Seagrass Ecosystems

Seagrass is NOT seaweed. The two are often confused.

Seagrasses are flowering marine plants with roots, stems and leaves that produce flowers and fruits. They are closely related to land plants like grasses, lilies, and palms. There are 72 different species of seagrass and they grow in coastal waters from the tropics to the arctic, on all continents except Antarctica.

With the exception of one species, seagrasses complete their entire lifecycle underwater. Not easy to do with salt water. They have evolved to have a thin layer which allows gas and nutrients to pass directly into their leaves.

They live in clear coastal waters that allow light to penetrate as they need photosynthesis to produce their food. Most seagrass can be found in 1-3 m of water (3-9 feet) but deep species have been found at 58m (190 feet). While most coastal areas support one or two species of seagrass, the tropical waters of the Indo-Pacific have the highest diversity with up to 14 species growing in one region.

In healthy conditions, seagrasses can form huge meadows some of which can be seen from space!

Seagrasses, and their contributions to the ocean, are often overlooked. But these plants have a lot to offer! Dense seagrass meadows trap sediment and nutrients creating one of the most productive ecosystems in the world. Seagrasses are foundation plant species because they modify the environments to create a unique habitat. Here are some of their contributions:

Seagrass beds provide a nursery habitat for thousands of species. Their underwater canopy shelters small invertebrates, small fish, and juveniles of larger species. A single acre (0.004 square km) of seagrass can support more than 40,000 fish and 50 million small invertebrates!

Food Source
Seagrass is a primary food source for manatees and dugongs as well as green sea turtles. These large animals graze on seagrass meadows like cows or sheep graze on open grasslands. An adult dugong eats 64 to 88 pounds (28 to 40 kg) of seagrass a day; an adult green sea turtle can eat about 4.5 pounds (2 kg) per day. Manatees, dugongs, and green sea turtles are keystone species in a seagrass ecosystem. Their foraging keeps the grasses cropped short, removes old grass, and encourages new growth while their waste adds nutrients to the food chain.

Seagrass absorbs nutrients and heavy metals from the water helping to keep it clean. Their roots also slow the flow of water and capture sand and silt which stabilizes the sediment filtering the water and improving water clarity.
Researchers recently learned that seagrass doesn’t just absorb nutrients, but also plastic. Clumps of a seagrass species in the Mediterranean sea were found to hold 1500 plastic particles per kilogram! This means that 900 million pieces of plastic could be trapped each year. Seagrass ecosystems can help to stop the flow of plastic into the open sea.

Coastal Protection
Like other vital coastal ecosystems (mangroves and coral reefs), seagrass habitats help to dissipate wave energy and buffer the coast from the effects of storms and floods. When their roots trap sediment, they not only filter the water, but help to reduce coastal erosion.

Carbon Storage
Possibly most impressive of all, seagrass absorbs 35 times more CO2 than the same area of rainforest, helping to reduce the effects of global warming. One acre of seagrass absorbs enough carbon per year to counteract a car traveling 3,860 miles (6,212 km). Just as trees absorb carbon from the atmosphere to build their trunks, seagrass uses carbon from the water to build its leaves and roots. While seagrass occupies only 0.1% of the ocean floor, it is responsible for up to 11% of the carbon stored in the ocean.

Seagrass has natural threats like extreme weather, intensive grazing, and disease. Coastal changes resulting in siltation or freshwater incursion can also harm seagrasses. However, human activity, in many different forms, is the primary threat to seagrass ecosystems.

Today, 25% of seagrass species are endangered or vulnerable. The rate of loss of seagrass meadows is estimated to be as high as 7% of their global coverage every year. That’s 2 football fields of seagrass disappearing every hour.

Coastal Development
More coastal regions are subject to urbanization and development. This can result in the direct removal of seagrass to dredge for a port, create a sandy beach, or build a jetty. Additionally, the sediment washing into the water from coastal construction can suffocate the leaves and block sunlight taking away its energy source.

Chemicals and nutrients from fertilizer wash into seagrass beds from surrounding coastal areas and cause algal blooms that block the sunlight necessary for seagrass growth.

