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
Gulfs and bays are large (greater than 200 km2 surface area), open (or already open) and shallow (less than 50 metres in depth) coastal waters; typically they are dominated by marine conditions except where rivers flow into them.
The sheltered nature of bays lends itself to the deposition of sediments, fine muds or sand. These soft bottoms form an important habitat for benthic species sensitive to extreme wave energy, but also act as a ' sink' for those pollutants which readily attach to solids.
Important studies conducted over the past five or more years that assessed the condition of bays include:
- Jervis Bay, New South Wales, baseline studies (CSIRO 1989),
- Storm Bay, Tasmania (Harris et al. 1991),
- the Port Phillip Bay Study, Victoria (Harris et al. 1996),
- the Southern Metropolitan Coastal Waters Study in Western Australia (DEPWA 1996), and
- the Moreton Bay Study, Queensland (Dennison and Abal 1999).
Only in the case of Storm Bay were natural processes the main determinants of condition, other bays being degraded over at least part of their area. Port Phillip Bay was found to be close to its critical nitrogen loading, which was also the case for parts of Moreton Bay. General degradation was greater in the west of Moreton Bay than in the east. The South Metropolitan Coastal Waters Study found that water quality problems had not been solved, Cockburn Sound in particular suffering tributyl tin (TBT) contamination and declining seagrass meadows. Jervis Bay maintained a rich and diverse fauna, despite the increased nutrients and turbidity that follow prolonged rainfall. Introduced organisms were identified as serious problems in both Port Phillip Bay and Cockburn Sound.
In the far north, fewer comprehensive studies of bays have been done, partly because of the lower pressures of population and pollution. However, Hervey Bay to the north of Brisbane is much larger than Moreton Bay, is more rapidly flushed and has less development, but has nonetheless seen dramatic loss of seagrass (Lee Long et al. 2000).
Moreton Bay is a medium-sized shallow lagoon east of Brisbane, separated from the open ocean by Moreton Island and North and South Stradbroke Islands. High sediment loads can be discharged to the estuary after rain, and there are high nutrient loads from agricultural and wastewater discharges. Seagrasses are found on both eastern and western shores, where they provide food for turtles and Dugong.
The objective of the Moreton Bay Study (Dennison and Abal 1999) was to develop an integrated strategy to improve water quality in Moreton Bay and the adjoining rivers. It was undertaken by several organisations (six local councils, State and Commonwealth governments, universities, and consultants). The overall conclusions of the study were that river estuaries and western portions of the bay showed degradation, but that rich and diverse ecosystems of eastern and northern Moreton Bay were ' essentially intact'.
A Moreton Bay ' report card' (Greenfield 2000) ranked the condition of the seven bays, from A to F. Eastern Bay received an A on the basis it was well flushed, and there were extensive seagrass beds supporting Dugongs and turtles. Southern Deception Bay received a C to D rating because of seagrass loss and high turbidity.
Productivity is limited mainly by nitrogen, except where turbidity reduced light levels in the water. In general seagrass declined as turbidity increased.
Much of the degradation is relatively recent. Nitrate concentrations in the Brisbane River increased twenty-twofold from 1950 to 1996, phosphate concentrations increased elevenfold, while suspended solid concentrations quadrupled. Nitrate and phosphate concentrations in Moreton Bay increased two to eightfold over the same period. Management strategies have focused on reducing nitrogen loadings to the Bay.
Stage 3 of the Study is assessing upper catchment issues as they relate to the quality of the rivers and estuaries. Responses to the conditions of some bays that have been affected by land uses have included:
- the establishment of a multi-agency group to coordinate improvements,
- the upgrade of wastewater treatment plants, and
- the protection of vulnerable habitat through a variety of mechanisms.