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Deviations of N2, O2 and Ar from equilibrium concentrations reflect biological and physical processes acting on the system.
‘Oxygen bubbles’ formed in situ from photosynthesis will cause loss of Ar and N2 from the dissolved phase.
N2 is approximately half as soluble as Ar and O2 is ~90% as soluble as Ar. Therefore, the potential for gas stripping is greatest for N2 followed by O2 and Ar.
Once bubbles begin to form, we expect a drop in the dissolved Ar concentration as measured by MIMS. In productivity (no denitrification) experiments, we expect more N2 vs. Ar to diffuse into the bubble.
For bubbles produced by photosynthesis, we need to evaluate the gas composition in the bubble because it may not be in equilibrium with the bulk phase dissolved gas concentrations.
[1] Kana, T.M., C. Darkangelo, D.M. Hunt, J.B. Oldham, G.E. Bennett, and J. C. Cornwell. 1994. Membrane inlet mass spectrometer for rapid high-precision determination of N2, O2, and Ar in environmental water samples. Anal. Chem. 66: 4166-4170.
This research suggests that dissolved Ar measurements can be a useful tool for evaluating the effects of bubble formation in experimental incubations. The observed non-equilibrium conditions will require development of a gas flux model to determine the degree of gas partitioning if the O2 productivity or N2 denitrification rate is to be determined during bubble formation.
The measurement of O2 productivity or N2 denitrification in aquatic systems is sometimes affected by the formation of bubbles inside the experiment chamber. We evaluated the use of Ar as an indicator of bubble formation because it is only affected by physical factors. Ar stripping from the dissolved phase was detected using membrane inlet mass spectrometry (MIMS) during photosynthesis of Ulva lactuca in a closed container. Simultaneous measurements of O2 and N2 exhibited distinctly different patterns relating to production (O2) and solubility differences among the gases.
Photosynthetic O2 production detected from the dissolved phase, was linear prior to bubbles forming and non-linear when bubbles were forming. The O2/Ar ratio increased in a linear fashion, but the slope exhibited an abrupt drop of 22% beginning at the point when bubble formation commenced. The direction and magnitude of the change in slope suggests that the gas composition inside the bubbles was not in equilibrium with the bulk dissolved phase.
This study suggests that Ar concentration data derived from MIMS can be used to detect and correct for the formation of bubbles in productivity and denitrification experiments.
This project was funded by an NSF Research Experiences for Undergraduates grant. We thank Dr. Fredrika Moser and Maryland Sea Grant for support of this project.
BACKGROUND
ABSTRACT
MATERIALS AND METHODS
CONCLUSIONS
REFERENCE
ACKNOWLEDGEMENTS
The abrupt change in slope of the O2/Ar ratio at the inception of bubble formation suggests a physical cause rather than it being due to a change in photosynthetic rate. It is therefore probable that the rate of production of O2 was constant during the entire incubation. Based on 10% higher solubility of Ar relative to O2, we would expect, if the gases were in equilibrium, that gas transport into the bubbles would cause a decrease in slope of ca. 10% when bubbles are forming versus the period prior to bubble formation (i.e. more O2 will partition in the gas phase than Ar causing the dissolved O2/Ar ratio to decline). The observed 22% decrease in slope indicates that the gas composition of the bubbles was not in equilibrium with the bulk water and was elevated in O2 by ca. 12% above the equilibrium value due to the bubble’s formation in the boundary layer of the leaf.
Hei
ght a
bove
leaf
Concentration
Ar O2 N2
Compared to surrounding water, the bubble is rich in N2 due to lower solubility and rich in O2 due to proximity to the photosynthetic O2 source. Ar is most soluble and relatively depleted in the bubble.
Differences in gas solubilities
Gas Bunsen coef. (25 C) Rel. diff.
N2 0.01442 46%
O2 0.02844 91%
Ar 0.03127 100%
…for changes in dissolved gases during photosynthetic bubble formation.
Prediction: Dissolved N2/Ar ratios will decline during bubble formation. Dissolved O2/Ar ratios will reflect the photosynthetic O2 flux and solubility partitioning between bubble and water.
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Dissolved Ar drop-off beginning here indicates loss of Ar to gas bubbles.
More pronounced drop in dissolved N2 is due to the lower solubility of N2 relative to Ar.
N2/Ar declines due to solubility partitioning between bulk water and gas phase.
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Dis
solv
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solv
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mol
l-1 )
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1. Dissolved O2 exhibits a progressive decline in rate due to loss in bubbles.
2. O2/Ar ratio is linear with two slopes. Slope during bubble formation is 76% of initial slope.
3. Expected O2/Ar ratio slope if bubble gases were in equilibrium with bulk water phase. See Interpretation.
Light on
Bubble formation starts
2. O2/Ar ratio is linear with two slopes. Slope during bubble formation is 22% lower than the initial slope..
RESULTS
Conceptual Model
DATA INTERPRETATION
Aquaculture and Restoration Ecology Laboratory at HPL
