High gas‐transfer velocity in coastal regions with high energy‐dissipation rates

Abstract

[1] We measured the gas-transfer velocity (k) and analyzed factors regulating k at coral reefs and an estuary at Ishigaki Island, Japan, using the floating-chamber method and the measured energy-dissipation rate (e) to represent turbulence in a small-eddy model. We confirmed the validity of the floating-chamber method quantitatively for the first time by the comparing e values inside and outside the chamber device. We also compared k to e and empirical parameters such as wind and current speeds. Measured k had a low correlation with the empirical parameters and a high correlation with e, as indicated by the small-eddy model. The high e values may have been regulated by topographic conditions, e.g., corals or seagrasses that generate wakes, and complex coastlines or large-scale (on the order of kilometers) topographic factors that generate horizontal current shear. Our measurements indicate that coastal k is regulated by e and cannot be accurately determined using wind or current speeds. Topographic conditions in coastal regions are important factors that regulate e; thus, a quantitative analysis of the effects of these conditions is necessary to accurately determine coastal air-water gas flux.Citation: Tokoro, T

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

“…Zappa et al (2007) found a good relationship when combining data from four different systems (rivers to estuaries). Similar results were obtained in a recent study (Tokoro et al 2008) in marine coastal systems that used the FC method. However, some difference appears between the slopes of these relationships.…”

“…These values are comparable to those of the upper mixed layer of other lakes (MacIntyre et al 1995) but lower than dissipation rates found under breaking wave conditions (Terray et al 1996), in estuaries with tidal currents (Zappa et al 2007), or in the turbulent coastal environment (Tokoro et al 2008). TKE dissipation rate inside the FC (e in ) averaged 1.5 3 10 24 6 1.1 3 10 24 m 2 s 23 (mean 6 SE, n 5 49) at the ER site and 1.1 3 10 24 6 9.6 3 10 25 m 2 s 23 (mean 6 SE, n 5 9) in ET lakes (Table 1).…”

“…Relationship between k 600 and turbulence-Many authors have pointed out that near-surface turbulence is a key factor governing gas exchanges across the air-water interface because it constitutes a direct proxy of the physical state of the mass boundary layer (Tokoro et al 2008). The gas transfer velocity estimation from turbulence has the advantage of being likely quasi-universal among the different system types, regardless of what drives that turbulence.…”

“…The predictive utility of a single turbulence-based model lies not only in its precision but also in its wide application to a variety of systems, from rivers and lakes to reservoirs and estuaries (Zappa et al 2007;Tokoro et al 2008). This is a major advantage of k 600 models based on turbulence over those based on a proxy such as wind speed.…”

“…Because e varies with depth, this relationship holds only if e is measured at the interface of interest (i.e., near the surface). This theoretical relationship has been recently examined in natural systems (Zappa et al 2007;Tokoro et al 2008) and was found to hold generally. The relationship between turbulence and gas exchange is also the basis for the many empirical functions relating k to wind speed, where wind speed is used as an integrative proxy for turbulence (Wanninkhof 1992;Wanninkhof and McGillis 1999).…”