GO-BC
Research Spotlight:
Challenges for Carbon Crediting in Zostera marine (Eelgrass) Meadows.
GO-BC spoke with Prof. Hillary Kennedy and Prof. Dorte Kraus-Jensen who gave us insights into the vision and take home messages from their latest publication which looked at how new findings on the carbon storage capacity of Zoster marina compared with other seagrass species has raised questions and produced new challenges for carbon crediting in seagrass meadows. Read on below to discover more on what the research revealed...
Eelgrass Meadows: Why Carbon Crediting Remains a Challenge
Eelgrass meadows (Zostera marina) are the most widespread seagrass ecosystems on Earth, supporting coastal biodiversity, stabilising sediments, and delivering a range of ecosystem services. However, new research shows that their capacity to store and accumulate carbon is substantially lower than often assumed, raising important challenges for their inclusion in carbon crediting frameworks.
Photograph: Danish seagrass meadows. Taken by Dorte Kraus-Jensen
Lower Carbon Accumulation than IPCC Tier 1 default for seagrasses
The study finds that the geometric mean global carbon accumulation rate (CAR) for eelgrass meadows is only around 21% of the IPCC Tier 1 seagrass default value. Crucially, that Tier 1 value is derived from just six measurements from Posidonia oceanica — a long-lived, persistent Mediterranean seagrass with exceptionally high carbon storage capacity.
In addition to lower accumulation rates, eelgrass sediment carbon stocks are also consistently below IPCC Tier 1 values, and in many cases are similar to or indistinguishable from nearby unvegetated sediments. This makes it difficult to demonstrate that eelgrass meadows provide large, clearly attributable gains in long-term sediment carbon storage — a key requirement for carbon crediting.
Species Traits Matter: Eelgrass vs. Posidonia oceanica
Direct comparison with Posidonia oceanica underlines a fundamental ecological distinction:
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Eelgrass is an opportunistic species, with relatively fast turnover, and composed of largely labile organic compounds that ultimately contribute little to long-term burial
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P. oceanica is a persistent, slow-growing species composed of largely refractory organic compounds that builds thick, long-lasting organic sediments over centuries to millennia.
These differences translate into orders-of-magnitude contrasts in blue carbon capacity, both in terms of carbon stocks and accumulation rates. Treating all seagrass species as equivalent in carbon accounting therefore risks systematic overestimation of climate mitigation benefits, particularly for eelgrass and species sharing similar traits.
Photograph: Danish seagrass meadows. Taken by Dorte Kraus-Jensen
Implications for Carbon Crediting and IPCC Guidelines
Although carbon credits can support eelgrass projects under certain circumstances, such as on the Virginia coast and potentially in other large restorable areas the potential per hectare is low. Because eelgrass meadows dominate the global distribution of seagrass, these findings have important implications beyond individual projects:
Carbon crediting methodologies that rely on generic seagrass defaults may significantly overstate eelgrass climate benefits.
Species- and functional-type differences need to be explicitly incorporated into future IPCC guideline updates.
Robust crediting will require site-specific baselines, improved sediment comparisons, and conservative accounting rules that reflect eelgrass ecology.
While eelgrass meadows provide many valuable ecosystem services — including habitat provision, coastal protection, and nutrient cycling — this study shows that their role in carbon markets must be approached with caution.
A More Nuanced Blue Carbon Framework
Eelgrass remains a vital coastal ecosystem, but this research highlights that not all blue carbon systems are equal. Recognising differences between opportunistic and persistent seagrass species is essential for ensuring that carbon accounting, IPCC guidance, and carbon markets remain scientifically credible.
As blue carbon policy evolves, incorporating species-specific evidence will be key to aligning conservation, climate mitigation, and market integrity.
Photograph: Danish seagrass meadows. Taken by Dorte Kraus-Jensen
Where can you find this paper?
The paper is published fully open access in Science of The Total Environment - see below: