Table 3. Overview of Potential Processes for Different Apparent k600 (Referred to as app. k600 in Table) Values for CH4 and CO2a.
| no. | process | effect | explanation | study |
|---|---|---|---|---|
| 1 | microbubble flux | app. k600CH4 > app. k600CO2 | based on hypothesis that microbubbles form that move faster than dissolved gases across the water surface boundary layer and that CH4 enters these bubbles to a greater extent than CO2. The apparent difference in k would in this case result because of a combination of different flux processes combined. | (5,13,18,20) |
| 2 | oxic surface water CH4 production | app. k600CH4 > app. k600CO2 | if there is oxic surface CH4 production above the depth where surface water CH4 and CO2 concentrations are measured, the true concentration gradient of CH4 is underestimated and apparent kCH4 will be overestimated. | |
| 3 | high surface primary production | app. k600CH4 > app. k600CO2 | if there is high primary productivity above the depth where surface water CH4 and CO2 concentrations are measured, the true concentration gradient of CO2 is overestimated and apparent kCO2 will be underestimated. | |
| 4 | chemical reactivity | app. k600CH4 < app. k600CO2 | chemical enhancement of CO2 due to equilibration reactions between CO2 and bicarbonate can alter the near-surface CO2 gradient. In cases for which this process is important, the k600CO2 should truly exceed k600CH4. | (7) |
| 5 | surface microfilm respiration processes | app. k600CH4 < app. k600CO2 | for cases with surface films enriched with organic matter where microbial or photochemical processes generate greater respiration and greater CO2 concentrations above the depth where surface water CH4 and CO2 concentrations are measured, the true concentration gradient of CO2 is underestimated and apparent kCO2 will be overestimated. | |
| 6 | possible biased gas concentration measurements using equilibrators | app. k600CH4 > app. k600CO2 | if gas concentration measurements do not fully account for the slower equilibration times for CH4, relative to CO2, between water and a gas headspace, the CH4 concentration will be underestimated and k600CH4 will be overestimated. | (6,18,20) |
| 7 | headspace extraction in waters where CwCO2 is undersaturated relative to CO2 concentrations in the atmosphere | app. k600CH4 < app. k600CO2 | if not accounting for the chemical equilibration of the carbonate system inside the water sample when calculating CwCO2 in undersaturated waters, CwCO2 will be underestimated relative to CwCH4. This in turn will overestimate apparent k600CO2. | |
| 8 | headspace extraction in waters where CwCO2 is supersaturated relative to CO2 concentrations in the atmosphere | app. k600CH4 > app. k600CO2 | if not accounting for the chemical equilibration of the carbonate system inside the water sample when calculating CwCO2 in supersaturated waters, CwCO2 will be overestimated relative to CwCH4. This in turn will underestimate apparent k600CO2. |
a
Here k600 represents the gas transfer velocity normalized to the Schmidt number 600. The explanations are considering processes when k600 is estimated from combined flux and surface water concentration measurements. Each process in the table is discussed further in the separate paragraphs in Section 4.2. The last column, study, is an attempt trying to link possible effects on apparent k600 from each specific mechanism presented in Section 4.2 to the studies that are shown in Table 2.