Assessment of carbon capture and storage in natural systems within the English North Sea (Including within Marine Protected Areas)

This report was commissioned by the North Sea Wildlife Trusts, Blue Marine Foundation, WWF and the RSPB to assess the extent, scale, distribution, and potential of the current blue carbon sinks in the English North Sea (i.e. seabed sediments, saltmarsh, kelp forests, seagrass beds and biogenic reefs). The focus was to i) review the current extent and distribution of each blue carbon habitat, ii) estimate the quantity of carbon currently stored within these habitats, iii) establish the average net sequestration rate (i.e. gC m-2 yr-1), and iv) estimate the potential net total sequestration (i.e. gC yr-1) of each blue carbon habitat.This analysis synthesises and reviews the most up-to-date scientific literature on fixation, processing, and storage of carbon in the English North Sea, including within Marine Protected Areas (MPAs). Carbon stock densities and rates of production and storage are combined with measures of habitat area to give estimates of total carbon stored in blue carbon habitats and their associated sediment stores. The results are intended to inform management decisions and identify opportunities to enhance the seabed and their carbon sequestration potential. Evidence of this nature will contribute to explore the potential of the English North Sea Marine Protected Area (MPA) network to help mitigate against the effects of climate change.Extents of blue carbon habitats for the North Sea region were derived from available sources. These include the EUNIS level 3 combined map from JNCC, Natural England Marine Habitats and Species Open Data, and recently published estimates of organic carbon (OC) and inorganic carbon (IC) stocks in surface sediments (Smeaton et al., 2021). Where maps of coastal habitats based on surveys were not available, including kelp and seagrass, extents of these habitats were estimated from models.Limitations of the estimates produced here link primarily to poorly constrained spatial extents of blue carbon habitats at the scales required for this report. For some habitats (intertidal and subtidal sediments), confidence in observational understanding of long-term sequestration is very low, as is that for transport and fate of carbon from macroalgae. Kelp forests in the region, for example, have received little attention compared to the rest of the United Kingdom. Furthermore, the science of understanding the effects of physical disturbance (including trawling) and climate change on these systems is very much in its infancy and new developments will allow a much better-informed outlook for the fate of these stocks and accumulation rates in a changing world and under varying management scenarios.Direct comparison between these North Sea carbon stores and those in terrestrial vegetation and soils are fraught with difficulty. Carbon stock sizes (MtC) and density per unit area (t/km2) are assessed differently, over different areas of habitats, and different timescales for storage of reported stocks. Carbon in living material may persist for years or decades, while that buried in soils and marine sediments may last for 100s to 1000s of years. Such lack of comparability renders straight numerical comparisons nearly meaningless. This is even more of a problem when comparing marine and terrestrial stocks, where soils and sediments and the nature of vegetated habitats are so radically different from each other. Depths of soils considered are a vital consideration. Here we consider marine sediments to a depth of only 10cm, while carbon in terrestrial soils is often reported to depths, typically 30cm to a metre or more. Given these caveats, conclusions that the total carbon reported for the area is 19% of that in UK forests (101 Mt vs 529 Mt) should be treated with extreme caution