Air–water gas exchange in lakes and reservoirs measured from a moving platform by underwater eddy covariance

Abstract. Exchange of gases, such as O2, CO2, and CH4, 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 O2 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 O2–temperature sensor mounted on a floating platform the 3-D water velocity, O2 concentration, and temperature were measured at high-speed (64 Hz). By combining these data, concurrent vertical fluxes of O2 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 (k600) in the range of published values. A 40 h long deployment revealed a distinct diurnal pattern in air–water exchange of O2 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 O2 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. 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 O2 sensor readings using high-speed parallel temperature measurements. Fast-responding O2 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 O2 fluctuations and bias the O2 eddy flux calculation.

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Continuous measurement of air-water gas exchange by underwater eddy covariance

10.5194/bg-2017-340 ◽

2017 ◽

Author(s):

Peter Berg ◽

Michael L. Pace

Keyword(s):

Gas Exchange ◽

Exchange Rates ◽

Eddy Covariance ◽

High Speed ◽

Heat Fluxes ◽

Water Interface ◽

O2 Concentration ◽

Air Water Interface ◽

Ecosystem Assessments ◽

Water Gas

Abstract. Abstract. Exchange of gasses, such as O2, CO2, and CH4, 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 use the aquatic eddy covariance technique – originally developed for benthic O2 flux measurements – right below the air-water interface (~ 5 cm) to determine gas exchange rates and coefficients. Using an Acoustic Doppler Velocimeter and a fast-responding dual O2-temperature sensor mounted on a floating platform, the 3D water velocity, O2 concentration, and temperature are measured at high-speed (64 Hz). By combining these data, concurrent vertical fluxes of O2 and heat across the air-water interface are derived, and from the former, gas exchange coefficients. Proof-of-concept deployments at different river sites gave standard gas exchange coefficients (k600) in the range of published values. A 40 h long deployment revealed a distinct diurnal pattern in air-water exchange of O2 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 is prevalent in lotic systems and adds uncertainty to common ecosystem assessments of biological activity relying on water column O2 concentration recordings. For example, in the 40 h deployment, there was close-to 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 vertical temperature-density gradient formation and erosion in the surface water driven by the heat flux into or out of the river that controlled 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. 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 O2 sensor readings using high-speed parallel temperature measurements. Fast-responding O2 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 O2 fluctuations and bias the O2 eddy flux calculation.

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Eddy covariance flux measurements of momentum, heat and carbon dioxide in Hudson Bay at the onset of sea ice melt

10.5194/egusphere-egu21-15711 ◽

2021 ◽

Author(s):

Richard Sims ◽

Brian Butterworth ◽

Tim Papakyriakou ◽

Mohamed Ahmed ◽

Brent Else

Keyword(s):

Carbon Dioxide ◽

Gas Exchange ◽

Sea Ice ◽

Eddy Covariance ◽

Open Water ◽

The Arctic ◽

Gas Transfer ◽

Hudson Bay ◽

Flux Measurements ◽

Ice Fraction

Remoteness and tough conditions have made the Arctic Ocean historically difficult to access; until recently this has resulted in an undersampling of trace gas and gas exchange measurements. The seasonal cycle of sea ice completely transforms the air sea interface and the dynamics of gas exchange. To make estimates of gas exchange in the presence of sea ice, sea ice fraction is frequently used to scale open water gas transfer parametrisations. It remains unclear whether this scaling is appropriate for all sea ice regions. Ship based eddy covariance measurements were made in Hudson Bay during the summer of 2018 from the icebreaker CCGS Amundsen. We will present fluxes of carbon dioxide (CO2), heat and momentum and will show how they change around the Hudson Bay polynya under varying sea ice conditions. We will explore how these fluxes change with wind speed and sea ice fraction. As freshwater stratification was encountered during the cruise, we will compare our measurements with other recent eddy covariance flux measurements made from icebreakers and also will compare our turbulent CO2&#160;fluxes with bulk fluxes calculated using underway and surface bottle pCO2&#160;data.&#160;&#160;

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