Air‐Sea Gas Exchange and CO₂ Fluxes in a Tropical Coral Reef Lagoon

Abstract

Coral reefs are found predominately in tropical and subtropical areas and are sites of active carbon cycling. Knowledge of gas transfer velocities is necessary for carbon cycle studies in coral reef environments, and past studies in these environments have used wind speed/gas exchange parameterizations intended for the open ocean. In order to determine the relationship between wind speed and gas exchange, and to assess the suitability of open ocean wind speed/gas exchange parameterizations for this environment, two 3He/SF6 tracer release experiments were conducted during two separate years in a tropical coral reef lagoon in Kaneohe Bay, Hawaii. The results show that because water depth is relatively deep in much of Kaneohe Bay, some parameterizations intended for the open ocean can be used to predict gas transfer velocities in that environment. The bay was a source of CO2 to the atmosphere during both studies, and the residence times of water in the bay during the two studies differed by a factor of two.

Parallels between stream and coastal water quality associated with groundwater discharge

“…Uncertainties associated with these parameters are propagated throughout the calculation of SGD. Gas transfer velocities calculated using wind speed [56] were in agreement with those found using 3 He/SF 6 in Kāneʻohe Bay [57]. Fresh and saline SGD fluxes were estimated using Eq 3, after [52].…”

Hawaii Coastal Seawater CO2 Network: A Statistical Evaluation of a Decade of Observations on Tropical Coral Reefs

“…The entire bay is relatively shallow, with a mean depth of 9.5 m. The depth of the barrier reef is quite variable with a significant area characterized by approximately 2 m water depth, and the mean tidal amplitude in the bay is 68 cm (Ringuet and Mackenzie, 2005). Water at the CRIMP-2 location has a variable residence time (hours to days: Lowe et al, 2009a,b;Ho et al, 2019), which is long compared to the two buoy locations on the South Shore of O'ahu. Generally, open ocean water crosses the barrier reef before reaching CRIMP-2, then enters the deeper lagoon and returns to the ocean through the shallow Sampan Channel (Lowe et al, 2009a,b).…”

“…Other studies have described uncertainties in coastal CO 2 flux estimates due to the physical differences between nearshore and open ocean wind and current stresses (e.g., Tokoro et al, 2014). Although the gas transfer velocity is derived from an open ocean relationship, recent work by Ho et al (2019) in Kaneohe Bay show its general applicability to coastal and lagoon waters. Further refinement of the gas exchange at shallow reef sites, however, would benefit from inclusion of the effect of current induced turbulence on k.…”

“…El Niño events are also associated with lower trade wind speeds (Collins et al, 2010, and references therein). Weakening of the trade winds increases the residence time of water on the Kaneohe Bay reef flat (e.g., Lowe et al, 2009b) and also reduces air-sea gas exchange rates (e.g., Wanninkhof, 1992;Ho et al, 2006Ho et al, , 2019. Thus, both increased residence time and decreased air-sea gas exchange lead to a stronger buildup of biogeochemical signatures of primary productivity and dissolution (drawdown of pCO 2 sw) and respiration and calcification (increase in pCO 2 sw), respectively.…”

Water temperature control on CO2 flux and evaporation over a subtropical seagrass meadow revealed by atmospheric eddy covariance

“…While wind is clearly an important driver of gas transfer in coastal waters (Upstill‐Goddard 2006), especially when water is relatively deep (Ho et al 2018 a ), other factors like bottom‐driven turbulence (Tokoro et al 2007; Ho et al 2016, 2018 b ), convective forcing (Rutgersson et al 2011; Czikowsky et al 2018; Van Dam et al 2019 b ), biological surfactants (McKenna and McGillis 2004; Ribas‐Ribas et al 2018), and wave slope (Wanninkhof et al 2009) may cause variations in gas transfer irrespective of wind.…”