Continuous measurement of air–water gas exchange by underwater eddy covariance
Exchange of gases, such as O<sub>2</sub>, CO<sub>2</sub>, and CH<sub>4</sub>, over the air–water interface is an important component in aquatic ecosystem studies, but exchange rates are typically measured or estimated with substantial uncertainties. This dimini...
| Main Authors: | , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-12-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/14/5595/2017/bg-14-5595-2017.pdf |
| _version_ | 1811195767504764928 |
|---|---|
| author | P. Berg M. L. Pace |
| author_facet | P. Berg M. L. Pace |
| author_sort | P. Berg |
| collection | DOAJ |
| description | Exchange of gases, such as O<sub>2</sub>, CO<sub>2</sub>, and CH<sub>4</sub>, over the
air–water interface is an important component in aquatic ecosystem studies,
but exchange rates are typically measured or estimated with substantial
uncertainties. This diminishes the precision of common ecosystem assessments
associated with gas exchanges such as primary production, respiration, and
greenhouse gas emission. Here, we used the aquatic eddy covariance technique
– originally developed for benthic O<sub>2</sub> flux measurements – right below
the air–water interface (∼ 4 cm) to determine gas exchange rates and
coefficients. Using an acoustic Doppler velocimeter and a fast-responding
dual O<sub>2</sub>–temperature sensor mounted on a floating platform the 3-D water
velocity, O<sub>2</sub> concentration, and temperature were measured at high-speed
(64 Hz). By combining these data, concurrent vertical fluxes of O<sub>2</sub> and
heat across the air–water interface were derived, and gas exchange
coefficients were calculated from the former.
Proof-of-concept deployments at different river sites gave standard gas
exchange coefficients (<i>k</i><sub>600</sub>) in the range of published values. A 40 h
long deployment revealed a distinct diurnal pattern in air–water exchange of
O<sub>2</sub> that was controlled largely by physical processes (e.g., diurnal
variations in air temperature and associated air–water heat fluxes) and not
by biological activity (primary production and respiration). This physical
control of gas exchange can be prevalent in lotic systems and adds
uncertainty to assessments of biological activity that are based on measured
water column O<sub>2</sub> concentration changes. For example, in the 40 h
deployment, there was near-constant river flow and
insignificant winds – two main drivers of lotic gas exchange – but we found
gas exchange coefficients that varied by several fold. This was presumably
caused by the formation and erosion of vertical temperature–density gradients
in the surface water driven by the heat flux into or out of the river that
affected the turbulent mixing. This effect is unaccounted for in widely used
empirical correlations for gas exchange coefficients and is another source of
uncertainty in gas exchange estimates. The aquatic eddy covariance technique
allows studies of air–water gas exchange processes and their controls at an
unparalleled level of detail.<br><br>
A finding related to the new approach is that heat fluxes at the air–water
interface can, contrary to those typically found in the benthic environment,
be substantial and require correction of O<sub>2</sub> sensor readings using
high-speed parallel temperature measurements. Fast-responding O<sub>2</sub> sensors
are inherently sensitive to temperature changes, and if this correction is
omitted, temperature fluctuations associated with the turbulent heat flux
will mistakenly be recorded as O<sub>2</sub> fluctuations and bias the O<sub>2</sub> eddy
flux calculation. |
| first_indexed | 2024-04-12T00:48:20Z |
| format | Article |
| id | doaj.art-8de95cffd83f4abca6c6bcda3d60655b |
| institution | Directory Open Access Journal |
| issn | 1726-4170 1726-4189 |
| language | English |
| last_indexed | 2024-04-12T00:48:20Z |
| publishDate | 2017-12-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Biogeosciences |
| spelling | doaj.art-8de95cffd83f4abca6c6bcda3d60655b2022-12-22T03:54:49ZengCopernicus PublicationsBiogeosciences1726-41701726-41892017-12-01145595560610.5194/bg-14-5595-2017Continuous measurement of air–water gas exchange by underwater eddy covarianceP. Berg0M. L. Pace1Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia, USADepartment of Environmental Sciences, University of Virginia, Charlottesville, Virginia, USAExchange of gases, such as O<sub>2</sub>, CO<sub>2</sub>, and CH<sub>4</sub>, over the air–water interface is an important component in aquatic ecosystem studies, but exchange rates are typically measured or estimated with substantial uncertainties. This diminishes the precision of common ecosystem assessments associated with gas exchanges such as primary production, respiration, and greenhouse gas emission. Here, we used the aquatic eddy covariance technique – originally developed for benthic O<sub>2</sub> flux measurements – right below the air–water interface (∼ 4 cm) to determine gas exchange rates and coefficients. Using an acoustic Doppler velocimeter and a fast-responding dual O<sub>2</sub>–temperature sensor mounted on a floating platform the 3-D water velocity, O<sub>2</sub> concentration, and temperature were measured at high-speed (64 Hz). By combining these data, concurrent vertical fluxes of O<sub>2</sub> and heat across the air–water interface were derived, and gas exchange coefficients were calculated from the former. Proof-of-concept deployments at different river sites gave standard gas exchange coefficients (<i>k</i><sub>600</sub>) in the range of published values. A 40 h long deployment revealed a distinct diurnal pattern in air–water exchange of O<sub>2</sub> that was controlled largely by physical processes (e.g., diurnal variations in air temperature and associated air–water heat fluxes) and not by biological activity (primary production and respiration). This physical control of gas exchange can be prevalent in lotic systems and adds uncertainty to assessments of biological activity that are based on measured water column O<sub>2</sub> concentration changes. For example, in the 40 h deployment, there was near-constant river flow and insignificant winds – two main drivers of lotic gas exchange – but we found gas exchange coefficients that varied by several fold. This was presumably caused by the formation and erosion of vertical temperature–density gradients in the surface water driven by the heat flux into or out of the river that affected the turbulent mixing. This effect is unaccounted for in widely used empirical correlations for gas exchange coefficients and is another source of uncertainty in gas exchange estimates. The aquatic eddy covariance technique allows studies of air–water gas exchange processes and their controls at an unparalleled level of detail.<br><br> A finding related to the new approach is that heat fluxes at the air–water interface can, contrary to those typically found in the benthic environment, be substantial and require correction of O<sub>2</sub> sensor readings using high-speed parallel temperature measurements. Fast-responding O<sub>2</sub> sensors are inherently sensitive to temperature changes, and if this correction is omitted, temperature fluctuations associated with the turbulent heat flux will mistakenly be recorded as O<sub>2</sub> fluctuations and bias the O<sub>2</sub> eddy flux calculation.https://www.biogeosciences.net/14/5595/2017/bg-14-5595-2017.pdf |
| spellingShingle | P. Berg M. L. Pace Continuous measurement of air–water gas exchange by underwater eddy covariance Biogeosciences |
| title | Continuous measurement of air–water gas exchange by underwater eddy covariance |
| title_full | Continuous measurement of air–water gas exchange by underwater eddy covariance |
| title_fullStr | Continuous measurement of air–water gas exchange by underwater eddy covariance |
| title_full_unstemmed | Continuous measurement of air–water gas exchange by underwater eddy covariance |
| title_short | Continuous measurement of air–water gas exchange by underwater eddy covariance |
| title_sort | continuous measurement of air water gas exchange by underwater eddy covariance |
| url | https://www.biogeosciences.net/14/5595/2017/bg-14-5595-2017.pdf |
| work_keys_str_mv | AT pberg continuousmeasurementofairwatergasexchangebyunderwatereddycovariance AT mlpace continuousmeasurementofairwatergasexchangebyunderwatereddycovariance |