Environmental management of shellsand dredging Owen Anchorage, Western Australia

A major programme of environmental management is being undertaken off the coast of Perth in association with a programme of dredging of marine carbonate deposits, where seagrasses proliferate. Detailed technical investigations and scientific studies are providing the basis for government decisions and community consultation. Parmelia and Success Banks are two unconsolidated and relatively uniform marine sedimentary deposits off the coast of Western Australia. They consist predominantly of calcium carbonate and are currently being dredged for use in lime and cement production. These banks have been formed over the last approximately 6,000-7,400 years mainly from sediment transported towards the shore from the adjacent shelf. In situ calcium carbonate production accounts for less than 15% of the volume of the bank. Dredging now takes place in the centre of Success Bank, in zones known as the short- and medium-term areas. Seagrasses proliferate on the surface of the banks, at depths to 10 m or more. A sophisticated numerical model indicates that dredging of the area over a period of approximately ten years will not measurably effect navigation or shipping, or influence sediment transport loads to the shore. The model has also shown that widening the existing shipping channel through Success Bank will produce similar results. Overall, the adjacent coastline of Owen Anchorage is accreting at the average rate of 60,000 m(3) of sand each year. Seagrasses are widely distributed along the south-west coast of Western Australia. None of the seagrasses and their associated flora and fauna found on Success Bank or its immediate surrounds are rare or endangered by the dredging. Historical aerial photography has shown that large-scale changes in seagrass cover have occurred on both Success and Parmelia Banks. On Success Bank, there have been progressive increases in the cover and density on the eastern side of the FPA channel, with some losses on the western side of the channel due to sediment movement. On Parmelia Bank, losses of seagrass have occurred in the south and east. Overall, between 1971 and 1995, and including losses due to dredging, there are now approximately 528 hectares more seagrass on Success Bank, and 36 hectares less seagrass on Parmelia Bank. This increase in cover and density of seagrass on Success Bank can be explained by the initial establishment of seedlings followed by clumping and tillering (extending of rhizomes). The ecological function of seagrass in Owen Anchorage is being measured using a suite of biological attributes of these seagrasses, such as plant biomass, and seagrass use by juvenile fish. Seagrass appears to have little influence on the large-scale patterns of sediment distribution and transport on Success Bank. An underwater machine (ECOSUB I) has been developed for the mechanical transplantation of seagrasses, and is the largest development of this kind in the world. It was first used in late 1996, ECOSUB I extracts sods of approximately 50 cm by 50 cm size, to a depth of 40 cm, ensuring full retention of root and canopy structure during extraction, transport and planting. Approximately 1,000 sods of predominantly the seagrass Posidonia coriacea and some Amphibolis have been transplanted to date on the eastern side of Success Bank. Survival of the sods has surpassed all expectations, and 80% are intact and alive with evidence of new leaf growth. A second generation transplantation programme is being developed for introduction in early 2000. This will facilitate the movement of sods of twice the current size, at a transplantation rate ten times the rate currently being achieved. Future dredging options for marine resources of calcium carbonate in the region will include a consideration of all of the resources in the region, as well as the feasibility of raising calcium carbonate levels by beneficiation of resources.