South-West England seagrasses: ecology, evolution and contribution to biodiversity and carbon sequestration

The EU Nature Restoration Regulation is the first continent-wide, comprehensive law of its kind and is a key component of the EU Biodiversity Strategy. This sets binding targets to restore degraded ecosystems, particularly those with the most potential to capture and store carbon and to prevent, and reduce, the impact of natural disasters. The EU Nature Restoration Regulation contains seven specific targets, including:

‘marine ecosystems – restoring marine habitats such as seagrass beds or sediment bottoms that deliver significant benefits, including for climate change mitigation…….’

The 800 hectares of seagrass meadows in South-West England are rightly regarded as important biodiversity ‘hot spots’, providing habitats for many species of juvenile fish, cuttlefish and sea horses. Many of the meadows in the region have been impacted by a ‘Seagrass Wasting Disease’ in the 1930s and the more recent damage caused by the anchoring of small boats (usually pleasure craft). This damage is being rectified by re-planting of seagrass, but many of those engaged in this work do not appreciate the full story behind the present distribution and development of the meadows.

The oldest seagrasses are known from the Maastrichtian of The Netherlands and are found in the Maastricht Chalk Formation (70 million years’ old). Between that time and the present day there are very few direct records of seagrass fossils, and this is because:

• Seagrass meadows from the tidal/inter-tidal boundary are not in an environment that is commonly preserved in the geological record; and
• In modern seagrass meadows, the plants are rarely – if ever – preserved and are rarely found in cores drilled into the meadows below ~30 cm.

In marine cores taken in Plymouth Sound and elsewhere in Southern England there are only low levels of carbon (spores, pollen, dinoflagellates, seeds, and organic debris) and not the high levels that would be required to suggest that sea grasses sequester high levels of ‘blue carbon’ for extended periods of time. The accumulation of low carbon levels can be explained by the enhanced sedimentation created by the seagrass, the so-called allochthonous carbon. The one element of carbon sequestration that is often ignored is that of foraminifera, ostracods and bryozoans, all of which are extremely abundant in meadow sediments. In many cases, the biodiversity of the foraminifera (>100 species) dwarfs the usual biodiversity counts of larger organisms. Such fixed calcium carbonate does have a long-term storage potential.

The other issue that can be ignored by those studying seagrasses is that the marine environment has undergone significant change throughout the 1 million years of the Pleistocene/Holocene and many of the seagrass meadows have only just re-established themselves following the Last Glacial Maximum, when sea levels were 120‒130 m below present-day levels. How seagrass migrated back into the present-day coastal areas is not yet fully understood, including the separation of the inter-tidal and sub-tidal taxa.