What are seagrasses
Seagrasses are a group of flowering plants that can live underwater. Like most land based plants, they produce flowers, fruit and seeds. They are not however related to terrestial grasses and in fact, are more closely related to lillies. Seagrasses should not be confused with seaweed (an algae), a more primitive plant than the seagrasses.
According to Seagrass-Watch, seagrass meadows are rated the third most vauable ecosystem globally after estuaries and wetlands (all three of these ecosystems co-exist in the Great Sandy Straits Marine Park and Ramsar listed wetlands).
There are over 30 species of seagrass in Australian waters out of around 60 species wordwide.
Seagrasses, their ecosystem & biology
Seagrasses prefer shallow marine coastlines and estuaries with sandy or muddy beds that are sheltered from wave action. They commonly inhabit intertidal zones in areas where they will not be exposed to so much sun that they would dry out. Some species can live at depths of up to 60m - the limiting factor will be the amount of light required for photosynthesis to occur. Conditions favourable to seagrasses include optimum salinity and temperature, and low turbidity.
Seagrasses do not have the stomata of land based leafy plants - instead they have thin cuticle for gas and nutrient exchange from the surrounding water and a vein system to carry those nutrients and water throughout the plant. Their root system serves to anchor them to the sea floor as well as to absorb some nutrients. The root system branches out horizontally into segments with leaves or stems growing from the joins. It is in this way that seagrass meadows can spread.
Seagrass leaves can be ribbon shaped, fern like, spaghetti like or oval and range in size from a centimetre up to 7m in some species.
Why are seagrasses important
Seagrasses are important in terms of wildlife protection and sustenance but are also commercially significant to the fishing industry for the role they play as nurseries for many varieties of fish and crustaceans.
- One of the more critical functions of seagrasses is as a breeding ground for fish and crustaceans. Juvenile fish and crustaceans also spend some time in the seagrass meadows before venturing into open waters.
- Seagrasses are food sources for grazing animals such as the endangered dugong and turtle.
- Seagrasses attract predators such as sharks, large fish, and birds to feed on the smaller marine animals that make seagrass meadows their habitat or which use the habitat as a refuge.
- Stabilisation of the seabed - sediments will settle as they pass through the seagrass meadow helping to prevent erosion.
- Decaying seagrasses are a food source for other important micro-organisms such as plankton, a food source itself for animals higher up the food chain.
- Nutrient sinks - seagrasses can absorb some coastal nutrient runoff that might otherwise result in algal blooms.
- Absorption of carbon dioxide and production of oxygen from photosynthesis assists with greenhouse gas reduction
How do seagrasses reproduce
Seagrasses produce flowers, fruits and seeds. They pollinate between male and female plants but this process occurs entirely in, on or above the water.
Threats to seagrasses
Seagrasses are threatened by a number of factors including:
- Coastal developments including marinas, ports and urban development which can all be damaging to seagrasses through removal of mangroves which stabilise sediments, changes in water movement created by marine structures, dredging activities and pollution
- Pollution from urban (e.g. sewage, rubbish and sediment) and agricultural runoff
- Floods and cyclones are natural events that can increase turbidity in the water (suspended sediments) that reduce light available for photosynthesis resulting in seagrass dieback
- Dredging can equally cause turbidity, dump sediments onto seagrass meadows, and also physically remove the seagrass
- Boating and associated activities can damage seagrass meadows with anchor and chain drag as well as propeller damage
The Great Sandy Strait and seagrasses
Over 5000 hectares of the 90,000 hectare Great Sandy Straits Marine Park contains seagrasses.
Based on an aerial survey undertaken in 2002 it was found that “approximately 90% of the seagrass meadows in the Great Sandy Strait were Zostera capricorni dominated communities. The remainder were a mixture of Halodule and Halophila dominated.” All these seagrasses are major food sources for the dugong and turtles in the Great Sandy Straits Marine Park
Seagrass-Watch
SSSMeg and seagrasses
SSSMeg volunteers will shortly undertake a training program to identify and monitor seagrasses in the southern section of the Great Sandy Straits Marine Park for Seagrass-Watch.
Stay tuned for future reports on the status of seagrass meadows in the Southern Sandy Straits from SSMeg
Sources and more information
The above information is summarised from the following sources - for further information visit the websites below:
|
Seagrass 
