Original photo: James St. John
Often ignored by divers and snorkelers who swim straight past them, seagrass meadows are perhaps the most under-appreciated marine habitat.
Today, we’ll look at the importance of seagrass, and what benefits we receive from these underwater meadows.
What is seagrass?
Seagrass is just grass that grows in the ocean, right?
This is what most people assume when you ask them if they know what seagrass is, but this assumption is incorrect.
The name “seagrass” can be misleading, as it’s not really grass – its closest terrestrial relatives are lilies and orchids. Seagrass is named because of its long, narrow leaves, which bear a resemblance to some terrestrial grasses.
Found in shallow seas and brackish waters throughout the world, seagrasses are flowering plants that have evolved to thrive where few other plants could survive. There are approximately 72 species of seagrass, and they can be found in tropical, temperate, and even Arctic waters.
Original photo: Claire Fackler, CINMS, NOAA.
Like most plants, seagrasses have roots (called rhizomes in seagrasses) that burrow into the sand or mud for nutrients and anchorage. The latter is especially important considering the strong currents they may have to endure daily.
These rhizomes not only feed and stabilise the seagrass, they also play a key role in reproduction.
Many plants are capable of asexual reproduction (where an offspring arises from a single organism), but seagrasses take it to a whole new level.
Seagrass can clone itself via a process known as rhizome extension, where the rhizomes can spread across the ocean floor with new shoots appearing above the sediment. By reproducing in this way, seagrass can cover vast areas of shallow seas. These vast areas of seagrass are known as seagrass meadows. Some of these seagrass meadows are so large, they can be seen from space.
Why is seagrass important?
It’s easy for us to underestimate the importance of seagrass. But, seagrass meadows are one of the most productive and multifunctional ecosystems on the planet, and their importance should not be overlooked.
Seagrass meadows tend to form over soft sediment. Their rhizomes weave together under the seabed, stabilising the sand or mud they grow on. This is important as it prevents the seabed being washed away in currents and potentially smothering neighbouring coral reefs.
Seagrass meadows are often referred to as nursery habitats because the dense layer of leaves slows the flow of water, to provide a safe shelter for juvenile fish, as well as some smaller species of fish and invertebrates.
Many fish are attracted to the seagrass meadows to feed – either on the smaller animals that are hiding, or on the seagrass itself.
The world’s only true herbivorous sea mammal, the dugong, lives almost exclusively on a diet of seagrass, with an adult eating up to 40kg of seagrass a day. The green turtle is also herbivorous (as an adult), and seagrass makes up a large part of their diet – an adult can eat around 2kg a day.
Original photo: P.Lindgren
What is blue carbon and why is it important?
You may not have heard the term blue carbon before, but it plays an immensely important role in curbing climate change.
To put it simply, blue carbon is the term used to describe carbon that is trapped and stored either in the ocean or closely related marine ecosystems, such as mangroves or salt marshes.
Being plants, seagrasses photosynthesise in the same way terrestrial plants do. They use sunlight to synthesise nutrients from carbon dioxide and water, releasing oxygen as a by-product.
Original photo: Jean-Marc Kuffer
Seagrasses are aptly nicknamed “the lungs of the sea” because they can generate enormous amounts of oxygen. A single square metre of seagrass can release as much as ten litres of oxygen a day through photosynthesis. When snorkelling or diving over a seagrass meadow, you can actually see the oxygen being released from the seagrass as it floats up to the surface!
Because they are so photosynthetically productive, seagrass can absorb huge amounts of carbon from the atmosphere.
Each square metre of seagrass is capable of absorbing 83 grams of carbon per year, and seagrass meadows hold around 15% of the carbon stored in the ocean. This number is particularly impressive considering they make up only 0.1% of the ocean floor.
The carbon is stored in the plant, and as the plant dies, the leaves sink to the seabed and decay. The carbon trapped inside the leaves and rhizomes of the seagrass will become buried by sediment, and trapped indefinitely — that is unless the seagrass meadow is interfered with.
Original photo: Andrew Harvey
Seagrass meadows are under threat
Seagrass meadows are declining at an alarming rate. On average, we are losing an acre of seagrass habitat every 30 seconds, and an estimated 29% of seagrass meadows have disappeared over the past century.
Although seagrass meadows are rapidly declining, seagrass is one of the most resilient plants on the planet.
Depending on their location, seagrass meadows may have to endure changing levels in salinity, extreme temperature fluctuations, waves and storms, and strong currents. Some species that grow in very shallow water may have to deal with moving between water and air up to twice a day (as the tide moves in and out). Very few other plants could survive such harsh conditions.
Along with mangroves (another extremely resilient semi-marine plant), seagrass meadows have become a vulnerable shallow water habitat.
Original photo: jayhem
Their biggest threats are pollution from agriculture and habitat destruction.
Rain over farmland can wash fertilisers into rivers and the ocean. This increase in nutrients can cause algal blooms that could block the sunlight that is necessary for seagrass to photosynthesise.
Seagrass meadows are often removed by hotels and resorts who believe their guests would rather have a white sandy seabed than a green (often slimy) meadow. Completely removing the seagrass can have devastating effects on neighbouring coral reefs, local fisheries, and even the shape of the seabed.
Bad boating practices can also have negative consequences on seagrass meadows. Boats throwing their anchors or sailing over too shallow water can affect large areas of seagrass beds, damaging the leaves and rhizomes, slowing further growth.
The significance of seagrass meadows in sustaining fisheries and maintaining marine biodiversity, and their role in helping to alleviate the effects of climate change, are poorly understood on a global level.
Aside from monitoring seagrass ecosystems, a key focus of such projects is to raise awareness of the importance of seagrass ecosystems. This involves explaining to both local and global communities the importance of seagrass meadows. Once people understand why seagrass is important, it will be easier to protect it.
If you are interested in learning more about these fascinating marine ecosystems, and seeing seagrass meadows firsthand, why not join one of our marine conservation programs?