Seagrass

What is it?

Seagrasses are the only flowering plants able to live in seawater and pollinate while submerged. They often grow in large groups giving the appearance of terrestrial grassland – an underwater meadow.

There are four species of seagrass in the UK: two species of tasselweeds and two zostera species, commonly known as eelgrass. 

A natural solution to the climate crisis

Seagrass plants and meadows have the potential to sequester and store huge amounts of carbon dissolved in our seas – this is known as ‘blue carbon’. Similarly to trees taking carbon from the air to build their trunks, seagrasses take carbon from the water to build their leaves and roots. As seagrass plants die and are replaced by new shoots and leaves, the dead material collects on the seafloor along with organic matter (carbon) from other dead organisms. This material builds up forming layers of seagrass sediment, which if left undisturbed, can store carbon in the seafloor for thousands of years.

Considering that seagrass captures carbon at a rate 35 times faster than tropical rainforests, and account for 10% of the ocean’s total burial of carbon (despite covering less than 0.2% of the ocean floor), they are one of our most important natural solutions to the climate change crisis.

Why is it like this?

Seagrass is dependent on high levels of light for photosynthesis to grow and can therefore only be found in shallow water to a depth of around 4 metres.

The plants’ roots are anchored in mud, sand or fine gravel, acting to stabilise the seabed and prevent erosion, which has the further effect of helping to stabilise and defend the wider coastline.

The leaves are narrow and long, forming a three-dimensional habitat allowing a wide range of species to inhabit the area. The density of the grasses causes the water currents to slow down and allows nutrients to settle, in turn attracting more wildlife.

Distribution in the UK

Seagrasses are found around the coast of the UK in sheltered areas such as harbours, estuaries, lagoons and bays.

What to look for

The swaying leaves can become covered in algae, anemones and rare stalked jellyfish, while the soft sediment surrounding the roots is home to molluscs, tiny amphipods, polychaete worms and echinoderms.

The sheltered conditions of a seagrass meadow are perfect nursery grounds for young flatfish, and they provide a home for both of the UK's native species of seahorse. At low tide, wildfowl like wigeon and brent geese feed on the exposed seagrass, huge flocks noisily foraging along the shore.

Conservation

A wasting disease was the cause of a drastic reduction of seagrass in the UK in the 1930s. The following recovery has been hampered by increased human disturbance such as pollution and physical disturbance from dredging, use of mobile fishing gear and coastal development.

Sewage discharge high in nutrients is extremely toxic to seagrass, but it also acts to stimulate algae growth which can outcompete the seagrass by reducing the available sunlight. Alien species, including Spartina anglica and Sargassum muticom, also have an impact through competition.

The ability of seagrass to reduce the speed of currents can result in pollutants accumulating in the seagrass bed. Several heavy metals have been found to reduce the plant's ability to fix nitrogen, reducing its ability to survive.

Globally, 30,000km2 of seagrass has been lost in the last couple of decades which is equal to 18% of the global area. The UK has lost approximately 90% of its seagrass meadows, half of which has been lost in the past 3 decades.

The Wildlife Trusts are calling for nature's recovery at sea. You can help by pledging your support and becoming a Friend of MCZs.