Quantifying Blue Carbon for the largest salt marsh in southern British Columbia

Salt marshes are highly valuable ecosystems that have recently been recognized for the climate change mitigation potential of their soil carbon sequestration. This ‘blue carbon’ is sequestered annually and can be stored for more than a century, but their storage potential has not been well studied on the Pacific coast of North America. This study collected sediment cores from high and low marsh zones in the western portion of Boundary Bay, Delta, British Columbia (BC), to assess carbon storage and carbon accumulation rates (CARs). Carbon stocks in the high marsh were significantly higher compared to low marsh, averaging 84.2 ± 30.9 Mg C/ha and 39.3 ± 24.2 Mg C/ha, respectively. CARs ranged from 19.5 to 454 g C/m2yr, with an average of 137 ± 162 g C/m2yr and a median of 70.1 g C/m2yr. Our CARs indicate that the marsh exhibits substantial variability. Both carbon stocks and accumulation rates were at least 45% lower than global estimates but were similar to other studies on the Pacific coast of North America. By controlling for marsh environment and dating method, we provide a new 210Pb estimate of CAR of 88 ± 20 g C/m2yr for the Pacific coast of North America. Our low carbon stock and accumulation rates in comparison to global estimates are likely due to the shallow depth of the marsh and the dominant type of vegetation. Despite historical modifications and disturbances to the marsh, our study suggests that the western portion of Boundary Bay marsh has been growing in areal extent since at least 1930. Current legislation in the province of BC does not adequately protect salt marshes. This study provides the first quantification of carbon stocks and CARs, which is an important step towards leveraging the co-benefit of salt marshes for improved management, restoration, and preservation for these ecologically and culturally important ecosystems. This study outlines subsequent steps and research needed for Boundary Bay marsh, or other salt marshes in BC, to be included in a voluntary carbon market in British Columbia.