Snakelocks Anemone on Seagrass Georgie Bull

Protecting seagrass in Plymouth Sound

3 minute read

We’ve been working to protect and restore vital seagrass habitats at Cawsand, in Plymouth Sound.

The project is led by Natural England in partnership with us at the Marine Conservation Society, Ocean Conservation Trust, Royal Yachting Association and Plymouth City Council/Tamar Estuaries Consultative Forum.

Seagrass plays an important role in absorbing and storing carbon, which can help us battle the climate crisis. It can absorb 10% of the carbon buried in ocean sediment every year, but only covers 0.1% of the ocean floor.

Seagrass is also a biodiversity hotspot. It acts as a nursery for small fish and provides feeding grounds and a home for many marine creatures, including the UK’s spiny seahorse and the short snouted seahorse.

However, over the last 40 years, at least 35% of seagrasses worldwide have been lost or damaged. In the UK, it’s estimated we’ve lost 92% of our seagrass in the past century.

92

%

of UK marine meadows may have been destroyed as a result of pollution, disease and damage caused by people

35

times

more CO2 is thought to be absorbed by seagrass than rainforests

10

%

of the carbon buried in ocean sediment each year is absorbed by seagrass

To reduce human impact on the seabed, we’re replacing traditional boat mooring systems which rest and drag their chains on the seabed, damaging and uprooting large areas of seagrass. We’ve been installing Advanced Mooring Systems, which use submerged buoys to raise the chain off the seabed.

To date, 15 of the devices have been fitted.

With support from organisations like NatureSave Trust, we're able to further our efforts and deliver projects to restore our most effective habitats at absorbing carbon.

Their funding enabled us, and Natural England, to install a further two Advanced Mooring Systems in October 2021. This technology has a lifespan of around 20 years and uses an elasticated material, which further reduces the impact of mooring systems on fragile seabed habitats.

“Seagrass restoration is set to play a key part in tackling both the climate and nature crisis, and this new mooring design will play a key part in that endeavour"

Nick Oldridge, The Naturesave Trust

Water quality and seagrass restoration

Although we’ve reduced the pressure placed on the seabed by anchoring and mooring systems, seagrass restoration could be compromised by water quality as a result of Combined Sewage Overflows (CSOs).

CSO pipes are used to relieve pressure on sewer systems, and in doing so release sewage, 'sewage-related debris' (like wet wipes) and other pollutants into the environment. These pollutants leach into our seagrass beds, compromising the water quality and reducing the ability for seagrass to regrow and recolonise.

Concerningly, many of the CSOs with the greatest discharge rates are near our most important estuaries such as Falmouth, Plymouth Sound and Solent, which are hotspots for seagrass. This makes our work to repair and restore seagrass more difficult.

PlymouthSound_and_Estuaries_CSO Overwflows

When the amount of nutrients in the water increases, so do the levels of plankton and bacterial matter. This lowers the amount of light the seabed receives, which reduces the growth of seagrass.

Although we’re seeding seagrass beds in areas that have been identified by Natural England as being 'good' for seagrass growth, the lower light levels may be restricting the success rate of seed germination.

The increased nutrients brought by CSO discharge also stimulates excess colonisation of bacteria and plant matter on seagrass blades, which further reduces the plants' ability to effectively photosynthesise and grow.

These factors prevent seagrass and the wider habitat from gaining enough energy to grow into the areas where we've removed the abrasion from traditional swing-moorings. We've seen very low re-growth rates around the five Advanced Mooring Systems we put in Cawsand and have monitored since May 2019.

If the nutrients from CSO discharges continue to leach into our ocean, and especially our more sheltered sites which house seagrass beds, its ability to grow, our work to help restore it and our ability to fight the climate crisis could be greatly impacted.

Seagrass near Weymouth Georgie Bull

Credit: Georgie Bull

Find out more about seagrass

Seagrass