Article by Ella Humphreys

Dugong & Seagrass

Dugong

Since my last article about turtles, I have become curious about the dugongs and seagrasses in Moreton Bay. I had to learn more and want to share what I researched and learned with you. 

Dugongs belong to the order Sirenia, which includes three species of manatees. Sirenia is a group of herbivorous mammals that evolved from land mammals around 47 million years ago. The oldest known fossil of the order Sirenia had four limbs and could walk on land. One of their closest evolutionary ancestors is, would you believe, the elephant. The dugong and manatee are the only completely aquatic, herbivorous mammals alive today.

One difference between the species is that dugongs, like elephants, grow tusks (males at puberty and sometimes older females), while manatees don’t. Manatees, unlike dugongs, have 3 to 4 toenails on their flippers, similar to their elephant ancestors. 

If you look at the bodies of the two animals side by side, you can see they have different tails. The manatee has a paddle-like tail, while the dugong’s tail is a fluke, like a dolphin’s. Manatees are slow-moving, and boat strikes are a significant threat to them. Because dugongs are faster, they have a better ability to avoid boats. 

Dugongs eat seagrass and a lot of it. They can eat between 25 and 40 kilograms of seagrass a day. There are around 70 different types of seagrasses worldwide, and seven species are found in Moreton Bay. The dugongs in Moreton Bay usually forage in waters around 1 to 3 metres deep, but have been observed at depths of up to 30 metres. They prefer to eat “pioneer species”, the smaller, fast-growing Halophila ovalis and Halodule uninvervis. Halophila ovalis is low in dietary fibre and high in sugars, amino acids and proteins. Dugongs will leave a grazing trail after vacuuming up the entire plant, including the roots and rhizomes below ground. 

A dugong named Pig lives at Sea Life Sydney Aquarium. As a young dugong, he was found separated from his mother. He was rehabilitated and released. They found him again washed up and decided to keep him in captivity. To feed him, his carers buy a large amount of Cos lettuce as a seagrass substitute, which is threaded into special racks and placed at the bottom of the pool—imagine 30 kg of Cos lettuce a day! The story of Pig makes me think of and admire the dedication of people who rescue and care for marine animals.

In his book The Seabird’s Cry, Adam Nicolson wrote, “We have to stretch our understanding to accommodate the understanding of others.” Every species has a different way of perceiving the world through its sense organs and physical body, which have evolved over time. We may not be able to experience the mind-body landscape of another species directly, but we can make the stretch towards understanding. 

Seagrass

Seagrasses comprise less than 0.2% of the ocean floor, yet they store more than 10% of total ocean carbon annually. Research results published in Nature Geoscience journal demonstrated that seagrass beds can store up to 83,000 tonnes of carbon per square kilometre in their soils. By comparison, terrestrial forests can store around 30,000 tonnes of carbon per square kilometre. Seagrasses can store carbon 35 times faster than rainforests.

What is known as “blue carbon” is carbon dioxide stored in the world’s oceans. A large amount of carbon dioxide is dissolved directly into seawater. Carbon is also stored in coastal ecosystems, in the soils and sediments of mangroves, salt marshes, and seagrass meadows, and in ocean life such as phytoplankton and whales. 

Apart from carbon capture and storage, seagrass has many ecological benefits. It is food for dugongs, turtles and some species of birds. Seagrass provides vital habitat for small and juvenile fish, seahorses and invertebrates such as shrimp and crabs. The leaves of seagrass improve water quality by filtering nutrients and pollutants, while the roots and rhizomes create strong networks within the sediment, reducing coastal erosion. Seagrass stabilises the sediment floor, creating a foundation for mangroves to grow, and it protects coral reefs from wave action and erosion. 

So, what is seagrass? Unlike kelp, which is a type of macroalgae or seaweed, seagrass is an angiosperm (flowering plant). Seagrass evolved from freshwater ancestors to adapt to a fully submerged ocean environment with high salinity and low light. Their closest relative is the water lily, which belongs to the monocot plant family. 

Like all angiosperms, seagrass has roots and a vascular system, which allows the plant to draw nutrients from seafloor sediment and transport nutrients and fluids from one part of the plant to another. 

Seagrass, like most plants, uses sunlight to perform photosynthesis, converting carbon dioxide and water into food (sugars), a process that produces energy for the plant’s growth and reproduction. Oxygen produced as a by-product of photosynthesis is then released into the seawater. If the water becomes turbid because of sediment discharge from increased rainfall or flooding, seagrass may not get enough sunlight to photosynthesise and grow.

Threats to seagrass include degraded water quality from land-based runoff and fertilisers, dredging, trawling, anchors or moorings, and warmer ocean temperatures and marine heatwaves.

The ocean absorbs approximately 30% of all carbon dioxide emissions from human activities and an estimated 91% of the heat created by those emissions. This heat absorption has led to increased ocean temperatures and marine heatwaves, which can stress seagrass and affect its ability to photosynthesise and grow.

Seagrass, coral and kelp dieback directly impact the marine animals that depend on them for food and habitat, and disrupt the cycling of nutrients throughout the ecosystem. So much more needs to be done to ensure these vital, life-sustaining seagrasses, kelp forests, coral reefs, and coastal ecosystems survive into the future.

Many scientists and engineers are working to develop carbon capture and storage technologies to remove carbon from the atmosphere. But what can we do on an individual level? We can support political representatives who have a vision for reducing emissions. We can get involved with or support seagrass and coastal ecosystem conservation projects, plant trees in our garden, or join community tree-planting projects. We can work to protect remaining forests and contribute to rebuilding forests and ecosystems. In short, we can plant trees for every living being. 

In nature nothing exists alone.
― Rachel Carson, Silent Spring

“In every outthrust headland, in every curving beach, in every grain of sand there is the story of the earth.”
― Rachel Carson

References

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Bates, K. (2022, August 25). New Research Tracks the History of Manatees Across Earth’s Oceans / Duke Today. Duke Today. https://today.duke.edu/2022/08/new-research-tracks-history-manatees-across-earth%E2%80%99s-oceans

Dahlman, L., & Lindsey, R. (2023, September 6). Climate Change: Ocean Heat Content. Climate.gov. https://www.climate.gov/news-features/understanding-climate/climate-change-ocean-heat-content

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