Challenges for carbon crediting in Zostera marina (eelgrass) meadows

The protection and restoration of seagrass meadows are recognised contributions to address the combined biodiversity-, climate- and pollution crises because the meadows are hotspots of biodiversity and soil organic carbon (OC) storage and have experienced major global declines in response to pressures. However, the OC storage capacity and the associated climate change mitigation and carbon crediting potential vary among and within seagrass species. Here we address the potential for carbon crediting in meadows of Zostera marina (eelgrass), the most widely distributed seagrass species, through a review of soil OC stocks, accumulation rates and net organic matter inputs (subtracting the inherent mineral-protected OC). Eelgrass soil OC stocks and accumulation rates display a wide range of values, but especially accumulation rates are typically lower than the average global values for seagrasses. Only 22% of the eelgrass soil samples in this compilation would return positive OC values after subtracting the inherent mineral-protected fraction, and OC stocks under eelgrass meadows were generally not significantly different from stocks of nearby unvegetated soils. These features may partly be due to potentially strong spatial heterogeneity and temporal dynamics of eelgrass meadows, general eelgrass traits as well as export of eelgrass carbon beyond the meadows. We discuss how the findings affect the implementation of effective policies and methodologies that are required for the conservation and restoration policies relating to seagrass meadows.