Australia Marine Conservation Society - KI Branch

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MARINE >> SEAGRASS MEADOWS

Seagrass meadows are critical to the health of the marine environment!

80% of the planet's seagrass meadows have vanished. More than 80% of South Australia's once rich metropolitan seagrass meadows have disappeared during the past 50 years.


Native Vegetation Doesn't Stop At The Beach


Many people tend to forget the whole other world awaiting to be explored under the sea. You don’t have to even get your feet wet to dabble in this world.

The flotsom and jetsom of the beach provides an amazing window into the innerworld of our seas and oceans. The marine waters of South Australia are some of the most biologically diverse waters along Australia’s temperate or cool water coastline.

South Australia’s marine flora and fauna has elements of both the cold temperate elements characteristic of Tasmania, Victoria and southern New South Wales and also, the transitional warm to cool temperate elements characteristic of southern Western Australia. This produces a rich diversity of organisms and underwater communities along the South Australian coast, which in many instances is unparalleled, both in Australia and the rest of the world.


Marine Plants are Native Plants Too

The large sheltered gulf ecosystems of South Australia are of particular ecological significance. Gulf St. Vincent and Spencer Gulf, together with the seagrasses of south-west Western Australia, represent one of the largest temperate seagrass ecosystems in the world.

It is not surprising therefore, that South Australia (together with Western Australia) has one of the most diverse seagrass floras in the world. In particular, in the family of seagrass commonly called "Tapeweed" or Posidonia species, southern Australia has recorded the greatest number of species in the world.


Threats to Seagrasses


Along the metropolitan coast seagrass loss has been linked to increased nutrient levels from sewage. High levels of nutrients (ie. nitrogen and phosphorus) promotes the excessive growth of seaweeds and planktonic algae on the blades of seagrasses smothering them.

Over 4000 hectares of seagrass area has been lost off theAdelaide coast. Metro coastseagrass cover has declined from 80% in 1949 to 28% in 1993. At this continued rate of loss it has been estimated there will be no seagrass left off metro Adelaide by the year 2014.

Other nutrient imputs such as stormwater and agricultural runoff containg fertilisers may also cause impacts to seagrasses

Aquaculture developments may also shade out areas of seagrass as well as adding nurients to surrounding waters


Areas of Concern

  • Holdfast Bay (Brighton to Outer Harbour)
  • Port River at Outer Harbour
  • areas between St Kilda and Port Gawler
  • Western Cove Kangaroo Island
  • Northern Spencer Gulf region (Tickera to Port Pirie), large scale loss of Amphibolis (and Posidonia) approximately 80km, causes of loss not yet determined.

Tropical Relics of our Coastline

Another flowering plant found in South Australian waters is the Grey Mangrove, Avicennia marina var. resinifera found in the upper intertidal area on sandy mud flats.

This is the only species of mangrove found along the temperate southern Australian coast and in South Australia reaches its maximum abundance, covering a total area of 230 km2, due principally to the extent of sheltered habitats within the gulf ecosystems.

These mangroves are remnants from a time when a more tropical climate. As one goes north along the tropical coasts of Northern Australia, Queensland and Western Australia the number of mangrove species approaches forty.

Mangroves are adapted to living in the soft mud, and have extensive roots to hold them up. The aerial roots of grey mangroves also help the tree to breathe by absorbing oxygen.


Temperate tolerance

Extensive areas of salt-tolerant coastal samphire communities, including plants such as Sarcocornia and Halosarcia (relatives of the more familiar saltbush) also occur in South Australia.

These communities form extensive zones in the upper intertidal to supratidal level, adjacent to the mangrove forests.

Salt marsh plants are adapted to living in very salty soils.


Threats to Mangroves


Historically, mangrove and saltmarshes have been cleared for industry, port acess, landfill and other coastal developments. Loss of mangrove ecosystems has also occured adjacent to sewage outfalls.

Here the increased nutrients have caused algal blooms of sea lettuce or Uvla. Ulva blooms retard growth of mangrove seedlings, smother and kill aerial roots of established mangroves. Out of influence of sewage effluent areas, mangroves have been advancing in some areas.

The Era' oil spill in August 1992 at Port Bonython was the largest coastal oil spill in Australia's maritime history. 300,000 litres (296 tonnes) of heavy bunker oil spilled in sensitive mangrove and seagrass communities of upper Spencer Gulf. 23 hectares of mangroves were killed dead or totally defoliated in heavily oiled areas. There is no sign of mangrove recovery.


