World Seagrass Day

Atlas Included in

13-05-2025 1593 words 8 min read

Contents

March 1 Nature

Introduction

Seagrasses are not seaweed, and the distinction is the key to understanding why they matter. They are true flowering plants, with roots, leaves, flowers and seeds, and they are among the very few plant families that have adapted to live their entire lives submerged in salt water. World Seagrass Day exists to raise awareness of these plants, of the ecosystems they build, and of the alarming rate at which they are being lost. It is an official UN observance, which gives it institutional weight, and it is marked by conservation groups, marine scientists, coastal communities and schools.

What seagrass is

Around a hundred million years ago, some land plants returned to the sea, reversing the evolutionary journey their ancestors had made when plants first colonised the land. Their descendants are the roughly seventy species of seagrass alive today. Unlike algae, which anchor themselves to rocks and absorb nutrients directly from the water, seagrasses have proper roots that draw nutrients from the sediment, internal tissues that transport water and gases, and flowers pollinated underwater by currents and, as scientists have recently discovered, by tiny marine invertebrates doing the work that bees do on land.

Seagrasses grow in shallow, sheltered coastal waters where enough sunlight reaches the seabed for photosynthesis. There they spread by underground stems into vast interconnected meadows. Covering only around a tenth of a per cent of the ocean floor, these meadows are among the most productive ecosystems on the planet, and they support a density of life out of all proportion to their size. A single hectare of healthy seagrass has been estimated to support tens of thousands of fish and many millions of small invertebrates, a hidden abundance that makes the meadows one of the busiest neighbourhoods in the sea.

Why seagrass matters

The case for seagrass rests on a handful of services it provides, each of them significant. First, it is a nursery. A great many fish species, including commercially vital ones, spend their vulnerable early lives sheltering among seagrass blades before moving out to reefs or open water, which means seagrass underpins fisheries that feed and employ millions of people. Second, it is food. Green turtles, and the gentle marine mammals known as sea cows, the dugongs and manatees, graze directly on seagrass, and some depend on it almost entirely.

Third, and increasingly the reason seagrass has climbed the political agenda, is carbon. Seagrass meadows are one of the planet’s great stores of “blue carbon”, locking away carbon dioxide in their roots and in the sediment beneath them, where it can remain for thousands of years. Per unit area they capture carbon far faster than tropical forests, and though they occupy a tiny fraction of the ocean, they are estimated to hold a disproportionate share of the carbon stored in ocean sediments. Fourth, seagrass filters and clears coastal water, trapping sediment and absorbing pollutants, and its roots bind the seabed and blunt the force of waves, protecting shorelines from erosion. Researchers have even found that seagrass meadows can reduce the abundance of harmful bacteria in the surrounding water, lowering disease risk for people and for the reef organisms living nearby, a service quietly protecting human health along crowded tropical coastlines.

The scale of the loss

Set against all that value is a steep and continuing decline. Seagrass has been disappearing worldwide for over a century, and the rate of loss accelerated through the twentieth century as coasts were developed, dredged and polluted. Estimates suggest a substantial fraction of the world’s seagrass has already been lost, with meadows continuing to vanish each year. The causes are familiar: coastal construction and land reclamation, dredging, nutrient pollution from farming and sewage that clouds the water and starves the plants of light, damage from boat anchors and propellers, disease, and the warming and acidifying of the oceans. A wasting disease devastated Atlantic seagrass in the 1930s and has flared since.

History and recognition

The road to a UN day was built by decades of quiet scientific work. Marine botanists and monitoring networks such as Seagrass-Watch, and conservation charities such as Project Seagrass, spent years documenting both the value of the meadows and the speed of their disappearance, largely out of public view. Sri Lanka, an island nation acutely aware of what healthy coastal ecosystems mean, championed the resolution at the United Nations, and its adoption in 2022 marked the point at which seagrass finally entered the mainstream of global conservation alongside reefs and mangroves.

