Australia State of the Environment Report 2001 (Theme Report)
Australian State of the Environment Committee, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06751 5
The effects of pollution on aquatic ecosystems are dependent on a range of factors, including the nature of the receiving ecosystem, the mixing and dispersion of pollutants, whether floods have transported materials, and on the bioaccumulation of contaminants in sediments, flora and fauna.
In general, eutrophication of coastal waters occurs where these waters receive significant industrial discharges or urban and agricultural runoff. For example, a 1991 Parliamentary Select Committee report (1991) noted that, of the 22 estuaries in Western Australia's south-west, only seven had low nutrient levels, and these had forested catchments.
The majority of the eutrophic estuaries are fed by agricultural catchments. The situation for Western Australia's south-west estuaries has not improved, and except for the Peel-Harvey coastal catchment, phosphorus levels have not decreased.
The effects of eutrophication are evident Australia-wide; for example:
- in South Australia, at least 250 hectares of mangroves adjacent to the Bolivar sewage outfall have been lost to dieback and smothered by large drifts of sea cabbage. The proliferation of sea cabbage, promoted by nutrient discharges, is also killing seagrasses off the Adelaide metropolitan coastline, where total seagrass loss since 1935 is estimated at 5000 hectares.
- in Western Australia, toxic algal blooms occur annually in the upper reaches of the Canning and Swan Estuaries, and extensive areas of seagrass have been lost to epiphytic algae in Cockburn Sound, Geographe Bay, and Princess Royal and Oyster Harbours. In February 2000, the Western Australian Health Department warned against eating mussels from the Peel-Harvey Estuary following an outbreak of a toxic algal bloom. At the same time, sections of the Swan River were closed to the public following the first recorded major bloom of the toxic blue-green alga Microcystis aeruginosa.
- in Victoria, toxic algal blooms have occurred in the Gippsland Lakes, causing extensive fish deaths and affecting fisheries and the local tourism industry.
- in New South Wales, between 1997 and 1999, two algal blooms potentially harmful to marine organisms and seven blooms potentially toxic to humans were recorded. These bloom frequencies were similar to those recorded between 1994 and 1996. Contaminated edible bivalves in the Ballina and Newcastle areas lead to 59 and 23 cases of gastroenteritis, respectively.
- an algal bloom in the Wagonga Inlet near Narooma in October 1999 caused a temporary halt to oyster harvesting in affected leases. There were also 50 algal blooms between 1997 and 1999 - double the number in the previous three years (EPA 2000a).
Increased sediment loads are affecting almost all of Australia's coastal waters. Even in the sparsely populated north-west of Western Australia, sedimentation is severe and is directly linked to pastoral activities and over-grazing. Though not a coastal water, Lake Argyle provides an opportunity to evaluate sediment transport from the pastoral areas. Sediment has accumulated in Lake Argyle (since it was created by the Ord River Dam in 1971) at an average rate of 24 million cubic metres per year, which by 1998 represented a little over 10% of the original volume (Environment Western Australia 1998). Rehabilitation of the areas of the Ord River catchment degraded by pastoral activities has been ineffective.
Turbidity from suspended sediment can have serious physical impacts on aquatic ecosystems, including clogging and damaging the gills of fish, and loss of shellfish and other invertebrates from sediment ingestion. Along with the direct physical impacts on marine organisms, sediment contributes nutrient and toxicant loads to coastal waters in both urban and agricultural catchments, and so is a major concern for coastal water quality.
The 1992 Western Australia State of the Environment Report observes that:
- a single summer flood (date unknown) delivered 100 000 tonnes of soil to Beaufort Inlet and reduced its depth by 25 millimetres,
- the depth of Stokes Inlet has been reduced by 20-25 cm in 30 years,
- sediment almost filled the Irwin Inlet in a flood event (date unknown),
- many other south-western estuaries have become shallower since clearing for agriculture began, and
- sediment loads to south-western estuaries are a major source of nutrients, with those estuaries demonstrating eutrophic conditions.
In its studies of Port Phillip Bay, the CSIRO (Harris et al. 1996) has shown that sediments are the primary factor affecting water quality in the Bay. The study uncovered the basic features of nitrogen cycling in the Bay, where biogeochemical processes in the sediments assimilate and release nutrient loads.
Faecal contamination is a major concern where it affects recreational or aquacultural environmental values.
Monitoring of Sydney's ocean beaches shows that ocean beach water quality has improved very substantially since the commissioning of the deep ocean sewage outfalls in the early 1990s (see http://www.epa.nsw.gov.au/beach/snapshot.htm [accessed 5 September 2001]), and most of Sydney's beaches are now suitable for swimming most of the time. The New South Wales EPA's Beachwatch and Harbourwatch programs operate throughout the Sydney metropolitan area and in the Hunter and Illawarra regions, and reports to the community on the risks of sewage and stormwater pollution at swimming areas (http://www.epa.nsw.gov.au/beach/index.asp [accessed 5 September 2001]).
Faecal contamination by stormwater and sewage overflows following heavy rains is still causing some beach closures in Sydney's harbour and estuarine beaches, the Derwent Estuary in Tasmania, and Port Phillip Bay in Victoria.
Faecal contamination can also affect aquaculture activities. For example, in February 1997 an outbreak of hepatitis A was linked to oysters grown in Wallis Lake in central New South Wales, where the contamination was caused by poorly treated sewage entering the lake.
Toxicant concentrations are known for some coastal lakes and estuarine areas, but their effects on biota are poorly understood. Reports on toxicant monitoring indicate some ' hotspots' of contamination, for example:
- the presence of hydrocarbon ' biomarkers' in fish sampled in Cockburn Sound, Western Australia. Sampling adjacent to the Fremantle Fishing Boat Harbour had the highest levels of these biomarkers (DEPWA 2000).
- in the Port River estuary, Adelaide, monitoring during 1995-96 showed that sediment heavy metal concentrations (especially of copper and lead) at all sites exceeded guideline concentrations, and found high levels at some sites (copper, lead, zinc, mercury and cadmium).
- heavy metal contamination of Macquarie Harbour in Tasmania continues to be of concern, with the continued drainage of acidic mine water containing heavy metals into the Harbour (see The condition of an estuary).
- tests on fish, crustaceans and cephalopods taken from Moreton Bay in Queensland in 1995 found that lead, inorganic arsenic, chromium and nickel concentrations were, in most cases, below the level of detection. No sample contained a concentration of zinc, mercury, selenium or cadmium in excess of the maximum permitted concentration (MPC), but more than half the crustaceans tested for copper exceeded the MPC (EPA 1999a).
Coastal acid sulfate soils (CASS) disturbance has been linked to major fish kills and outbreaks of red spot disease in fish, and to the increased incidence of acid-tolerant, disease-carrying mosquitoes. The products of acid leachate from these soils have also been implicated in the increasingly severe Lyngbya blooms experienced in Deception Bay and Pumicestone Passage, Moreton Bay (although the blooms are also triggered by high phosphorus loads (Watkinson et al. 2000).
In April 2001, acid plumes with a pH as low as 3 have been detected in the Richmond and Macleay Rivers in northern New South Wales (Collins, pers comm.). Although recent acid flows and effects are not as great as in the 1994 floods, frequent low-level acidic discharges will continue to kill benthic organisms and affect estuarine ecosystems. The annual loss of fish catch in New South Wales as a result of disturbed acid sulfate soils is estimated to be $1 million, and a similar figure is estimated for the oyster industry (Department of Natural Resources 1999).