Home to some of our rarest creatures, our underwater seagrass meadows are some of the most threatened wetland habitats on earth.
Dip beneath the sun-dappled shallows of our coastal waters and a whole new world awaits. Rippling meadows of seagrass sway in the gentle currents, their leaves reaching up to catch the sunlight at the water’s surface.
These bountiful underwater grasslands can stretch for up to 3,000 square miles, creating a vast vibrant-green seascape as far as the eye can see. And they teem with life of all kinds. Around the UK’s coastlines, young fish shelter among the grasses, while delicate sea horses drift among the stems. Anemones cling to their leaves, as crabs scuttle by. And during the winter, at low tide, migrating birds feast on the exposed sea meadows.
Seagrasses are flowing plants that have adapted over millions of years to life in the sea. They’re the only flowering plant able to live and pollinate while fully submerged in saltwater. They have bright green, strap-like leaves and can form vast meadows when unimpeded. There are about 50 different species worldwide, but in the UK we have four types: two species of tasselweeds and two zostera species, both commonly known as eelgrass.
Seagrasses thrive in sheltered, shallow bays and estuaries, where they’re protected from the extremes of storms. Because they need sunlight to photosynthesise, they’re found in shallow water up to around four metres deep. One of the best places to find seagrasses is at WWT Castle Espie at Strangford Lough, where during winter on a low tide you can find a large proportion of the world’s entire population of light-bellied brent geese feasting on eelgrass.
Seagrass meadows are amazing for biodiversity. Their long, narrow leaves provide shelter for young fish like cod, plaice and herring, as well as pipefish, cuttlefish and dogfish. In the UK, young seahorses wrap their prehensile tails around seagrasses to avoid drifting away. The swaying leaves of these underwater sea meadows also provide a home to anemones and jellyfish, while molluscs and other invertebrates bury themselves in the soft nutrient-rich sediment trapped around their roots. At low tide in winter, huge flocks of migrating brent geese and whooper swans arrive to forage noisily on the exposed eelgrasses.
Seagrass meadows are really important for biodiversity, supporting thousands of species. They act as ecosystem engineers, literally creating the foundations of life. Their extensive root systems bring stability to the ocean floor creating a welcoming home for marine creatures that find shelter among their leaves and roots.
They’re also effective at carbon storage and are one of the top four ecosystems for carbon sequestration alongside tundra, mangrove forests and salt marshes. They absorb carbon dioxide faster than trees and because it decomposes slowly on the seabed, it’s a longer lasting carbon sink.
The dense seagrass leaves slow down waves and dissipate their energy, protecting coasts from storm surges and cyclones. The seagrass roots which are anchored in mud, sand or fine gravel also act to stabilise the seafloor which helps prevent coastal erosion.
Seagrasses also help keep our ocean water clear. As seawater gets trapped among the grasses, sediment and particles suspended in the water become trapped too and drop to the seafloor, creating beautiful clear, clean water.
Many communities around the world, particularly those from island nations, have depended on the resources that seagrass provide for millenia, harvesting them for fishing and shelter. However the benefits that seagrass provide us are far greater if left to grow in the ocean. Many seagrass dwellers are the inspiration for local myths and legends in several cultures. The mysterious dugong, an aquatic mammal that feeds exclusively on seagrass, is said to be the inspiration behind several tales of mermaids. The word 'dugong' means mermaid in Malay. Although mermaids are entirely mythical, the dugong is just as captivating, although its numbers have declined due to animal trafficking, pollution of their seagrass habitats and boat collisions. It is now classed as Vulnerable by the IUCN. A dugong can eat around 30kg of seagrass a day, so its fate is closely tied to that of the seagrass meadows.
These underwater meadows once covered vast areas around the UK’s coasts. But a wasting disease in the 1930s caused a massive reduction. Although efforts are now underway to restore them, their recovery has been hampered by increased human disturbance from agricultural and industrial pollution, to boats dragging their anchors across the seagrass meadows, as well as dredging, fishing and coastal development. Sewage discharge high in nutrients is also extremely toxic to seagrass, and stimulates algae growth which can outcompete seagrass by reducing the amount of sunlight available.
Restoring or creating a seagrass meadow is actually quite hard compared to other wetland habitats like saltmarsh. It’s effectively like transplanting seedlings in the sea. And there are a lot of things that can go wrong. The sediment might be too fine meaning the seeds get buried too deep, or they might simply get washed away in a storm. But when the seedlings take root, it doesn’t take long for a seagrass meadow to grow. In the UK large scale restoration projects include those at Plymouth Sound and at Dale in West Wales where a million seagrass seeds have been planted.
WWT works around the world to protect, create and restore wetlands for wildlife and people. At WWT Castle Espie we conserve zostera, a specific species of seagrass (also known as eelgrass), for the flocks of brent geese who migrate there in their thousands to feed on it every winter.
WWT creates, protects and restores wetlands for wildlife and people, including seagrass, at our sites across the UK and internationally. Join and support WWT now to support our work and access all our wetland sites for free.