Seagrasses are marine plants with the same basic structure as terrestrial (land) plants. They have tiny flowers and strap-like or oval leaves. They form meadows in estuaries and shallow coastal waters with sandy or muddy bottoms. Most closely related to lilies, they are quite different from seaweeds, which are algae.
Algae also grows in the sea, but seagrasses are different from algae in several ways.
- Seagrasses produce flowers, fruit and seeds - algae produce spores.
- Seagrasses, like terrestrial grasses, have separate roots, leaves and underground stems called rhizomes. These can form an extensive network below the surface. Algae rarely have 'roots' below the surface.
- Unlike algae, seagrasses are vascular plants - they have a network of veins to move nutrients and dissolved gases around the plant.
There are over 500 species of algae but only 14 species of seagrasses on the Great Barrier Reef.
To grow, seagrasses need nutrients, often obtained from nearby mangroves, and good light, which means clear water. They cannot grow easily where they dry out at low tide. They therefore thrive in shallow coastal waters where there is shelter (such as a sand bar) from drying winds and from wave action and strong currents which could create turbulent muddy water.
Although normally found in shallow water they can grow at depths of 32m and have been found in clear water at 68m.
Seagrasses are adapted to their marine habitat. Different species have different salinity tolerance.
While the roots of seagrasses serve to anchor the plants they are not necessary for water intake. They share the task of nutrient collection with the leaves which can absorb food and water directly from the surrounding water.
Flowering generally takes place in winter or early spring. The flowers are very small. Water carries the pollen from the male to the separate female flowers. The resulting fruit are often carried some distance from the parent plant before the seeds are released. Flowering, however, is not common for most tropical species and the spread of seagrasses is largely through vegetative propagation by the growth and branching of rhizomes.
Seagrass beds are believed to rival rice paddies in their photosynthetic productivity (the amount of the sun's energy they convert to plant sugars) and are very important as nurseries and habitat for many commercially important species of fish and prawns.
Tiger prawns settle there at the post-larval stage (3–4 weeks) and remain until they become adults - juveniles are found nowhere else. Many endeavour prawns also spend their youth among the seagrasses. Together these species have an average annual landed value of $1.2 million in the Cairns area alone.
Seagrasses are also home to a number of fish species valued by fishers (as catch or bait) and by aquarists. Other fish species in seagrasses are part of marine food chains which lead to commercially fished species.
But the value of seagrass meadows cannot simply be calculated in dollars. They are an essential part of the marine environment. Not only do the plants stabilise sand and mud banks (keeping water clear) but they form the basis of a complex ecosystem supporting forms of life from dugong to plankton.
Seagrasses are central to a web of life. Only a few animals - dugong, green turtles, sea urchins and some fish - have the ability to digest cellulose and feed directly on the leaves themselves. However, their usefulness does not end there. The leaves support an array of attached seaweeds and tiny filter-feeding animals like bryozoans, sponges, and hydroids as well as the eggs of ascidians (sea squirts) and molluscs. These provide food for small fish which feed the larger fish.
While living seagrasses might not be a popular item on the menu, dead seagrasses are a sought-after delicacy, forming the basis of lengthy food chains. Detritus from bacterial decomposition of dead seagrass plants provides food for worms, sea cucumbers, crabs and filter feeders such as anemones and ascidians.
Further decomposition releases nutrients (nitrogen, phosphorus) which, dissolved in water, are re-used by seagrasses and phyto(plant)plankton. Plankton, both plant and animal, is a food source for juvenile prawns and fish, as well as other filter feeders.
In mid-1992, fishers in the Hervey Bay area began to report large numbers of dead dugong in the area. Researchers set out to check on the seagrass beds but found that where in 1988 there had been 1000km of this dugong food, now there was none.
In March 1992, the adjacent Mary River had flooded twice, carrying loads of silt into the sea. Following this event, a cyclone caused a suspension of the sediment. This extended period of clouded water, it is now assumed, killed the seagrasses by cutting out sunlight. Without food the dugongs disappeared. This event probably also led to a chain of disasters for numerous, less obvious, animals.
As this experience showed, seagrasses are vulnerable. With or without human impact, they come and go seasonally but certain human activities put them at greater risk. Urban, industrial and agricultural runoff can have detrimental effects on seagrasses and the communities they support.
Repeated trawling and outboard motors may damage the meadows while destruction of mangroves may disrupt the supply of nutrients.
The removal of sandbanks can expose the plants to sediment-stirring waves and may cause the beds to drain and dry out at low tide.
Now that the value of the marine grasslands has been recognised, care is needed to ensure that coastal development takes this precious resource into account.
Hervey Bay Marine Park protects extensive seagrass beds growing in the shallow waters of eastern and southern Hervey Bay.
(Source: This information has been adapted from Tropical Topics - an interpretative newsletter for the tourism industry. Vol 1 No. 16 November 1993. Department of Environment.)