A synthesis of soil carbon stocks from coastal wetlands in western North America

Tidal wetlands sequester and store large quantities of organic carbon in their sediments and plant biomass, termed blue carbon. Assessing spatial variability in blue carbon storage and determining the environmental factors that influence that variability is important for wetland conservation and restoration planning as well as development of blue carbon policy. Over the last five years, the Pacific Northwest Blue Carbon Working Group has been building a database of sediment cores from blue carbon ecosystems along the Pacific coast of North America. Focusing on a subset of 1247 cores from 69 data sources characterizing least-disturbed coastal ecosystems in the region, our team also conducted a regional synthesis to examine differences in stocks between blue carbon ecosystem types, variability across scales, and relationships with key environmental gradients. Carbon stocks were highest in sediments from mangroves in Mexico (median value, 1m depth = 329 Mg/ha) and tidal swamps distributed from the San Francisco Bay-Delta to the Pacific Northwest (344 Mg/ha). Carbon stocks were also high in emergent tidal marshes (230 Mg/ha), but several-fold lower in seagrass meadows (83 Mg/ha) and un-vegetated estuarine tideflats (133 Mg/ha). For the most well-sampled west coast blue carbon ecosystem, marshes, much of the variability in stocks tended to occur at a smaller spatial scale (e.g., within estuaries), suggesting that local gradients such as wetland elevation may be more important drivers than regional differences in climate or other factors. In seagrass and mudflats, carbon stocks increased with the proportion of fine-grained sediments. Our database is supporting regional analyses and policy development by Pacific coast end-users. Moreover, our synthesis of west coast blue carbon stocks (and other syntheses made possible by the database) will increase understanding of drivers of variability in blue carbon within and between coastal ecosystems, thereby better informing wetland conservation and restoration planning.

Impact/Purpose

"Blue Carbon" is the carbon captured and stored by living coastal and marine organisms in coastal ecosystems, such as marsh and seagrass habitats.  There is increasing interest at the State, National, and International level to develop estimates of the carbon sequestered by these habitats and to develop market based incentives to conserve and restore these important habitats. Over the last five years, the Pacific Northwest Blue Carbon Working Group has been building a database of sediment cores from blue carbon ecosystems along the Pacific coast of North America.  Carbon stocks were highest in sediments from mangroves in Mexico (329 Mg/ha) and tidal swamps distributed from the San Francisco Bay-Delta to the Pacific Northwest (344 Mg/ha). Carbon stocks were also high in emergent tidal marshes (230 Mg/ha), but several-fold lower in seagrass meadows (83 Mg/ha) and un-vegetated estuarine tideflats (133 Mg/ha). Our database is increasing the understanding of drivers of variability in blue carbon within and between coastal ecosystems, thereby better informing wetland conservation and restoration planning.

Citation

Janousek, C., K. Poppe, J. Krause, J. Drexler, J. Holmquist, E. Watson, E. Peck, H. Diefenderfer, Cheryl A. Brown, C. Cornu, J. Apple, AND K. Buffington. A synthesis of soil carbon stocks from coastal wetlands in western North America. Coastal and Estuarine Research Federation, Portland, OR, November 12 - 16, 2023.