Controls on methane and carbon dioxide exchange in a Phragmites-invaded, impounded coastal wetland

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Rebecca Sanders-DeMott, Meagan Eagle, T. Wally Brooks, Jennifer O'Keefe Suttles, Adrian Mann and Sydney Nick, United States Geological Survey Woods Hole Coastal and Marine Science Center, Woods Hole, MA, Jianwu Tang, The Ecosystems Center, Marine Biological Laboratory, The University of Chicago, Woods Hole, MA, Faming Wang, South China Botanical Garden, CAS, Guangzhou, China, Faming Wang, Ecosystems center, Marine Biological Lab, Woods Hole, MA, Kevin D. Kroeger, Woods Hole Coastal & Marine Science Center, US Geological Survey, Woods Hole, MA

Background/Question/Methods
Tidally influenced coastal wetlands are important sites of carbon sequestration due to rapid carbon dioxide uptake and slow organic matter decomposition, and produce relatively low levels of methane due to regular inundation with saline seawater. Yet, widespread construction of structures to manage coastal hydrology has resulted in vast areas of coastal wetlands with restricted tidal exchange. These impounded ecosystems often undergo freshening via altered hydrology, which in turn causes vegetation shifts, especially invasion by Phragmites australis, that affect ecosystem carbon balance. Understanding controls and appropriate scaling of carbon exchange in these understudied ecosystems is critical for quantifying their carbon sequestration potential and for informing restoration and/or management interventions. Here we 1) examine how methane and carbon dioxide flux vary across a salinity gradient within Phragmites-dominated wetlands in impounded and unrestricted states using static chambers, 2) probe drivers of methane and carbon dioxide flux within an impounded coastal wetland using eddy covariance at the Herring River in Wellfleet, MA, USA, and 3) compare methods for assessing carbon fluxes at management-relevant scales.
Results/Conclusions
We found that decreasing salinity in impounded wetlands leads to a 50-fold increase in methane emissions across the measured salinity gradient (4 to 25 psu) while freshening generally enhanced carbon dioxide uptake but effects were much less pronounced. The freshened, impounded wetland at Herring River was a strong carbon dioxide sink, averaging -3.00 ± 4.58 g CO2-C m-2 day-1 from June-November 2020, but was offset by methane emissions of 0.11 ± 0.06 g CH4-C m-2 day-1. There was little variation in water table height or salinity at the impounded wetland, and methane flux was driven primarily by air temperature and displayed a strong diurnal cycle with a mid-day peak more than 3-fold higher than nighttime flux. However, we did not observe any reduction in methane emissions between opaque and transparent static chambers during the day, indicating that dark chambers do not reflect nighttime conditions in this system. This finding suggests caution is warranted in scaling daytime chamber measurements of methane to daily values, as this could overestimate methane emissions where there is a strong diel cycle. Taken together, these results suggest that although freshened, impounded wetlands can be strong carbon sinks, enhanced methane emission with freshening can offset their net radiative cooling. Restoration of saline tidal flow to these ecosystems could limit methane production and enhance their climate regulating benefits.