Seagrasses are Huge Stores of Blue Carbon

What are seagrasses? Where do they grow? Why are they so important? What are the main threats to seagrass meadows? How can we protect them? We give you the full story.
Seagrasses Ocean Floor
Seagrass meadows are nursery habitats for many important fish species. Photo: Heather Dine/NOAA

Seagrasses are marine plants that grow in seawater. When they photosynthesize, they release vast amounts of oxygen which is why they are sometimes called the ‘lungs of the sea’.

These wonderful seagrass plants also serve as one of the most efficient natural carbon capture and storage systems on the planet. They can sequester carbon much faster than any land forest. And by reducing the amount of carbon dioxide in the atmosphere, they help to minimize the effects of global warming on both land and sea.

The problem? Seagrasses are in rapid decline because of human activities. Yet another justification for renaming our present era the ‘Anthropocene Epoch.’

In this article, we discuss the importance of seagrass meadows and why their protection and restoration is so crucial for the battle against global warming, and for the health of the ocean on which we rely for food, jobs and recreation.

Did You Know?

Florida’s coastal bays and lagoons are home to an estimated 2.7 million acres of seagrasses. There are 7 main species of seagrass in the area, including turtle grass, manatee grass and shoal grass.

In temperate climates, usually only one or a few species of grasses dominate – like eelgrass in the North Atlantic. In hotter tropical climates, seagrass beds are usually more diverse.

Some fancy Latin names for seagrasses you may come across include: posidonia oceanica, posidonia australis, enhalus, syringodium, halophila johnsonii, halodule pinifolia, halophila minor and phyllospadix torreyi.

What Are Seagrasses?

Seagrasses are like secret gardens under the sea.

They are flowering (angiosperms) underwater plants. In fact, seagrasses are the only flowering plant able to live in salty seawater. They often grow in large patches, densely covering the seafloor creating an ecosystem called a seagrass meadow or seagrass bed. In fact, some seagrass meadows are large enough to be viewed from space.

Seagrasses originally descended some 70 to 100 million years ago from terrestrial plants, before eventually migrating back into the ocean. Other aquatic plants like mangroves also adapted to saline conditions and migrated in a similar manner.

Like terrestrial plants, seagrasses have roots, seeds and flowers. They make their food through the process of photosynthesis during which they release oxygen into surrounding sea water. However, unlike terrestrial plants their stems are not strong enough to hold them up – instead they are supported by the buoyancy of the water around them.

Seagrasses should not be confused with marine algae like phytoplankton, seaweed or beachgrass (which is a terrestrial plant).

There are about 72 species of marine seagrasses which belong to four major groups: Hydrocharitaceae, Posidoniaceae, Zosteraceae and Cymodoceaceae. Their common names like spoon grass, turtle grass, eelgrass and tape grass often reflect their shape and role in the marine ecosystem.

Seagrasses photograph from space
Seagrasses seen from Space. Astronaut photograph shows part of the Al Wadj Bank, located along the northern Red Sea coast. This area has a healthy and diverse coral reef system and extensive seagrass bed. You can see several islands (tan), surrounded by dark green seagrass – the largest is at top left. Image: NASA

Scientists are tracking the genome of certain types of seagrasses in order to discover how they evolved originally from algae in the sea – then migrated to land and back to the sea again as flowering plants. Understanding this evolutionary process may help us understand how crops may respond to climate change, and how they could become salt tolerant.

Are You A Student?

Did you know – marine botany is the study of plants and algae that live in seawater. It is a branch of marine biology and botany.

If you are interested in botany, you might enjoy these articles:

Where Do Seagrasses Grow?

Seagrasses need lots of sunlight for photosynthesis to grow. That is why they are generally only found in shallow coastal waters, up to depths of about 4 meters (13 feet).

They are found in every continent in the world, except Antarctica. Based on estimates 1 seagrasses are reported to occur in 191 countries, across 6 bioregions spanning temperate and tropical seas.

Most coastal regions are dominated by one type of seagrass, but in tropical waters off the western Pacific and Indian oceans you can find as many as 14 different varieties growing together.

