By Rowland Benjamin

Seagrasses is a term used to describe flowering plants that reproduce by pollination and that have evolved to live in seawater. Seagrasses were named because many resemble the appearance of grass, with most having long ribbon-like leaves. But it is not a 'grass' in the true sense and many different types of seagrasses do not look like grass at all. All seagrasses have common features; they consist of roots, stems, leaves, and form miniature flowers, fruits and seeds. Seagrasses grow where there is abundant sunlight and moderate water current. They mainly grow in shallow coastal areas, such as salt marshes, estuaries and mangroves, but may grow at depths of 40 metres below the ocean surface in sand, mud or rock.

Where are they found?

There are 57 species of seagrasses growing in mostly tropical, but also temperate regions. Seagrasses are found in south Florida, mainly in Florida Bay, Florida Keys and Tampa Bay; in the Red Sea, along the southern Sinai coast; along the Arabian Gulf coastline in Saudi Arabia; throughout the region of East Africa; southern India and many estuaries of Sri Lanka; and in South-East Asia coastal waters off south-west Sulawesi. More than half of the species of seagrasses found in the world exists in Australian waters: in the Gulf of Carpentaria; in Shark Bay, Western Australia; in the Spencer Gulf and the Gulf St Vincent in South Australia; and in the waters off north-eastern Queensland. The largest seagrass population lies along the temperate coast of Western Australia, particularly in Cockburn Sound near Perth.

Where and when did seagrasses originate?

It is thought that seagrasses originated on land and adapted to life underwater around 100 million years ago. About 45 million years ago, seagrasses were widely distributed, particularly in the Asia-Pacific region.

Why are seagrasses important?

Without seagrasses our inshore coastal areas would be like deserts. Seagrass communities are one of the most productive breeding grounds because they provide a valuable nursery for commercially important fish and crustaceans, such as prawns and rock lobsters. Overall, seagrasses play a key role in the marine food web as they provide the basis of a habitat for many marine animals that would otherwise not survive there.

Their function includes slowing down the water current, maintaining water clarity by trapping sediments to allow light penetration and providing shade and habitats for small marine species. These marine species include microscopic plants such as seaweeds, diatoms and macroalgae and animals such as sea squirts and epizoites. These provide a food source for other species such as small shrimps and snails. These are in turn eaten by larger creatures, such as fish, rock lobsters and crabs, that inhabit the dense seagrass beds. In tropical waters, dugongs and green turtles feed on the leaves and roots of the seagrasses. Manatee and bottlenose dolphins and a variety of diving and wading birds use areas of seagrasses for feeding.

Seagrasses are continuously subjected to the inflowing and outflowing of the tides, and to avoid being washed away the seagrass has a root system and stems. The roots help in stabilising the seabed against powerful water currents.

What threatens seagrasses?

Seagrasses are sensitive to changes in the environment and human-induced activities. Increases in nutrients from sewage, fertiliser runoff, pollution and industrial waste, increases in sediment from the erosion of top soil from land developments and boating activities, including recreational fishing and net trawling, have contributed to the loss of seagrass communities in areas of high urban density and development.

Loss and degradation

In 1996 it was found that 45 000 hectares of seagrasses had been lost in Australia and 90 000 hectares of seagrasses were lost in the United States. By 1996, scientists stated that the enormous loss of seagrasses in the Mexican gulf and South-East Asia was due to heavy pressure from humans.

In the last 50 years, seagrass beds have declined in areas around Florida due to a rapid increase in urbanisation and industrial growth. Countries in the South Asian region, such as Thailand, India and Sri Lanka, suffer from major loss of seagrasses from over-fishing, explosives fishing, tourism, mining and industrial growth.

Loss of seagrasses means losses in marine ecosystems and species that depend on seagrass survival.

Dredging

Dredging and filing of the seabed removes and destroys seagrass communities that lead to the decline of fish and other marine species. Dredging causes the sediments to stir up from the sea bed and cover the seagrasses, and causes decreasing water clarity which blocks light from reaching them. In Cockburn Sound, off Western Australia, 90% of seagrasses have been lost in the last 40 years due to the dredging of shell-sand. Once destroyed some seagrasses may never grow back.

Pollution

High levels of nutrients from agricultural and urban runoff into the marine environment can cause an unnatural growth of microscopic, single-celled plants known as an algal bloom which can accumulate into dense, visible patches near the surface of the water. During a bloom, the algae smother the seagrasses on which they live and block out the light needed by the plants to grow by producing their own food. Without light the seagrasses will die.

Increased loads of organic and domestic wastes into waters containing seagrasses have been the cause of losses in south Florida. In 1987 a massive area of 40 square kilometres of seagrasses was lost in Florida Bay along the reef tract.

What you can do

Practice safe and responsible fishing and boating activities.

Reduce the amount of fertiliser you apply on your lawns and gardens to reduce pollution in our waterways.

Support ecologically sensitive tourist activities along coastal areas.

Write a letter to the editor of your local newspaper about your concerns for seagrass loss and degradation.

Links:

Bibliography

Search our database for the contact details of organizations that directly address Seagrasses