Testing two potential fates for coastal marshes: Greenhouse gas emissions from native, Phragmites australis-invaded, and permanently inundated zones

Abstract:

Global changes such as biological invasions and sea level rise can significantly affect GHG emissions from coastal wetlands by changing plant community composition and/or environmental conditions. To first characterize GHG fluxes across major plant-defined marsh zones, CO₂, N₂O, and CH₄ fluxes were compared between S. patens- dominated high marsh and S. alterniflora low marsh during 2012 and 2013 growing seasons in 3 New England marshes. To test how these fluxes may change in response to biological invasions and sea level rise, GHG fluxes were then compared between native, P.australis- invaded, and permanently inundated marsh zones at these sites in 2013 and 2014. GHG emissions were analyzed simultaneously from marsh ecosystems using infrared-based spectrometers connected to static flux chambers. Daytime CO₂ uptake rates (ranging on average between -2 and -21 μmol CO₂ m^-2s^-1) were generally greater in S. alterniflora low marsh zones than in S. patens high marsh among all 3 sites. Methane fluxes were generally low in both native marsh zones (< 50 μmol CH4 m^-2 h^-1) and N₂O emissions were rare. However, CO₂ uptake and CH₄ emissions from P. australis zones were typically more than an order of magnitude greater than those of either native marsh zone. In contrast, permanently inundated marsh soils had similar GHG emissions to native marsh zones. . Though large, the P. australis CH₄ emissions are estimated to offset less than 5% of observed CO₂ uptake rates based on a global warming potential of 25 (100 years). These results suggest that two alternative fates for coastal marshes in the future- conversion to P. australis marshes or to standing water with sea level rise- will substantially affect CO₂ and CH₄ emissions. Net impacts on climatic forcing of these ecosystems will depend on how long term C sequestration is affected as these emissions shift.