The Coastal Squeeze: Rising seas and upland plant invasions differentially affect vertical exchange of greenhouse gases

Biological invasions and sea level rise may significantly alter greenhouse gas fluxes from coastal marshes and their roles as major global carbon sinks. A spatial gradient in a coastal wetland was used to test how greenhouse gas fluxes may vary in response to either invasion by Phragmites australis or inundation by sea level rise. Net fluxes of CO₂, N₂O, and CH₄ were compared between four zones of a New England coastal marsh (Sage Lot Pond, MA): the native low (Spartina alterniflora) and high marsh vegetation zones (Distichlis spicata and Juncus gerardii- dominated), invasive Phragmites australis zones, and permanently inundated, bare ponds. To test for potential proxies of greenhouse gas fluxes, plant properties were analyzed for relationships to CO₂ or CH₄ fluxes using a multivariate non-linear data-analytics model. Gas fluxes were also measured from a range of differently sized ponds and compared to die-back areas in two additional RI marshes. High precision infrared-based spectrometers were used to measure the gas fluxes in flux chambers. Among the native marsh zones, greatest CO₂ uptake rates were found in S. alterniflora low marsh zones (averaging from -1 to -14 µmol CO₂ m^-2 s^-1). Further, invasive Phragmites zones displayed significantly larger CO₂ uptake rates (-7 to -15 µmol CO₂ m^-2 s^-1) than the native (high) marsh zone (< 2 µmol CO₂ m^-2 s^-1), while, in contrast, unvegetated ponds were typically small sources of CO₂ to the atmosphere. Methane fluxes were generally low (< 50 µmol CH₄ m^-2 h^-1) and did not significantly offset CO₂ uptake in any vegetated marsh zones. No significant N₂O fluxes were observed (neither sinks nor sources). Among the plant properties in this study, belowground biomass was the strongest proxy for CO₂ fluxes in native marsh zones, while abiotic properties are more likely to drive shifts in methane fluxes. Gas fluxes in multiple ponds and adjacent die back areas suggest a successional transition from strong C sinks in vegetated marshes to C sources. These findings signal clear potential for two alternative ecosystem fates- either inundation by rising seas or alteration by biological plant invasions- to produce opposite impacts on marsh carbon cycling.