Rapid carbon accumulation at a saltmarsh restored by managed realignment exceeded carbon emitted in direct site construction

Abstract

Increasing attention is being paid to the carbon sequestration and storage services provided by coastal blue carbon ecosystems such as saltmarshes. Sites restored by managed realignment, where existing sea walls are breached to reinstate tidal inundation to the land behind, have considerable potential to accumulate carbon through deposition of sediment brought in by the tide and burial of vegetation in the site. While this potential has been recognised, it is not yet a common motivating factor for saltmarsh restoration, partly due to uncertainties about the rate of carbon accumulation and how this balances against the greenhouse gases emitted during site construction. We use a combination of field measurements over four years and remote sensing to quantify carbon accumulation at a large managed realignment site, Steart Marshes, UK. Sediment accumulated rapidly at Steart Marshes (mean of 75 mm yr-1) and had a high carbon content (4.4% total carbon, 2.2% total organic carbon), resulting in carbon accumulation of 36.6 t ha-1 yr-1 total carbon (19.4 t ha-1 yr-1 total organic carbon). This rate of carbon accumulation is an order of magnitude higher than reported in many other restored saltmarshes, and is somewhat higher than values previously reported from another hypertidal system (Bay of Fundy, Canada). The estimated carbon emissions associated with the construction of the site were ~2–4% of the observed carbon accumulation during the study period, supporting the view that managed realignment projects in such settings may have significant carbon accumulation benefits. However, uncertainties such as the origin of carbon (allochthonous or autochthonous) and changes in gas fluxes need to be resolved to move towards a full carbon budget for saltmarsh restoration.

Maximizing blue carbon stocks through saltmarsh restoration

“…Saltmarshes in southeastern England (the study region for this research) appear to have limited sediment supply (Ladd et al, 2019). In the Blackwater Estuary, suspended sediment concentrations (5 0 -150 mg/l), and thus sediment supply, are relatively low in comparison to other UK estuaries such as the Parrett Estuary in southwestern England which has a suspended sediment concentration in the range of 1,000 -10,000 mg/l (Manning et al, 2010;Spearman, 2011;Mossman et al, 2022). Tollesbury MR was implemented at initial elevations lower in the tidal frame than Northey Island and Orplands MR (Wolters et al, 2005).…”

“…Identifying local scale controls on blue carbon variability within MR sites will ascertain how the increasing number of MR schemes globally contribute to addressing the Climate Crisis. Drivers of blue carbon stock in saltmarshes include environmental characteristics such as elevation and local hydrology, that influence the supply of sediment and allochthonous carbon into a site (Wollenberg et al, 2018;Rogers et al, 2019a;Mossman et al, 2022), sedimentary characteristics such as particle size that affect the preservation of carbon stocks (Kelleway et al, 2016;Ford et al, 2019), and vegetation characteristics that include vegetation community composition and diversity, which affect both the supply of autochthonous carbon through primary production as well as the trapping of allochthonous carbon (Ouyang and Lee, 2014).…”

Blue carbon benefits from global saltmarsh restoration

“…Natural flood protection is an important driver of marsh restoration in many global regions and has great potential for co‐benefits to carbon and biodiversity (e.g. Barbier et al, 2011; Mossman et al, 2022), but it can also result in trade‐offs of other ecosystem services, depending on design (Auerswald et al, 2019; Loon‐Steensma and Vellinga 2013). While trade‐offs from flood protection projects are relatively well‐studied (see e.g.…”

“…The search returned 3874 results from Web of Science and 29,253 from Scopus. Duplicate results were removed, and two additional studies were added (ABPmer, 2021; Mossman et al, 2022: these were not available on online search engines at the time of the literature search) following consultation with the Saltmarsh Code Consortium (https://www.ceh.ac.uk/our-science/projects/uk-saltmarsh-code), yielding a final list of 29,182 published studies prior to screening. Publications were screened first by title (3443 retained), then by abstract (930 retained) and finally by full text (431 retained: listed in Supplementary Materials, Table S1).…”

“…global regions and has great potential for co-benefits to carbon and biodiversity (e.g. Barbier et al, 2011;Mossman et al, 2022), but it can also result in trade-offs of other ecosystem services, depending on design (Auerswald et al, 2019;Loon-Steensma and Vellinga 2013)…”