Coast & Estuaries
Page 226
The ACWS Final Report identifies that management actions need to work
towards improved environmental outcomes by reducing inputs to Adelaide's
coastal waters. The study has 14 recommendations; the first five are focussed
on the reduction of inputs (EPA 2011).
The ACWS has a focus on the area of Gulf St. Vincent from Port Gawler in the
north to Sellicks Beach in the south and extends approximately 20 km offshore.
The Port Adelaide River and associated estuary and wetlands are not a primary
focus for the Adelaide Coastal Waters Study, however the input of nutrients and
other contaminants from these sources to the coastal strip was included in
investigations.
Pressure Indicator:
Ecosystem decline of seagrasses (by %)
Seagrass loss
Coastal urbanisation and development within river catchments have resulted in
the reduction in health of near-shore marine systems, particularly seagrass
communities. Seagrass has an important role in terms of primary production,
nutrient recycling, sediment and sand stabilisation, modification of wave energy
and provision of nursery habitat for fish and invertebrates. Increased nutrient
loads in near-shore systems are most frequently identified as the cause for
seagrass decline.
The Adelaide Coastal Waters Study Technical Report No 2 studies the extent of
seagrass loss on the Adelaide metropolitan coast. There are up to 13 species of
seagrass along Adelaide’s coastline and at least a third of these species are
endemic (i.e. they do not live anywhere else). The species along the coast
have affinities ranging from cool temperate to warm temperate, almost all are
perennial and flower in the spring and summer. They grow across various
depths from shallow intertidal to 40 metres, and vary in size from 5cm to 150cm
in length. There are an estimated 5000 km
2
of seagrass beds in South
Australia - 3,700 km
2
in Spencer Gulf and 1,530 km
2
in Gulf St Vincent.
There are seven species found in Zone 2 adjacent to Semaphore and Lefevre
Peninsula.
The loss of seagrass was first observed in 1968 near the Patawalonga outlet
and Glenelg Wastewater Treatment Plant outfall. The documentation of
seagrass loss has been comprehensive since this time, with an estimated 4,000
to 5,000 hectares lost. The most significant seagrass losses have occurred in a
1-2 km wide strip, parallel to the shore (Outer Harbor to Brighton), with smaller
but significant areas located around the former Port Adelaide sewage sludge
outfall point (about 5 km from Semaphore beach - operational from 1978 -1993)
and at the dredge spoil dumping ground off Outer Harbor. The decline of
seagrass has been measured by using aerial photographs.
Peak losses of seagrasses occurred between 1970 and 1977 along the
Northern metropolitan coast with differences in rate of loss between species
