Turbulence and Gas Transfer Velocities in Sheltered Flooded Forests of the Amazon Basin

Abstract

Seasonally flooded forests along tropical rivers cover extensive areas, yet the processes driving air‐water exchanges of radiatively active gases are uncertain. To quantify the controls on gas transfer velocities, we combined measurements of water‐column temperature, meteorology in the forest and adjacent open water, turbulence with an acoustic Doppler velocimeter, gas concentrations, and fluxes with floating chambers. Under cooling, measured turbulence, quantified as the rate of dissipation of turbulent kinetic energy (ε), was similar to buoyancy flux computed from the surface energy budget, indicating convection dominated turbulence production. Under heating, turbulence was suppressed unless winds in the adjacent open water exceeded 1 m/s. Gas transfer velocities obtained from chamber measurements ranged from 1 to 5 cm/hr and were similar to or slightly less than predicted using a turbulence‐based surface renewal model computed with measured ε and ε predicted from wind and cooling.

Enhanced Turbulence in the Upper Mixed Layer Under Light Winds and Heating: Implications for Gas Fluxes

“…Zappa et al (2007) found the coefficient c 1 to average 0.419, Katul et al (2018) determine on theoretical grounds that it will be ∼0.4, and Wang et al (2015), who present their own results and review those of others, found the coefficient to increase for ε above 10 −6 m 2 s −3 . Given that the upper bound for dissipation estimates with the SCAMP is ∼10 −5 m 2 s −3 , and the relative invariance of the coefficient in Wang et al (2015) for typical dissipation values in the upper mixed layer of lakes, we assume the coefficient is a constant, and given the uncertainty, use the value 0.5 as in MacIntyre et al (1995MacIntyre et al ( , 2019MacIntyre et al ( , 2021.…”

Carbon Dioxide Fluxes to the Atmosphere From Waters Within Flooded Forests in the Amazon Basin

“…The low CO 2 efflux reported by Dalmagro et al () is likely related to their use of a wind‐based equation to estimate the CO 2 fluxes. While wind speeds in flooded forest sites are low, other processes can increase fluxes, as noted above and in MacIntyre et al (). The large range in our study indicates the need for a sampling over diel cycles on a seasonal basis (Table ).…”

“…Wind‐induced currents or internal waves within the stratified waters of the flooded forests and neighboring habitats may have transported or mixed water with elevated CO 2 concentrations into the surface waters within the flooded forest. Near‐surface turbulence in the forest may have increased in the day due to advective flows generated outside the forest and higher wind speeds resulting in higher gas transfer velocities (MacIntyre et al, ). Values of k 600 under these conditions will be higher than those expected from convection at night when winds are negligible.…”

“…The variability of k 600 was higher at the WE site as were mean values (6.7 + 4.6 SD ) relative to the WP site (2.3 + 1.6 SD ; Figure S3). The higher k 600 values are likely associated with increases in shear in the surface waters of the flooded forest caused by wind‐induced water currents from nearby open water (MacIntyre et al, ). Ho et al () obtained similar results in the Everglades.…”

“…Vegetated aquatic habitats, such as flooded forests, are likely to experience lower wind speeds than open water areas. However, diel variations in cooling or heating of surface waters and associated horizontal water motions and convective mixing can enhance gas exchange even at low wind speeds (MacIntyre et al, ). Direct measurements of k , water–air CO 2 concentration gradients, and meteorological parameters in flooded forests are needed to better understand the mechanisms associated with CO 2 outgassing from these aquatic habitats and to evaluate their role in the carbon cycle.…”