On Factors Influencing Air-Water Gas Exchange in Emergent Wetlands
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PUBLICATIONS Journal of Geophysical Research: Biogeosciences RESEARCH ARTICLE On Factors Influencing Air-Water Gas Exchange 10.1002/2017JG004299 in Emergent Wetlands Key Points: David T. Ho1 , Victor C. Engel2 , Sara Ferrón1 , Benjamin Hickman1, Jay Choi3, and • Wind speed is not a good correlate for 3 gas exchange in emergent wetlands, Judson W. Harvey and parameterizations developed for 1 2 3 lakes and the ocean are inappropriate Department of Oceanography, University of Hawaii, Honolulu, HI, USA, U.S. Forest Service, Fort Collins, CO, USA, U.S. • Rain contributes significantly to gas Geological Survey, Reston, VA, USA exchange in emergent wetlands • Gas exchange in emergent wetlands could be parameterized by rain rate, Abstract Knowledge of gas exchange in wetlands is important in order to determine fluxes of climatically water flow, and outgoing heat flux and biogeochemically important trace gases and to conduct mass balances for metabolism studies. Very few studies have been conducted to quantify gas transfer velocities in wetlands, and many wind speed/gas exchange parameterizations used in oceanographic or limnological settings are inappropriate under Correspondence to: conditions found in wetlands. Here six measurements of gas transfer velocities are made with SF6 tracer D. T. Ho, [email protected] release experiments in three different years in the Everglades, a subtropical peatland with surface water flowing through emergent vegetation. The experiments were conducted under different flow conditions and with different amounts of emergent vegetation to determine the influence of wind, rain, water flow, Citation: Ho, D. T., Engel, V. C., Ferrón, S., Hickman, waterside thermal convection, and vegetation on air-water gas exchange in wetlands. Measured gas transfer À À B., Choi, J., & Harvey, J. W. (2018). On velocities under the different conditions ranged from 1.1 cm h 1 during baseline conditions to 3.2 cm h 1 factors influencing air-water gas when rain and water flow rates were high. Commonly used wind speed/gas exchange relationships would exchange in emergent wetlands. Journal of Geophysical Research: overestimate the gas transfer velocity by a factor of 1.2 to 6.8. Gas exchange due to thermal convection Biogeosciences, 123, 178–192. https:// was relatively constant and accounted for 14 to 51% of the total measured gas exchange. Differences in rain doi.org/10.1002/2017JG004299 and water flow among the different years were responsible for the variability in gas exchange, with flow accounting for 37 to 77% of the gas exchange, and rain responsible for up to 40%. Received 10 NOV 2017 Accepted 19 DEC 2017 Accepted article online 4 JAN 2018 Published online 24 JAN 2018 1. Introduction Corrected 25 JAN 2019 Knowing the rate of gas fluxes in wetlands is important for understanding the air-water cycling of climatically This article was corrected on 25 JAN important trace gases (e.g., CO2,CH4,CH3Br, and N2O), and for aquatic metabolism studies using O2 mass 2019. See the end of the full text for fl details. balances. Gas uxes occur through three main pathways in wetlands. They could occur through (1) the aerenchyma of emergent vegetation (e.g., Dacey and Klug, 1979), (2) ebullition (e.g., Happell & Chanton, 1993), and (3) diffusive gas exchange across the air-water interface. The study here is focused on the latter, and on assessing processes that control the gas transfer velocity in emergent wetlands. In most aquatic environments, the air-water gas transfer velocity is controlled by near-surface turbulence. Whereas wind is the dominant mechanism for generating turbulence in lakes and in the ocean (Ho et al., 2011; Wanninkhof et al., 1987), sources of energy for near surface mixing in large wetlands may also include rain, flow around emergent vegetation, and wind-induced movement of emergent vegetation (e.g., Foster- Martinez & Variano, 2016; Ho et al., 1997; Poindexter & Variano, 2013). Because very few systematic laboratory and field experiments have been conducted specifically to exam- ine gas exchange in wetlands (e.g., Happell et al., 1995; Poindexter & Variano, 2013; Variano et al., 2009), wind speed/gas exchange parameterizations developed for lakes have been used to determine gas trans- fer velocity in these environments (e.g., Hagerthey et al., 2010). However, the relationship between wind speed and momentum flux has been found to be significantly different between open water environ- ments, such as lakes and the ocean, and wetlands with emergent vegetation (Tse et al., 2016). Because of the existence of vegetation (floating, emergent, and submerged), limited fetch, and shallow depths, the relationship between wind speed and gas exchange in wetlands could be significantly different than in open waters. Therefore, unlike lakes and the ocean, wind might not be the dominant factor driving gas exchange in wetlands. For example, waterside thermal convection has been found to be important in the absence of wind (MacIntyre et al., 2010; Poindexter & Variano, 2013). Rainfall increases