Climate Variability in the Oceans
Regional weather cycles – such as the Indian Ocean Dipole (IOD) between East Africa and SE Asia, and the El Nino-Southern Oscillation (ENSO) in the Pacific, account for much of the climate variability in the tropics. Their mechanisms are not thought to be linked to climate change, but their overall effects are.

Why Are Seagrasses So Important?

1. Important Carbon Sinks

Seagrasses are ‘wonder plants’ that help to mitigate climate change by removing carbon dioxide from the atmosphere. They absorb about 10 percent of carbon dioxide in circulation, despite only populating 0.2 percent of the seafloor.

In fact, seagrasses are 35-times faster than trees at removing carbon. 2

Seagrasses take carbon which has dissolved in seawater and store it in their leaves and roots – similar to land plants. When the plants die, the carbon they contain becomes buried in the sea floor sediment, sealing it away for hundreds and even thousands of years. The carbon stored in this way, is called blue carbon because it is stored in the sea. One acre of seagrass can sequester 740 pounds of carbon per year, which is about the same as a car traveling 6,212 km (3,860 miles). For more on this, see: How Blue Carbon Reduces Global Warming.

2. Keep Oceans Healthy

Seagrass ecosystems are highly productive and help keep the ocean healthy. Known as the ‘lungs of the sea’, one meter (3.2 feet) of seagrass can generate 10 liters of oxygen every day through photosynthesis. 3 Marine animals absorb dissolved oxygen through water and use it to breathe, grow and reproduce.

Seagrasses act like a big filter. They clean oceans by absorbing nutrients and by capturing drifting dirt and sand particles. Their roots trap sediment, which improves water clarity and quality and also reduces erosion along coastlines.

A recent study by Cornell University showed that seagrasses provide a filtration service which could counteract the effects of sewage water pollution. Unwanted ‘nasty’ bacteria are reduced when they pass through seagrass meadows.

3. Foundation of Marine Food Web

Seagrass meadows are important feeding grounds for thousands of species of marine wildlife.

Originally scientists believed very few species fed on seagrass leaves, partly because of their low nutritional content, but this view has subsequently changed. Now we know that seagrass herbivory is an important link in the marine food web and that the plants provide food for hundreds of species including: geese, swans, fish, dugongs, octopuses, sea turtles, shrimp, krill, blue crabs, oysters, sponges, sea urchins, clams and squid. For example, an adult sea turtle can chomp it’s way through 4.5 pounds (2kg) of seagrass a day.

Certain epiphytic algae which live on seagrass leaves – are another food source for marine life. These algae extract nitrogen from their environment, making it available to larger animals like snails, when they themselves are eaten.

Marine Microbes Drive the Aquatic Food Web
Zooplankton and the Marine Food Chain

4. Nursery For Younger Fish

Like coral reefs, seagrass meadows provide critical nursery grounds for baby fish, including commercially important fish species like cod, plaice and pollock. They also offer shelter from predators to invertebrates like crabs and shrimp.

Some species are permanent residents among the meadows. While others are temporary visitors who come to feed but raise their young in nearby coral reefs and mangrove forests. A single acre of seagrass can support up to 40,000 fish and 50 million invertebrates.

Seagrass beds are biodiversity hotspots, which is why fishermen often seek them out.

5. Fishing Industry

Seagrass meadows are essential to the fishing industry because they provide the nursery ground and food for their catch. Catch such as the Walleye pollock, Atlantic herring and cod and Pacific herring and cod.

Even if commercial fisheries catch stock off shore in deeper waters, many of those fish still rely on seagrasses for rearing their young.

Thus, seagrasses are a vital source of food security for billions of people who rely on fish as their main source of protein. In the Mediterranean, seagrass covers less than 2 percent of the sea floor, for example, but is associated with 30 to 40 percent of hauls for commercial fisheries. 4 There is also increasing appreciation for the fact that small‐scale fisheries make up 25 percent of global total catch. 5

Yet, when official bodies talk about fishery management, seagrass meadows are often overlooked. This is why marine scientists have been pushing for formal acknowledgement of the significance of seagrass meadows as nursery grounds.