Climate Change
As temperatures rise species that cannot move, like seagrass beds, are at risk from warmer waters. Recently, marine heatwaves have been occurring more frequently than in the past. These sudden spikes in temperature have taken a toll on some of the world’s seagrass.

Boats and Anchors
As boats approach the shore and drop anchors they kill sections of seagrass meadows. As boat traffic increases, so does the number of anchors causing damage and scarring the meadows below. As the seagrass bed is divided from these scars, erosion around the edges increases further reducing the size of the seagrass meadows.

In a balanced ecosystem, the removal of one element can cause a chain reaction and have multiple effects. For example, overfishing reduces the number of fish who normally eat sea urchins and keep their population in check. Without these fish, the sea urchin population increases and overgrazes the seagrass. The same happens when sharks are removed from the system; herbivore populations increase and overgraze. Other large predators eat intermediate predators, when their population goes unchecked, they overeat small organisms which are essential to clean seagrass blades. Each member of the ecosystem has an important role to play.

Invasive species
Algae can compete with native seagrass species for resources. In the Mediterranean in the 1980s, “killer algae” was released from aquariums and the population exploded. This very successful invasive species overgrew and replaced the native Neptune seagrass, covering more than 131 square kilometers (50 square miles) of the Mediterranean coastline.

As with most issues affecting the ocean, spreading awareness is important. The more people understand the value of seagrass ecosystems, the more protection will be prioritized.

Since so many of these issues are interconnected, the small life changes you make can add up and help in many ways.

  • Reduce the waste you and your household generate.
  • Limit the chemicals and fertilizer you use.
  • Reduce your personal and household carbon emissions (ride a bike, walk, use public transportation, attend virtual meetings and conferences, use renewable energy, reduce your meat consumption)
  • Think about what you put down the drain in your home.
  • When boating near seagrass areas, slow down, avoid getting too shallow, and always use a mooring instead of dropping an anchor.
  • If you build a dock, use grating rather than planks to allow light to penetrate to the seagrass below

Support Local and Global Seagrass Conservation Efforts

Help to Identify Remaining Seagrass Areas
Project Seagrass has developed an app, Seagrass Spotter, for people to use to identify and create a global database of these important habitats. Since seagrass isn’t as exciting as coral reefs, the location of remaining seagrass is not always known. A solid knowledge of where seagrass is located will make protecting it easier. The app is free, easy to use, and you can help to preserve this essential ecosystem.

Contribute to Seagrass Restoration
SeaGrass Grow is a carbon calculator that allows you to determine your carbon footprint and donate to offset it. Your donation helps to restore wetland ecosystems that will absorb the carbon you are responsible for releasing into the atmosphere.
It is important to note that this is not a substitute for reducing your personal carbon emissions. We should all reduce our global footprint and use offsetting only for the carbon generated by the things we cannot avoid.

There are a number of organizations doing great work in protecting the remaining seagrass meadows and restoring essential seagrass ecosystems to areas where they were formerly abundant.

Learn more about these organizations and the importance of seagrass below.

Association for Coastal Ecosystem Services
The Ocean Foundation
Project Seagrass

Natural Seagrass Recovery, Virgina, USA
Eelgrass once covered Virginia’s coastal lagoons but was wiped out in the 1930’s by overfishing, harvesting, disease and severe weather. Now, ninety years later, a marine ecologist discovered that the seagrass seeds, which rely on currents, weren’t making it into the lagoons. Seeds from surrounding seagrass areas were dropped into the lagoons and the eelgrass beds returned. Now they are self-sustaining and are bringing crabs, fish, and scallops with them!

Seagrass Fights Against Acidification
In 2020, scientists in the Chesapeake Bay, US, made a fortunate discovery. The seagrass beds were as they photosynthesize, were turning some of the carbon from the water into tiny crystals of calcium carbonate. These crystals were reducing the acidity of water, as much as 60 miles away by 0.6pH (four times what it otherwise would have been). These vital marine ecosystems are joining in the fight against ocean acidification!