GO-BC
Research Highlight:
Methane Fluxes in Tidal Marshes of the Conterminous United States
GO-BC interviewed Dr Ariane Arias-Ortiz who gave us insights into her vision and take home messages from her latest meta-analysis research which looked at maethane fluxes from tidal marshes across the contiguous United States. Read on below to discover what her vision was for the paper, what difficulties she encountered, and what she hopes to do following on from this research milestone in greenhouse gas fluxes from blue carbon ecosystems.
About the author
Dr Ariane Arias Ortiz is a member of GO-BC’s science technical working group and a research scholar specialising in carbon-biogeochemical cycling and ecosystem-atmosphere interactions at the Universitat Autonoma de Barcelona. Her work, and the work of her team, focus on studying how carbon flows in and out of coastal ecosystems, and how disturbances and management impact the direction and magnitude of these fluxes.
Back in 2019, the Coastal Carbon Methane Working group was created as part of the Smithsonian Coastal Carbon Network through funding support from the NSF. This group brings together some of the world’s best research scholars working on coastal ecosystems, and who have a vested interest in studying the fluxes of methane therein. Dr Ariane Arias-Ortiz spearheaded a big community paper to understand the magnitude and variability of methane fluxes in tidal marshes and swamps across the conterminous United States, along with their predictor variables. This work resulted in an open-source dataset on coastal wetland greenhouse gas fluxes that is well-documented and analysis-ready; the first of its kind.
Photograph: Tidal saltmarsh in Mayport, Florida. Photo by Mark Bias.
Vision of the paper...
Over the next two to three years, the Methane Working Group worked tirelessly to collate, process and analyses huge amounts of methane flux data from chamber and eddy covariance sites across the conterminous US. In total, methane flux data was synthesised from over 100 sites, 100 of which were chamber sites, and nine of which were eddy covariance sites.
With the data assimilated in one place, the team were in a prime position to quantitatively analyse methane flux data from across the conterminous US for the first time, examining patterns across space and at various timescales thanks to the numerous chamber sites and the continuous datasets from eddy covariance towers, respectively.
Prior to this paper being published, salinity was the best established predictor for methane emissions, but it poorly predicted emissions below the 18 ppt threshold. Fresher < 18 ppt wetlands could be emitting over 200 g of methane into the atmosphere every year, per square meter.
Through hard work and many hours of data compilation and analysis, Dr Arias Ortiz and the rest of the Methane Working Group, managed to discover the types of wetlands most likely to emit large quantities of methane. They found that the warmest, freshest wetlands—those with a salinity of 5 ppt or less and an average daily maximum temperature of at least 25°C—had the highest methane emissions. Cooler freshwater wetlands that experienced frequent flooding also emitted substantial methane, though less than half the amount of the warmer sites. In contrast, freshwater wetlands that were neither warm nor frequently flooded emitted almost none. Salinity, maximum annual daily temperature, and inundation frequency emerged as the primary factors influencing methane fluxes from tidal marshes in the US
By using both chamber and flux tower measurements, the authors were able to distinguish, for the first time, between the spatial variability of methane fluxes and its drivers, and the temporal variability and its drivers.
Graphical abstract of Arias-Ortiz et al., 2024 "Methane Fluxes in Tidal Marshes of the Contiguous United States.
What are the three take home messages?
1. Salinity is a critical factor in methane emissions across sites.
2. Temperature and flooding drive methane emissions in brackish tidal wetlands.
3. Within site temperature, gross primary productivity (GPP) and tidal height controlled variability at the seasonal and diel scales, respectively.
What's next for the Methane Working Group?
Several ideas were identified and suggested whilst discussing with Dr Ariane Arias-Ortiz, what the next steps of the Methane Working Group could be.
The first step would be to integrate the methane flux database within the existing Coastal Carbon Atlas app for soil cores, a data repository for coastal data sets across the globe.
Secondly, they are looking for additional funding so that the working group can either; a) accept new GHG flux data submissions to be added to the Coastal Carbon Atlas; b) expand data submissions to include mangroves and seagrasses within the conterminous U.S., providing a comprehensive view of methane fluxes across blue carbon habitats; or c) extend the study to cover tidal marshes, globally with the ultimate goal of including all three blue carbon habitats worldwide.
This expansion will need to be implemented in phases. However, each of these ideas would provide a great deal of critical information on methane fluxes across one or multiple blue carbon habitats, providing the opportunity to more accurately quantify their potential contributions towards climate mitigation.