Seagrass is a group of flowering plants occurring underwater. Since it has roots, like land plants, it is able to grow in sandy sediments. Seagrass must not be confused with seaweed, which is a type of algae, not a true plant, and requires hard surfaces where it attaches to grow. Seagrass reproduces both by producing seeds that sprout into new plants or by cloning. One individual can grow several clones connected by an underground stem called rhizome. Clones can span more than a hundred kilometres and live for thousands of years (1).

Seagrass meadows can be found in shallow areas near the coast, in association with the coral reefs or deeper in the water column (2) up to depths of 60 meters (3). The maximum depth at which seagrass is found is possibly determined by the availability of sunlight to photosynthesise (4). Globally, there are 72 species of seagrass (4) spanning from tropical to subarctic waters (5,6). The Pacific turtlegrass (Thalassia hemprichii) is one of the most widespread seagrass species in Southeast Asia (8). It is found from the eastern coast of Africa, across the Indian Ocean up to the western Pacific Ocean (9,10).


 An Essential Marine Ecosystem

Photo: Ben Jones / Ocean Image Bank

Seagrass provides multiple ecosystem services, from coastal protection to provision of seafood and climate change mitigation. It serves as coastal protection by reducing the energy of waves and currents and stabilizing the sediments in the ground preventing erosion (11,12). In contrast to human engineering, healthy seagrass beds are self-sustained, providing long-term solutions to coastal protection (13). Furthermore, seagrass captures particles in the water, improving the water quality and promoting the capture of carbon in the seafloor (12). It also captures carbon by making photosynthesis in its leaves, producing biomass that is stored underground in its roots and rhizome (14). Although seagrass meadows cover only 0.2 % of the world’s oceans, they bury carbon in the seafloor at a rate of 27.4 Gigatonnes of carbon per year (GtC yr-1), accounting for about 10 % of the global ocean carbon burial (14,15). Therefore, seagrass restoration is a potential measure for carbon sequestration (16) and climate change mitigation.

Seagrass beds provide food as well as temporary and permanent habitat for different species of marine invertebrates and commercially important species of fish. As temporary habitat, seagrass beds are used as spawning and nursery grounds where juveniles spend part of their life before moving to a different habitat (17). In addition, seagrass meadows are inhabited by large marine vertebrates, including seabirds, dolphins and sharks, as well as herbivores that feed on seagrass such as sea turtles, manatees and dugongs. Finally, seagrass beds support adjacent ecosystems such as coral reefs, mangrove forests and even the deep sea by transferring food and nutrients, for instance, through the drift of seagrass leaves and the movement of animals that come to seagrass beds to feed (20).



 Habitat and Food Source for Marine Turtles

Photo: Jayne Jenkins / Ocean Image Bank

Photo: Jayne Jenkins / Ocean Image Bank

All species of marine turtles swim to the open ocean after hatching, where chances of encountering a predator are lower, and most return to habitats near the coast, such as coral reefs, estuaries, seagrass meadows, rocky reefs and kelp forests as they become older (21,22,23,24,25,26,27).

Green turtles are a common species of marine turtle found in seagrass meadows, distributed globally in the tropics and subtropics. They shift from their life in the open ocean to seagrass beds when they reach a size of 30 to 40 cm (22). From this stage, they are mostly herbivorous feeding on seagrass, seaweed and sometimes invertebrates (28,29,30). Immature turtles continue to grow in shallow ecosystems until they reach adulthood. Mature females migrate between nesting beaches and their feeding grounds (30).

Green turtles play an important role in maintaining the health of seagrass ecosystems(31). By eating seagrass, green turtles remove organic matter preventing eutrophication and enhancing water circulation, illumination, and oxygenation(31). On the other hand, too much grazing by sea turtles, especially when the turtles are concentrated in small areas or when the seagrass meadows are already unhealthy, might also damage the seagrass meadows and the turtles themselves(32,33,34). It has been observed that the presence of tiger sharks prevents green turtles and dugongs from consuming excessive amounts of seagrass in certain areas of the meadows, improving the quality of the plants(35).

Seagrass beds are also important for other species of marine turtles. Olive ridley and flatback turtles may feed on organisms that are associated with seagrass beds. In the case of olive ridley, these include crustaceans, molluscs, fish and algae; while flatback prefers softer organisms like soft corals, jellyfish and sea cucumbers and pens(37,38). Furthermore seagrass is also classified as habitat for loggerhead and hawksbill turtles by the IUCN(26,21).

The critically endangered hawksbill turtles are a tropical species whose adults and juveniles from sizes of 25 cm or less(22) live in coral reefs(39) feeding mostly on sponges(40) and, to some extent, other soft organisms such as soft corals and seaweed(28,26). However, healthy groups of hawksbills have also been found in seagrass beds(41,33) and these ecosystems might become an important habitat for hawksbills in the future as coral reefs continue to decline(41).

Loggerhead turtles have a global distribution, from tropical to temperate waters(28,21) and have been found nesting around seagrass beds(42). They usually leave their fully oceanic life of hatchlings to frequent coastal ecosystem when they are about 50 cm in size or bigger and between 7 and 10 years old(22). Large populations of loggerhead and green turtles inhabitant Shark Bay in Western Australia(43), a relative pristine ecosystem where some of the largest seagrass beds in the world have been reported(44). Loggerhead turtles have a varied omnivorous diet(45) with a preference for slow or immobile invertebrates such as mussels(42,28,46) which can be found in the seabed. They dig in the sediments when looking for food, losing and mixing the soil(46). Among other benefits for the ecosystem, this redistributes the nutrients in the soil which are necessary for primary producers, such as seagrass and seaweed to grow.



 Conservation and Threats

Seagrass meadows are disappearing at a rate of 110 Km2 yr-1, which is roughly 7 % of the global seagrass coverage every year(47). The removal of seagrass has been estimated to release up to 0.3 GtC yr-1(14). Due to its proximity to the coast, seagrass meadows are susceptible to a wide range of human activities like destructive fishing practices, coastal development, and damage by the propellers of boats. Seagrass is also sensitive to low water quality which can be induced by eutrophication, pollution, and turbidity. Global threats to seagrass ecosystems include overfishing, aquaculture, pandemics, and climate change(47). Seagrass die-off compromises the survival of marine turtles, posing a conservation concern for these endangered species. For example, a series of die-off events in the Coco Islands, an atoll in the Indian Ocean, since 2006 have remove 80% of the original seagrass coverage(33). Several incidents may be responsible for the loss of seagrass in the Atoll, including high temperatures, coastal developments, dredging and cloudier water with less available light for seagrass to photosynthesise(33). The atoll is inhabited by hawksbill and green turtles which depend on the seagrass of this remote atoll for feeding. Therefore, seagrass loss is expected to have profound impacts on their populations(33,48). The international community has started to recognize the importance of seagrass for the environment and the urgent need to protect and restore them. "UNEP" provides global leadership on seagrass ecosystems, promoting international cooperation and science-based and ecosystem-based management approaches, supporting regional and global assessments […] developing best practice manuals, and working with partners to implement conservation and restoration projects on the ground”(49). The CMS Memorandum of Understanding on the Conservation and Management of Dugongs (Dugong dugon) and their Habitats throughout their Range (Dugongs MOU) has established an online platform compiling information on seagrass research and conservation. The Dugongs MOU also has partners working in collaboration with coastal communities to empower them for the protection of their seagrass ecosystems. Furthermore, the CMS COP14 Resolution on Seagrass Ecosystems recommends the Parties to recognize and raise awareness on the importance of seagrass ecosystems for marine species and as carbon sinks, and to support their conservation.








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