Abstract
Other chapters underline the importance of seagrasses to our nearshore ecosystems, for example in supporting fish populations and modifying sediment movement wave energy, and it is clear that seagrasses are important components of major Australian ecosystems. While in the natural environment a variety of factors affect the dynamic processed involved in the establishment, maintenance and erosion of seagrass meadows in an unstable, often high-energy environment, one can only view with concern the more or less irreversible loss of meadows, often over extensive areas, which have taken place as a consequence of man's activities. It is this 'cultural' decline in seagrass meadows which is the subject of this chapter, which addresses case studies drawn from several States, reviews mechanisms which may be responsible for seagrass decline, and addresses management considerations.
The most extensively documented example of seagrass decline on the western coast has occurred in Cockburn Sound, a marine embayment 30kmsouth of the capital city, Perth (Figure 12.1). A decision was taken to locate a major industrial development around the Sound, which is the only extensive, sheltered, deep-water area close to Perth on an otherwise inhospitable and high-energy coastline. A basin almost 20m deep, the Sound is protected on the east by Garden Island, and on the north and south by shallow banks covered by 3-5 m of water; these features restrict the exchange of water between the basin and the open ocean. A dredged channel through the northern bank allows access by deep-draught vessels.
Development began in 1954. On the mainland several major industries were established, including an oil refinery, blast furnace and steel-rolling mill, superphosphate factory, and processing plants for alumina and nickel. Garden Island became the site for a major naval facility. Industrial effluents began to enter the Sound, and to the north a sewage treatment plant began discharging in the mid 1960's.
Ten species of seagrasses were associated with the Sound. Posidonia sinuosa (formerly included in Posidonia australis) formed extensive meadows, with Posidonia australis, Amphibolis antarctica and Amphibolis griffithii at the edges of P. sinuosa meadows or in more turbulent, disturbed areas. Posidonia angustifolia is included tentatively; though no longer found there it was probably present around limestone rocks as it is in adjoining waters.