turbulence on the fl ©2018. American Geophysical Union. water surface and also changes the heat ux because rain is often colder than the water surface on which All Rights Reserved. it falls (Ho et al., 1997, 2000). HO ET AL. 178 Journal of Geophysical Research: Biogeosciences 10.1002/2017JG004299 Vegetation in wetlands can affect air-water gas exchange in multiple ways. Emergent vegetation can affect air-water gas exchange by (1) sheltering the water surface from wind and creating a shear-free region, (2) increasing turbulence as water flows around the plant stems and through wind-induced plant movement, and (3) influencing large-scale water movement. Floating vegetation (e.g., Nymphaea spp.) may act as a barrier for air-water gas exchange. Submerged vegetation (e.g., Utricularia spp.) in the Everglades ridge-slough habitat has been shown to be a significant factor controlling velocity in the upper part of the water column (Larsen et al., 2009; Leonard et al., 2006), which could affect air-water gas exchange. Linkages between emergent vegetation patterning, surface water flow vectors, and water depth fluctuations are well documented in emergent wetlands (Eppinga et al., 2008; Larsen et al., 2007; Rietkerk et al., 2004). For example, in undisturbed portions of the Everglades, the dominant emergent vegetation, sawgrass (Cladium jamaicense), forms elongated and slightly elevated ridges that are oriented in patterns parallel to the prevail- ing flow direction. The shallow (<1 m) surface water flows in this system are therefore concentrated between ridges in the deeper-water sloughs, which are characterized by an order of magnitude lower emergent plant stem densities (He et al., 2010; Leonard et al., 2006). Recent results from simulation models that couple surface water and vegetation dynamics suggest that the ridge-slough patterns in the Everglades, and the prevailing slough flow velocities and water depths, are the result of a complex set of feedback between the landscape-scale water mass balance, Cladium flooding tolerance, variable ridge-slough resistance to overland flow, and nutrient and sediment transport and redis- tribution (Cheng et al., 2011; Harvey et al., 2017; Kaplan et al., 2012; Larsen et al., 2007). These feedbacks, in turn, influence many of the factors (e.g., wind fetch and water flow velocities) that govern air-water gas exchange. The goals of the study presented here are (1) to measure gas transfer velocity using sulfur hexafluoride (SF6) evasion experiments in the Everglades and (2) to determine how wind, rain, water flow, waterside convection, and vegetation contribute to air-water gas exchange in a shallow water environment populated by floating, emergent, and submerged vegetation. The results also provide a better understanding of how changes in wetland hydrology and vegetation patterns resulting from management practices or climate variability may impact dissolved gas dynamics. 2. Methods 2.1. Study Site The study was conducted in the Everglades of south Florida, USA, in an area known as “the pocket,” between the L-67A and L-67C levees and canals in water conservation area (WCA) 3A (Figure 1). Specifically, SF6 tracer release experiments (TREs; see below) were conducted at sites designated RS1, RS2, and C1. The area is characterized by peat-based ridges dominated by emergent sawgrass (Cladium jamaicense) that are inundated during periods of seasonally high water, separated by deeper water sloughs (generally <1 m depth) containing floating and submerged vegetation. These wetlands are also characterized by the presence of large amounts of periphyton, which covers floating vegetation, such as Utricularia spp., as well as the organic substrate that is exposed between patches of thick emergent or floating vegetation. In 2013, a culvert (called S-152) was installed at the L-67A levee upstream of the study sites to experimentally increase flow across the pocket, in order to test hypotheses about landscape formation and ecosystem restoration as part of a project knows as the Decompartmentalization Physical Model (DPM; Larsen et al., 2017). The installation of this culvert created an opportunity to examine gas exchange under different flow conditions. Experiments conducted in 2009, 2011, and 2014 are reported here. The results from the 2009 experiment represent baseline conditions, such that the vegetation and water depth patterns at the study site were typi- cal of conditions prior to the culvert installation. A fire burned through the study site in June 2011, before the experiment reported here, during which emergent vegetation burned to the waterline. In 2014, the emergent vegetation had returned, and the S-152 culvert was opened, thereby increasing the flow in the wetland. The experimental conditions are summarized in Table 1. Sampling events are referred to below using a year and location nomenclature (e.g., 2011 C1). HO ET AL. 179 Journal of Geophysical Research: Biogeosciences 10.1002/2017JG004299 Figure 1. Map showing the locations of the SF6 tracer release experiments (RS1, RS2, and C1), the S-152 culvert installed as part of the DECOMP Physical Model (DPM), and the 3AS3WX weather station.