How it is observed

The day is marked with seagrass planting and restoration events, snorkelling and citizen-science surveys, school programmes, and scientific gatherings. Restoration has become a particular focus, as projects around the world experiment with replanting meadows using seeds and shoots, some of them on impressive scales. It sits comfortably alongside the other ocean observances that ask people to value what lies beneath the surface: the reefs of World Reef Awareness Day, which seagrass meadows often protect and feed, the sea cows celebrated on World Manatee Day, for whom seagrass is the staple food, and the free-flowing rivers of World Fish Migration Day, whose clean waters seagrass helps to keep clear.

Seagrass and people

Long before anyone spoke of blue carbon, coastal communities put seagrass to practical use. Dried eelgrass, cast up in great banks by winter storms, was gathered across northern Europe and used to stuff mattresses and cushions, to insulate walls, and famously to thatch roofs on the Danish island of Læsø, where a handful of these thick, mossy seagrass roofs have survived for centuries and are now protected heritage. In parts of the world seagrass was ploughed into fields as fertiliser, packed around fragile goods for shipping, and even used in early upholstery and soundproofing. These uses have largely faded, but they leave a trail of local names and traditions, and they remind us that the plant was once a familiar working material rather than an obscure conservation cause.

World variations

Seagrass grows on the coasts of nearly every continent, and the day carries local colour accordingly. In Australia, home to some of the world’s largest and most diverse seagrass meadows, the plant is bound up with the survival of dugongs and green turtles and with the health of the waters around the Great Barrier Reef. In the United Kingdom, where an estimated majority of historical seagrass has been lost, ambitious replanting projects along the Welsh and English coasts have become flagship restoration stories. In the Mediterranean the ancient Posidonia meadows are both a carbon treasure and a tourism casualty, chewed up by boat anchors dropped by summer yachts. In East Africa and Southeast Asia, seagrass meadows are a direct source of food security, gleaned at low tide for shellfish and sheltering the fish that coastal families depend on.

Fun facts

A single seagrass meadow in the Mediterranean, a stand of the species Posidonia oceanica growing between the islands of Ibiza and Formentera, has been estimated to be around a hundred thousand years old, making it one of the oldest and largest living organisms on earth. Seagrasses flower and are pollinated entirely underwater, and researchers only confirmed in the last decade that small crustaceans and other tiny animals help carry the pollen, a marine echo of bees. Seagrass produces oxygen so vigorously in bright sunlight that you can see the bubbles streaming off the blades. And a healthy meadow can slow water so effectively that it traps and buries organic carbon at a rate that has led scientists to describe seagrass as one of the most efficient natural carbon sinks known, hectare for hectare outperforming rainforest. The largest clonal seagrass meadow ever documented, a single self-cloning plant of the species Posidonia australis in Shark Bay, Western Australia, was found in 2022 to sprawl across some two hundred square kilometres, having spread from one seedling over an estimated four and a half thousand years. Seagrass also shelters seahorses, which wrap their tails around the blades to anchor themselves against the current, so a decline in meadows is felt directly by one of the ocean’s most beloved animals.

A closing reflection

Seagrass suffers from a peculiar disadvantage in the competition for human attention: it is green, it is common-looking, and it grows in the murky shallows rather than the photogenic deep. It has none of the coral reef’s colour or the whale’s grandeur, and so it has been drained, dredged and forgotten while more charismatic habitats were being saved. World Seagrass Day is a correction to that neglect. It asks people to look again at the ordinary green meadow swaying in the shallows and see it for what it is: a nursery for the fish we eat, a pasture for turtles and sea cows, a shield for the coast, and one of the quiet engines holding carbon out of the sky.

Written by Atlas

Writes vo.rs's calendar of special days and the stories of the people, places and curiosities behind them. Endlessly nosy about why we mark the dates we do, from solemn remembrances to gloriously silly food holidays, Atlas digs up the origins, the traditions and the odd fact worth repeating at dinner.