They are called ‘seagrass’ because most have ribbon-like, grassy leaves, but none is a true grass. There are many different kinds of seagrasses and some do not look like grass at all. For example, they may have oval leaves( see drawing to the right). Seagrasses have roots, stems and leaves. They also form tiny flowers, fruits and seeds. Most seagrasses reproduce by pollination – the pollen is transported to other plants by water.
The roots and horizontal stems (rhizomes), often buried in sand or mud, anchor the grasses and absorb nutrients. Leaves, usually green, are produced on vertical branches and also absorb nutrients. The stems and leaves of seagrasses contain veins and air channels so they can carry fluid and absorb gases. Seagrasses rely on light to convert carbon dioxide into oxygen (photosynthesis). The oxygen is then available for use by other living organisms.
Where are seagrasses found?
Worldwide, there are about 12 major divisions, consisting of approximately 57 species of seagrass. They are mainly found in bays, estuaries and coastal waters from the mid-intertidal (shallow) region down to depths of 50 or 60 metres. Most species are found in shallow inshore areas.
Seagrasses inhabit all types of ground (substrates), from mud to rock. The most extensive seagrass beds occur on soft substrates like sand and mud. Seagrasses cover areas in coastal waters from tropical (hot) to temperate (cool) regions. The number of species is greater in the tropics than in the temperate zones. Only two species, Halophila ovalis and Syringodium isoetifolium , occur in both regions.
Over 30 species can be found within Australian waters. The most diverse seagrass communities are in the waters of north-eastern Queensland and are an important part of the flora in the Great Barrier Reef region.
How are seagrasses important to the marine ecosystem?
Seagrass communities are one of the most productive and dynamic ecosystems. They provide habitats and nursery grounds for many marine animals, and act as substrate stabilisers.
In northern Australia, seagrass meadows are important as they provide sheltered refuges and feeding areas for prawns and juvenile fish. In some coastal areas, entire fisheries may depend on the productivity of these seagrass beds.
The rhizomes and roots of the grasses bind sediments on the bottom, where nutrients are recycled by microorganisms back into the marine ecosystem. The leaves of the grasses slow water flow, allowing suspended material to settle on the bottom. This increases the amount of light reaching the seagrass bed and creates a calm habitat for many species.
Seagrass meadows are a major food source for a number of grazing animals in the Great Barrier Reef region. The dugong ( Dugongdugon ) and the green turtle ( Cheloniamydas ) mainly feed on seagrass. An adult green turtle eats about two kilograms of seagrass a day while an adult dugong eats about 28 kilograms a day.
What threatens seagrass?
A number of problems face the long-term survival and health of seagrass populations in our coastal zone.
Human pollution has contributed most to seagrass declines around the world. The greatest pollution threat to seagrass populations is from high levels of plant nutrients. High nutrient levels, often due to agricultural and urban run off, cause algae blooms that shade the seagrass. Reduction in light decreases seagrass growth and can kill whole populations.
Suspended sediments also reduce light. This sediment can come from land development run off and through drains. Boating activity may also stir up sediment, reducing light levels.
Other threats to seagrass include damage to the leaves, stems and roots by boat propellers, trawlers’ nets, and dredging.
Loss of seagrass habitats will mean losses in marine ecosystem productivity as well as extinction of species that depend on seagrass for survival.