The relationship between near‐surface turbulence and gas transfer velocity in freshwater systems and its implications for floating chamber measurements of gas exchange

Abstract

We performed a series of gas exchange measurements in 12 diverse aquatic systems to develop the direct relationship between near-surface turbulence and gas transfer velocity. The relationship was log-linear, explained 78% of the variation in instantaneous gas transfer velocities, and was valid over a range of turbulent energy dissipation rates spanning about two orders of magnitude. Unlike wind-based relationships, our model is applicable to systems ranging in size from less than 1 km 2 to over 600 km 2 . Gas fluxes measured with our specific model of floating chambers can be grossly overestimated (by up to 1000%), particularly in low-turbulence conditions. In high-turbulence regimes, flux overestimation decreases to within 50%. Direct measurements of turbulent energy dissipation rate provide reliable estimation of the associated gas transfer velocity even at short temporal and spatial scales.

Oxic water column methanogenesis as a major component of aquatic CH4 fluxes

“…Air-water CH 4 fluxes in the mesocosms and the surrounding lake were calculated by combining the measured surface water pCH 4 and the CH 4 gas exchange coefficient (k CH4 ), the latter derived from the measured gas exchange coefficient for CO 2 (k CO2 ) in mesocosms supersaturated in CO 2 . k CO2 was measured in one mesocosm per treatment, 3-5 times daily during days 6 and 7 and 27-28 of the experiment, using the floating chamber method following Vachon et al 58 Briefly, chambers were connected via a closed loop system to an EGM-4 infrared gas monitor (PP Systems). Fluxes were calculated as the rate of change of chamber pCO 2 per min during a 15-min interval.…”

Increases in terrestrially derived carbon stimulate organic carbon processing and CO2 emissions in boreal aquatic ecosystems

“…Excess pCO 2 was calculated by subtracting ambient atmospheric pCO 2 at the corresponding sites. CO 2 flux, and the associated diffuse gas exchange coefficients for CO 2 (k CO2 ), were measured for every site on the basis of changes in pCO 2 with time in floating chambers 38 . The chamber was connected in closed loop to an EGM-4 infrared gas analyser (PP-systems), pCO 2 in the chamber was measured every minute for 10 min and accumulation rate was calculated by linear regression.…”

“…Roughly half of the variation in pCO 2 was explained by factors other than DOC degradation, and, considering that fluxes are a direct function of concentration, the relationship between excess pCO 2 and fCO 2 was unexpectedly weak (r 2 ¼ 0.22). Physical forcing, however, strongly mediates the pCO 2 versus fCO 2 relationship, with low-turbulence (expressed by the gas exchange coefficient, k CO2 in m d À 1 ) sites sustaining higher average CO 2 concentrations for a given CO 2 production or input rate than systems that typically have higher turbulence 38 . In this regard, there was a clear bimodal distribution of the empirically determined k CO2 around a median of 1 m d À 1 across all sampled sites (Fig.…”

Magnitude and regulation of bacterioplankton respiratory quotient across freshwater environmental gradients

“…Readings of CO 2 concentration in the chamber headspace were taken each minute for duplicate periods of 10 min. Day-time flux measurements from these floating chambers are overestimating daily fluxes because of induced water turbulence (Vachon et al, 2010), implying that a calculated contribution (%) to the measured CO 2 efflux by BR becomes conservative.…”