Variability of UK seagrass sediment carbon: Implications for blue carbon estimates and marine conservation management

Abstract

Seagrass meadows provide a multitude of ecosystem services, including a capacity to sequester carbon dioxide (CO2) within their sediments. Seagrass research in the UK is lacking and there is no published data on sediment carbon (C) within UK seagrass meadows. We sampled 13 Zostera marina meadows along the southwest coast of the UK to assess the variability in their sedimentary organic carbon (OC) stocks. The study sites were considered representative of sub-tidal Z. marina meadows in the UK, spanning a gradient of sheltered to exposed sites, varying in formation, size and density, but found along the same latitudinal gradient. OC stocks (Cstocks) integrated across 100cm depth profiles were similar among all sites (98.01 ± 2.15 to 140.24 ± 10.27 Mg C ha-1), apart from at Drakes Island, which recorded an unusually high Cstock (380.07 ± 17.51 Mg C ha-1) compared to the rest of the region. The total standing stock of C in the top 100cm of the surveyed seagrass meadows was 66,337 t C, or the equivalent of 10,512 individual UK people’s CO2 emissions per year. This figure is particularly significant relative to the seagrass area, which totalled 549.79 ha. Using estimates of seagrass cover throughout the UK and recent UK C trading values we approximate that the monetary value of the UK’s seagrass standing C stock is between £2.6 million and £5.3 million. The C stock of the UK’s seagrass meadows represent one of the largest documented C stocks within Europe and are, therefore, of important ecosystem service value. The research raises questions concerning the reliability of using global or regional data as a proxy for local seagrass C stock estimates and adds to a growing body of literature that is looking to understand the mechanisms of seagrass C storage. When taken with the fact that seagrass meadows are an important habitat for commercially important and endangered species in the UK, along with their declining health and cover, this research supports the need for more robust conservation strategies for UK seagrass habitats.

High Seasonal Variability in Sediment Carbon Stocks of Cold‐Temperate Seagrass Meadows

“…This study highlights the within‐year dynamics of sedimentary carbon content and the influence of seasonality on carbon stock levels in cold‐temperate seagrass meadows. Not only does Z. marina meadows display a large spatial variation in carbon storage (Dahl, Deyanova, Gütschow, et al, ; Green et al, ; Kindeberg et al, ; Röhr et al, ), but as shown in this study, there is also a high seasonal variability (at all sediment depth layers down to at least 30 cm) where sedimentary carbon levels both increased and decreased over the year. Our findings show a peak carbon density in June (0.0139 ± 0.0009 g cm −3 ) and the lowest in August (0.0089 ± 0.0008 g cm −3 ), and the PLS model indicated that there was a negative correlation between TOC stocks and NCP.…”

“…The seagrass Zostera marina is a common species in the temperate zone of the Northern Hemisphere, creating meadows with a high carbon storage potential where the seagrass areas in the Skagerrak‐Kattegat strait and in the Mediterranean show particularly large carbon stocks, exceeding the stocks found in many terrestrial environments (Mcleod et al, ; Röhr et al, ). There is, however, a large variation in carbon storage efficiency among Z. marina habitats (Dahl, ; Green et al, ; Kindeberg et al, ; Röhr et al, , ), which is mainly related to the location of the meadow, where water depth, salinity, and exposure to hydrodynamic forces are regulating the sedimentary carbon accumulation (Dahl, Deyanova, Gütschow, et al, ; Prentice et al, ; Röhr et al, ). Understanding this variability is a key issue for conservation and protection of these important carbon sinks as some areas are deemed to have a lower accumulation of carbon than others, such as in the Baltic Sea, where Z. marina is mainly found in less sheltered sites compared to the Skagerrak area (Dahl, Deyanova, Gütschow, et al, ; Jankowska et al, ; Röhr et al, ).…”

Reduced water motion enhances organic carbon stocks in temperate eelgrass meadows

“…The low OC stocks and high variability found in our study are not uncommon to other Z. marina meadows located in temperate regions (Hodgson and Spooner ; Jankowska et al ; Röhr et al , ; Kindeberg et al ; Poppe and Rybczyk ; Postlethwaite et al ). These low and variable values, along with a lack of data on meadow extent, present challenges for including seagrasses in national and international blue carbon policies (Hejnowicz et al ; Green et al ; Needelman et al ). Building on this first regional assessment of seagrass carbon stocks in Canada, future work should continue to refine regional and local estimates for Pacific Northwest eelgrass meadows.…”

“…Previous studies have highlighted variability in OC stocks and accumulation rates at both the local level, within meadows, and at regional scales, among meadows (Lavery et al ; Ricart et al ; Oreska et al ; Green et al ). Such heterogeneity has been attributed to a wide array of habitat characteristics, including species composition, hydrodynamic regimes, and seagrass and sediment characteristics (Rozaimi et al ; Serrano et al , ; Samper‐Villarreal et al ; Kindeberg et al ; Mazarrasa et al ; Santos et al ).…”

Ecological performance differs between range centre and trailing edge populations of a cold-water kelp: implications for estimating net primary productivity

“…More generally, our estimate of spatial extent suggests that L. digitata stands occupy ~ 85 km 2 (i.e. 8500 ha) of coastal habitat, which is ~ 6 times that of maerl beds (Burrows et al 2014), ~ 1 times that of seagrass meadows (Green et al 2018) and ~ 1/5 that of salt marsh (Beaumont et al 2014) coverage in the UK. Given that these stands store and release significant amounts of particulate C (i.e.…”