Blue carbon stocks and sources in four restored UK saltmarshes

There is interest in using saltmarsh restoration, and that of other so-call blue carbon ecosystems, as a natural climate solution owing to the ability of these wetlands to sequester and store high amounts of organic carbon in their sediments. Given this, it is important to consider if restored saltmarshes come to function in the same way as natural, established habitats of the same type, and to determine the permanence and origin of the carbon stored in their sediments.

Several existing restored UK saltmarshes have not had their carbon stocks and sequestration rates assessed. Furthermore, the numerous studies of saltmarsh carbon stocks that do exist rarely consider the permanence of the carbon found in the sediment, and only sometimes assess whether the carbon comes from in-situ or external marine or terrestrial sources. This information is needed for carbon crediting schemes – how restoration projects can secure funding to ensure their implementation. To convert the carbon stored in a restored saltmarsh to carbon credits, it is necessary not only to quantify the amount of carbon being stored but also to establish whether it was buried due to the restoration and whether it is stored for a meaningful length of time.

Four restored UK saltmarshes were assessed as a part of this study. Firstly, carbon stock estimates were calculated for these sites that have so-far gone unquantified. This was achieved by developing a novel and inexpensive technique using foraminifera to gain an estimate of how much carbon can be attributed to restoration actions. Secondly, the quality/degradability of organic matter was determined using thermogravimetric analysis. And finally, the sources of organic matter were assessed using stable isotopes. Results show that much of the organic matter present in the sediment of the restored marshes is stable and recalcitrant, suggesting that it originated externally and was washed onto the marsh surface by the tide.

This project will add to our knowledge of UK blue carbon stocks while also implementing techniques that provide information additional to what is usually considered in saltmarsh studies. Developing a better understanding of the functioning of restored saltmarshes will allow judgements to be made about the value of restoration as a means of mitigating climate change.