Methane emissions in Port Susan Bay: the missing link in carbon accounting

Coastal wetlands have the ability to sequester large amounts of “blue carbon” in sediments that would otherwise act as a harmful greenhouse gas in the atmosphere. The recently restored marsh in Port Susan Bay, Washington sequesters 231gC m-2 yr-1 and accretes 2.75cm of sediment per year. While this restored marsh stores nearly twice as much carbon as the nearby natural marsh, it can also emit methane, a potent greenhouse gas that has the potential to outweigh the benefits of carbon storage. It is critical to measure these emissions to determine if this site is a net source or sink of greenhouse gas potentials. The main objective of this study was to measure carbon sequestration and estimate methane emissions to complete carbon accounting in the restored area of Port Susan Bay. To calculate the most recent rate of carbon sequestration at this site, I used surface elevation table (SET) data from 2021 along with site-specific sediment characteristics and a pre-established 210Pb accretion rate correction factor. Direct, static chamber, methane measurements in the field were unsuccessful due to lack of instrument sensitivity and limited access to the lab. Estimates of methane emissions were made using proxies and published values based on environmental data. The emission estimates were then converted to carbon dioxide equivalents using a global warming potential of 25 and multiplied by the carbon fraction. Environmental data were taken alongside the direct methane measurements at four sites in the restoration area. Samples were taken approximately bimonthly during low tide from Fall 2020 through Summer 2021. I measured salinity, sulfate, redox, sediment temperature, pH, and assessed vegetation percent cover, the results of which all suggest negligible emissions. Average redox was well above -200mV year-round, the level at which methane would be produced. Salinity and sulfate (correlation R-value: 0.76) were high at all four sites in the summer months. The high salinity and sulfate suggest low methane emissions, especially in combination with the redox values. In these conditions, sulfate reduction is more energetically favorable than reduction to methane and thus suppresses emissions. While salinity and sulfate were low in the Winter (0ppt and 50ppm respectively), the low sediment temperature (below 5°C) and high redox conditions were likely not conducive to extensive, methanogenic microbial activity. Based on the environmental data, the methane emissions from this site would be between 19.37gCH4 m-2 yr-1 and 16.4 gCH4m-2yr-1. In terms of greenhouse gas equivalents, this would be between 110.7gC m-2 yr-1 and 130.75gC m-2 yr-1. The most recent estimate of carbon sequestration is nearly double the estimated emissions at 206.04gC m-2 yr-1 suggesting that this restored site is a net sink of greenhouse gas carbon equivalents. The information from this study will fill an important data gap to determine if this Pacific Northwest coastal wetland is a source or sink of greenhouse gas carbon equivalents and help inform future restoration decisions