Seagrasses
Seagrasses
are flowering plants that have evolved to live in
sea water
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 below). 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.
 |
The drawing on
the left is of a 'composite' seagrass
representing different types. |
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 (Dugong
dugon) and the green turtle (Chelonia
mydas) 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.
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Last updated -
December 18, 2008
Copyright ©1996-2005 Australian Institute of
Marine Science
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