Manatee grass underwater photo
Manatee grass (Syringodium filiforme) in southeastern Graham’s Harbour, San Salvador Island, Bahamas. It is the favorite food of the manatee, more commonly known as the sea cow. Image: James St John. CC-BY-2.0

What Are The Threats To Seagrass Meadows?

Despite their importance – seagrasses are disappearing at the rate of 7 percent a year since 1990. 6 That’s about 2 football fields worth of meadow lost every hour.

What’s worse, with an estimated 44 percent of the world’s population now living within 100 miles (150 km) of the ocean, this rate of decline is likely to accelerate.

Seagrasses are very sensitive to water quality, and so where they thrive is a good indicator of the overall health of a coastal ecosystem. Since they produce energy through photosynthesis, that means the water needs to be clear enough to allow sunlight through to reach them. This is why the main threat to seagrasses is eutrophication, a type of water pollution which frequently occurs around coastal urban areas.

Eutrophication is the over-enrichment of coastal waters from fertilizer run-off and sewage waste. This excess of nutrients shifts the balance away from seagrass meadows and in favor of competing faster-growing algae like phytoplankton. Algae blooms smother seagrass, decreasing their ability to absorb light and photosynthesize. This contributes towards another problem – ocean deoxygenation – decreasing levels of oxygen in the water which can cause fish to suffocate.

Other effects of global warming on the oceans that affect seagrass populations include:

Warming oceans – episodes of severe ocean warming known as marine heatwaves are becoming more frequent. This poses a direct threat to seagrasses since temperatures affect how plant enzymes and metabolism work.

Higher waves and storm activity – linked to extreme weather events and climate change. All this disrupts plant habitats and water quality.

Rising sea levels – as sea levels rise, this increases the risk of coastal flooding and dislodgement of coastal plants.

Changing water chemistry ocean acidification is affecting marine animals and plant life.

How Can We Protect Seagrasses?

There is no international legislation to protect seagrasses, so typically protection comes from local agencies. However, countries that have signed up to the Paris Climate Agreement are placing increased focused on restoring lost seagrass meadows as a way of meeting their National Determined Contributions (NDC’s).

For example, in 2020 Britain started planting a million seagrass seeds, in an effort to grow 20,000 sq m (215,280 sq ft) of meadow. Scientists hope this will help mitigate greenhouse gas emissions, as well as support marine wildlife and boost fish numbers. Other successful restorations have taken place in Chesapeake Bay, Virginia and Tampa Bay, Florida.

There are difficulties in managing seagrass meadows, mainly because of the remoteness of some coastlines and the costs involved in managing them. Furthermore, there is still little basic understanding of the importance of seagrass as a vital resource. Governments, policy makers and the general public require educating in order to save these habitats.

Unfortunately, we are losing seagrasses at an accelerating rate. To slow down this rate of loss, we need to do two things. (1) We need to raise global awareness of the environmental and climatic value of these carbon reservoirs. (2) We need to continue our efforts to resolve the climate crisis we have created.

Some simple steps everyone can take to help seagrasses and other marine habitats include: don’t litter, be careful with the amount of fertilizer and pesticides you use in your garden, don’t dump any hazardous liquids down the drain and be careful when boating by going slow and avoiding shallow areas.

Further Reading

Seagrass Research in Southeast Asia, Indonesia and Philippines – a biologically diverse region threatened by anthropogenic activities. Jillian Lean Sim Ooi et al, 2018.

Sowing the Seeds of Seagrass Recovery Using Hessian Bags, Richard K. F, Unsworth et al. August 2019. Growing seagrass seeds of the species Zostera marina using a new, simple method “Bags of Seagrass Seeds Line (BoSSLine).”

References

  1. “The global distribution of seagrass meadows.” Len J McKenzie et al. 2020. []
  2. “The ocean, a carbon sink.” La Plateforme Océan et Climat (POC) ocean-climate.org/ Dec 3, 2016. []
  3. Sea grasses and sea grass beds – Ocean []
  4. “Seagrass meadows support global fisheries production”. Richard K.F. et al.
    2018 []
  5. “Global significance of seagrass fishery activity”. Lina M Nordlund et al. November 2017 []
  6. “Disappearing Seagrass Threatening Future Of Coastal Ecosystems Globally” University of Maryland Center for Environmental Science. 2009 []
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