Areas of Concern


St Kilda and Port Gawler, adjacent to the Bolivar sewage outfall. Dieback began six years after outfall started. 68 hectares dead in vicinity of outfall since 1969.

‘Seaweeds play a part of the utmost importance in the scheme of Nature, and incidentally may be of direct service to man.’


Seaweed

Most marine plants are simple, microscopic plants that float in the sea as plankton. The larger algae or seaweeds are found on rocky coasts, stuck to the hard surfaces by sucker-like holdfasts. Many seaweeds are bright green, because the major light -catching pigment is chlorophyll. Other algaes have dark brown or bright or dark red colours that help catch sunlight in the deeper waters. These colours are used as the basis of classification for seaweeds, greens, browns and reds.

South Australia boasts some of the greatest seaweed diversity in the world, especially for brown seaweeds ( over 220 species) and red seaweeds (over 800). While many of the 1200 recorded species of macroalgae in South Australia extend into the cooler waters of Victoria and Tasmania and the warmer waters of Western Australia, by far the highest concentration of species is found in the transitional waters of South Australia. In addition, the greater majority of these species are found only in southern Australian waters. Over 75% of the red seaweeds recorded in this region are endemic.

Text references:

Dr Karen Edyvane, in press, SOMER,

John, E.W., , Smith, J.H. , et al, 1979 Below High Water


Ecologically Representative Areas

On inspection many habitats and bioregions are under-represented as MPAs in South Australia. In particular, kelp communities, soft-bottom benthos, estuaries, beach habitats and wave-exposed cliffs habitats are under-represented. Biogeographically, no habitats, communities or ecosystems have been reserved as MPAs in one of the two major biogeographical regions in South Australia, ie. the cold temperate Maugean Subprovince (east of Robe) in the south-east of South Australia.

As such, no MPAs have been established to protect the dominant habitats which characterise this major bioregion, ie. the large kelp forests (ie. Macrocystis pyrifera) and the Bull Kelp (Durvillea potatorum) communities.

Further, the marine habitats associated with the limestone cliff formations and dune transgressions characteristic of the west coast of South Australia (ie. from southern Eyre Peninsula, west to the Western Australian border), are also poorly represented as MPAs. As such, Point Labatt Aquatic Reserve (230 hectares) represents the only Aquatic Reserve on the entire west coast of South Australia (Figure 1).

Of the estuaries reserved as MPAs, the Onkaparinga estuary is particularly significant. Located in the Port Noarlunga Reef and the Onkaparinga Estuary Aquatic Reserve the estuary represents the largest estuary between the Glenelg River in southwestern Victoria and Blackwood River in southwestern Western Australia. The halophytic samphire community within the reserve is the only example of this community type south of Adelaide along the eastern shores of Gulf St Vincent (Johnson 1988). The estuary type is uncommon elsewhere in Australia and is also a known spawning area for Black Bream (Acanthopagrus butcheri) and a nursery area for Yellow Eye Mullet (Aldrichetta forsteri).

A number of Aquatic Reserves in South Australia have been established in order to protect mangrove and seagrass communities, (and in particular, their role as fish nursery and feeding areas). These reserves include Whyalla-Cowleds Landing (Spencer Gulf), South Australia's largest Aquatic Reserve; Blanche Harbour-Douglas Bank (upper Spencer Gulf); Yatala Harbor); American River Inlet (Kangaroo Island); Barker Inlet-St.Kilda and St.Kilda-Chapman Creek (Adelaide).

Within these reserves, a number of specific habitat-types are generally represented: supra-tidal samphire flats, intertidal mangrove areas (dominated by the Grey Mangrove, Avicennia marina var. resinifera), intertidal sand and mudflats (often colonised by the intertidal and shallow-water seagrasses, Heterozostera tasmanica and Zosteria muelleri) and subtidal seagrass meadows (generally dominated by Posidonia spp.). In American River Inlet, Halophila ovalis also occurs, whilst in Blanche Harbor-Douglas Bank six seagrass assemblages have been described (Shepherd 1983).

In some reserves (such as American River Inlet and the Barker Inlet - St.Kilda aquatic reserves) extensive, intertidal and shallow subtidal seagrass-dominated areas, also represent significant feeding areas for many resident and migratory aquatic birds (Johnson 1988).


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