Environmental Controls, Emergent Scaling, and Predictions of Greenhouse Gas (GHG) Fluxes in Coastal Salt Marshes

Abstract

Coastal salt marshes play an important role in mitigating global warming by removing atmospheric carbon at a high rate. We investigated the environmental controls and emergent scaling of major greenhouse gas (GHG) fluxes such as carbon dioxide (CO 2 ) and methane (CH 4 ) in coastal salt marshes by conducting data analytics and empirical modeling. The underlying hypothesis is that the salt marsh GHG fluxes follow emergent scaling relationships with their environmental drivers, leading to parsimonious predictive models. CO 2 and CH 4 fluxes, photosynthetically active radiation (PAR), air and soil temperatures, well water level, soil moisture, and porewater pH and salinity were measured during May-October 2013 from four marshes in Waquoit Bay and adjacent estuaries, MA, USA. The salt marshes exhibited high CO 2 uptake and low CH 4 emission, which did not significantly vary with the nitrogen loading gradient (5-126 kg · ha À1 · year À1 ) among the salt marshes. Soil temperature was the strongest driver of both fluxes, representing 2 and 4-5 times higher influence than PAR and salinity, respectively. Well water level, soil moisture, and pH did not have a predictive control on the GHG fluxes, although both fluxes were significantly higher during high tides than low tides. The results were leveraged to develop emergent power law-based parsimonious scaling models to accurately predict the salt marsh GHG fluxes from PAR, soil temperature, and salinity (Nash-Sutcliffe Efficiency = 0.80-0.91). The scaling models are available as a user-friendly Excel spreadsheet named Coastal Wetland GHG Model to explore scenarios of GHG fluxes in tidal marshes under a changing climate and environment.

Methane Emissions From the Salt Marshes of Doñana Wetlands: Spatio-Temporal Variability and Controlling Factors

“…Our data show a direct and strong relationship between the levels of dissolved methane, organic matter content and water temperature, as it has been observed in other brackish coastal wetlands (Welti et al, 2017). Methanogenesis is substantially driven by temperature (Martin and Moseman-Valtierra, 2017) and it is therefore the most dominant environmental driver of CH 4 emissions in wetlands (Yvon-Durocher et al, 2014) specifically in coastal salt marshes (Abdul-Aziz et al, 2018). The overall pattern found here therefore suggests a methanogenesis control by temperature in the sediment, which has been described as muddy with numerous stems and roots and scarce micro-fauna (Ruiz et al, 2004).…”

“…The overall pattern found here therefore suggests a methanogenesis control by temperature in the sediment, which has been described as muddy with numerous stems and roots and scarce micro-fauna (Ruiz et al, 2004). In addition, vegetation in the marshes may have supported a methanogens population that could contribiute to an increase of CH 4 concentrations during warmer conditions, which has been found in Spartina dominated wetlands (Burke et al, 2002;Abdul-Aziz et al, 2018). The organic-matter rich sediment in conjunction with the organic inputs from the heterotrophic Guadalquivir estuary (Flecha et al, 2015) and those generated in situ by the phytoplankton productivity, as indicated by the association found between Chl a and DOC, provide optimal conditions for sedimentary methanogenesis to proceed, with temperature acting as a major regulating driver.…”

“…Few studies have quantified and investigated air-water CH 4 exchange and dynamics in tidal flats (Dean et al, 2018). The fluxes measured in our study are slightly lower than those observed in subtropical tidal wetlands (Tong et al, 2013;Welti et al, 2017), coherent with a temperature control on methanogenesis (Abdul-Aziz et al, 2018). Lower emissions can therefore be expected from temperate coastal wetlands compared to tropical and subtropical coastal wetlands.…”

Salt Marsh Greenhouse Gas Fluxes and Microbial Communities Are Not Sensitive to the First Year of Precipitation Change

“…The complexity of the salt marsh response may also be related to tidal forcing, which is a control on water availability and solute content that exists in salt marshes, but not terrestrial upland ecosystems or even most wetlands. In salt marshes, rates of productivity and GHG fluxes vary between high and low tides (Abdul-Aziz et al, 2018;Kathilankal et al, 2008;Moffett et al, 2010) and between the neap and spring tide astronomical extremes (Guo et al, 2009;Tong et al, 2013;this study). Therefore, the carbon cycling response of salt marshes to precipitation change likely depends on tidal context, with a given precipitation change potentially having different or even opposite effects depending on local tidal inundation regime and tidal phase.…”

Enhanced Carbon Uptake and Reduced Methane Emissions in a Newly Restored Wetland

“…The seasonal change of daytime CO 2 uptake by wetland plants is mainly driven by temperature, photosynthetically active radiation, and day‐length (Moseman‐Valtierra et al, ). Previous studies reported that higher ambient temperature and light intensity could accelerate the activity of the primary C 4 photosynthetic enzyme (RuBisCO), resulting in higher CO 2 uptake (Abdul‐Aziz et al, ; Guo et al, ; Inglett et al, ). Moreover, methanogenesis is substantially driven by temperature (Martin & Moseman‐Valtierra, ), which could contribute to higher CH 4 emission during high temperatures.…”