0.33 m/sec

Probe 2

Location 2, probe 2

Location 2, probe 2

  • Average CO2: 1,021.1 ppm
  • Water temperature: 9 C
  • Stream depth: 0.22 m
  • Stream width: 4.4 m
  • Flow rate: 0.33 m/sec
Location 3 Description
  • Featured another bridge, both probes sampled downstream of the bridge in partial sunlight
  • Footpath close by
  • Stream emptied into a stagnant marsh

Probe 1

Location 3, probe 1

Location 3, probe 1

  • Average CO2: 1,996.4 ppm
  • Water temperature: 13 C
  • Stream depth: 0.17 m
  • Stream width: 8.5 m
  • Flow rate: 0.5 m/sec

Probe 2

Location 3, probe 2

Location 3, probe 2

  • Average CO2: N/A*
  • Water temperature: 13 C
  • Stream depth: N/A
  • Stream width: N/A
  • Flow rate: N/A

*Sampling time was relatively short (20 mins), which did not allow for proper equilibrium of concentration measurements.

Location 4 Description
  • Both probes sampled downstream of a culvert, where water flowed through to empty into the ocean, in direct sunlight
  • Secondary highway close by

Probe 1

Location 4, probe 1

Location 4, probe 1

  • Average CO2: 311.8 ppm
  • Water temperature: 9 C
  • Stream depth: 0.16 m
  • Stream width: 13.1 m
  • Flow rate: N/A

Probe 2

Location 4, probe 2

Location 4, probe 2

  • Average CO2: 596.9 ppm
  • Water temperature: N/A
  • Stream depth: N/A
  • Stream width: N/A
  • Flow rate: N/A
Conclusions

Our results appear to be consistent with Butman and Raymond’s (2011) findings, where measured CO2 concentrations started out higher closer to the headwater, and decreased along the stream system, with the exception of Location 3. This trend is likely due to the decrease in gas velocities with increasing stream order. A spike in CO2 was measured at Location 3, which was likely due to the stream emptying into a low flowing marsh area, where high carbon content (detritus, biological processes) likely led to increased respiratory activity. It is important to note that this stream’s source was a different lake from the previous two locations and that Location 3 was the site at which both streams converged. Using Forerunner’s CO2 gas probes, we observed high saturation of headwater streams in CO2 relative to the atmosphere of the Cow Bay River Watershed. We also observed that sites sampled further downstream became less saturated, which is likely due to higher offgassing rates in the headwater streams.

 

References

Aufdenkampe, A. K., E. Mayorga, P.A. Raymond, J. M. Melack, S. C. Doney, S.R. Alin, R. E. Aalto, and K. Yoo (2011), Riverine coupling of biogeochemical cycles between land oceans and atmosphere, Front. Ecol. Environ. 9, 53-60.

Butman, D., and P. A. Raymond (2011), Significant efflux of carbon dioxide from streams and rivers in the United States, Nat. Geosci. 4, 839-842.

Leith, F. I., K. J. Dinsmore, M.B. Wallin, M.F. Billett, K. V. Heal, H. Laudon, M. G. Öquist, and K. Bishop (2015), Carbon dioxide transport across the hillslope-riparian-stream continuum in a boreal headwater catchment, Biogeosciences 12, 1-12

Raymond, P. A., J. Hartmann, R. Lauerwald, S. Sobek, C. McDonald, M. Hoover, D. Butman, R. Striegl, E. Mayorga, C. Humborg, P. Kortelainen, H. Dürr, M. Meybeck, P. Ciais, and P., Guth (2013), Global carbon dioxide emissions from inland waters, Nature 503, 355-359.

Strahler, A. N. (1957), Quantitative analysis of watershed geomorphology, Trans. Am. Geophys. Union 38, 913-920.