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 crises because the meadows are hotspots of biodiversity, sediment organic carbon (OC) stocks and carbon accumulation rate (CAR) and have experienced major global declines in response to pressures. However, OC storage varies among seagrass species. Here we assess carbon storage and CAR in meadows of Zostera marina (eelgrass), the most widely distributed seagrass species, thereby evaluating their potential for inclusion in carbon markets. We review sediment OC stocks, CAR, sources and stability (mineral associated organic matter - MAOM) of the organic matter. We compare our findings for eelgrass, which is a fast-growing colonizing-opportunistic seagrass species, with those for Posidonia oceanica, typifying slow-growing, persistent seagrass species. Eelgrass sediment OC stocks and CAR display median stocks 40 % and 50 % lower, respectively, than those of P. oceanica and median eelgrass CAR is only 21 % of the Tier 1 emission factor for seagrass used in the Intergovernmental Panel on Climate Change (IPCC) greenhouse gas accounting guidelines. The OC stocks in most (60%) vegetated areas were not significantly different from stocks of nearby unvegetated sediments and only 27 % of the eelgrass sediment samples in this compilation would return positive OC values after subtracting the MAOM fraction, a requirement of some carbon market methodologies. These features may partly be due to the strong spatial heterogeneity and temporal dynamics of eelgrass meadows, eelgrass traits as well as export of eelgrass carbon beyond the meadows. We discuss implications for carbon market restoration projects and encourage considering all the ecosystem services the meadows provide.