Natural Ecosystems - Seagrass Beds
|Seagrasses meadows cover the seafloor in many of our bays and inlets and also occur in sheltered patches along the open coast. Seagrasses are true flowering plants (angiosperms), rather than seaweeds (macroalgae). Their ancestors made the move from the land to the sea early in the development of flowering plants. Some present-day seagrasses are ancient, being known from fossils up to 26 million years old. Their distributions have sometimes become fragmented with the movement of the tectonic plates. Many associated animals and plants have evolved to live exclusively in seagrass beds.|
Although there are only seven seagrass species in Victoria, the extensive seagrass beds support a diverse marine community and contribute greatly to biological production in coastal areas. The four major species are eelgrasses (Zostera and Heterozostera), Sea Nymph (Amphibolis sp.), and Fibre-ball Weed (Posidonia sp.). Eelgrasses grow along the entire Victorian coastline and are common in bays, inlets and estuaries. Zostera forms beds on intertidal mudflats, covering very large areas in Western Port, while Heterozostera is more commonly subtidal on both mud and sand. Sea Nymph is found on sandy bottoms in slightly more exposed conditions, typically surrounding rocky reefs. Fibre-ball Weed forms large meadows in Corner Inlet and Nooramunga.
Seagrass beds typically grow in sheltered waters on silt or sand. The beds bind together unstable sediments and provide substrate, habitat, and food sources for many other organisms. Numerous small seaweeds and lace corals (bryozoans) grow on the stems and leaves, some growing only on particular seagrasses. Abundant smaller invertebrates shelter in the leafy canopy or in the sediment amongst the roots. The most diverse of these groups are marine worms (polychaetes), small crustaceans (amphipods, cumaceans and harpactacoids), snails and bivalves (molluscs). Shrimps, crabs and seastars inhabit the seafloor.
A significant decline in seagrass beds in recent decades has occurred throughout the world. In Victoria the most affected areas are Western Port Bay and Corner Inlet. No single cause has been identified. Some of the decline may be due to natural changes in populations, but other factors appear to be sediment and nutrients from urban and agricultural run off, dredging, and small boat activity.
The large seagrass meadows in Corner Inlet and Nooramunga are within Marine and Coastal Parks. Small open-coast seagrass beds are within reserves at Wilsons Promontory and on the Bunurong coast.