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| Estuaries Overview of Tasmania's Estuaries
General Characteristics and Classification
The Importance of Estuaries
Conservation Significance
Threats to Tasmanian Estuaries
Water Quality in Tasmanian Estuaries
References
Links
An estuary is “a semi-enclosed or periodically closed coastal body of water in which the aquatic environment is affected by both freshwater and marine systems”. Tasmania has many types of estuaries including coastal inlets, drowned river valleys, barrier estuaries, river estuaries and coastal lagoons.
A study of Tasmanian estuaries recognised approximately 111 medium to large estuaries on the Tasmanian mainland and Bass Strait islands (Edgar et al. 1999). In addition, the convoluted coastline results in a number of marine embayments, many of which have estuaries associated with them. Each estuary possesses a unique environment due to its shape, size, depth, degree of tidal variation and catchment characteristics such as rainfall and runoff and vegetation cover. The water within an estuary can be stratified, with fresh water flowing over the top of the heavier marine water. Human activities such as agriculture, forestry and urban development can all affect water quality within the estuary.
Based on geomorphology, 5 general types of estuaries are recognised in Tasmania.
1) Coastal Inlets (e.g. West Inlet, East Inlet)
Enclosed marine embayments with wide mouths that lack large riverine inputs but have detectable reduction in salinity from small creeks after heavy rainfall. Generally well mixed and can be hypersaline in summer.
2) Drowned River Valleys (e.g. Tamar Estuary, Derwent Estuary, Huon Estuary)
Estuaries with wide river mouths, rocky headlands and deep channels and can be stratified.
3) Barrier (or Bar) Estuaries
Estuaries with sandbars across their mouths. Can be permanently-open (e.g. Prosser River, Ansons Bay) or seasonally-closed (e.g. Wanderer Estuary, Browns River) and can be stratified.
4) River Estuaries (e.g. Don River, Pieman River)
Estuaries where fast flowing rivers discharge into the sea with little bar or lagoon development and poor water mixing.
5) Coastal Lagoons (e.g. Grants Lagoon, Cameron Inlet)
Saline lagoons with irregular input and infrequent openings to the sea. Incursion by seawater generally occurs only after extreme runoff events or tidal or artificial breaching of the sand barrier. Can be hypersaline in summer.
Catchments in the west, north-west and south are characterised by high rainfall and runoff, resulting a predominance of river estuaries, whereas catchments in the east and north-east are relatively dry and barrier estuaries and coastal lagoons dominate. Estuaries in the north possess much greater tidal ranges (approximately 3 m) than those on the east, south and west coasts (1 m).
Estuaries are extremely important both ecologically and particularly for human settlements. Because estuaries occur at the 'bottom' of the catchment, they have been identified as appropriate monitoring locations to detect human induced impacts within catchments and from land based activities. They are an irreplaceable natural resource that requires careful management on a catchment wide scale.The sheltered tidal waters of estuaries support unique communities of plants and animals which are specially adapted for life at the margin of the sea and estuarine environments are among the most productive on earth. A wide range of habitat types is found in and around estuaries. In temperate regions such as Tasmania, these include beaches and dunes, rocky foreshores, marshes and other wetlands, mud and sand flats, seagrass meadows, kelp forests and rocky reefs.
Estuaries are essential for the survival of many birds, fish and mammals. Estuaries have been referred to as the "nurseries of the sea", as they provide many species of fish with sheltered waters for spawning and safe habitat for juveniles to develop. Many commercially valuable fish species, depend on estuaries during some point in their life cycles. Some migratory wader birds rely on estuaries as resting and feeding grounds during their long journeys.
People derive recreational, economic and cultural benefits from estuaries. Most of Tasmania’s major cities and towns are situated on estuaries, from which they receive many direct economic benefits. Tourism, fisheries, aquaculture and other commercial activities thrive on the natural resources supplied by estuaries. The protected coastal waters of estuaries also support important public infrastructure, providing ports and harbours vital for shipping, transportation and industry.
Boating, fishing, swimming and bird watching are just a few of the many recreational pursuits people enjoy in and around estuaries. Estuaries are often cultural and historical centres for coastal communities, serving as focal points for celebrations, customs and heritage. In Tasmania, many of the remaining aboriginal sites are situated along estuarine shorelines. In addition, estuaries provide considerable aesthetic enjoyment for the people who live around them.
A study of Tasmanian estuaries classified estuaries and associated catchments into different conservation significance classes (Edgar et.al. 1999). Estuaries with the least human disturbance and greatest proportion of catchment areas under statutory protection were considered to possess the highest conservation status. Estuaries were also assigned a high level of conservation significance if they possessed exceptional species diversity or included species with restricted distributions.
Five conservation classes were identified:
Class | Conservation Significance | Number Identified | A | Critical | 10 | B | High | 38 | C | Moderate | 34 | D | Low (moderately degraded) | 21 | E | Low (severely degraded) | 8 |
Refer to the Conservation Significance Classifications for a breakdown of Tasmanian estuaries.
Human activity in estuaries and their catchments has resulted in the deterioration of water quality, increased siltation and subsequent habitat loss. Humans commonly use estuaries as a means of disposing of urban waste (i.e. sewage and stormwater) and industrial waste. Upstream, the deforestation of catchments for agriculture and forestry results in increases in run-off and in peak flow rates, thereby increasing sediment and nutrient loads to rivers and thus estuaries (Saenger 1995; Edgar et al. 1999). Estuarine degradation caused by increased sediment loads, eutrophication and changed flows was identified in the Commonwealth State of the Marine Environment Report (SOMER) as one of the most serious marine environmental issues facing Australia (Saenger 1995).
The study of Edgar et al. (1999) identified nine potential threats to the biological resources and conservation value of Tasmanian estuaries. These were:
i. |
Increased siltation resulting from land clearance and urban and rural runoff |
ii. |
Increased nutrient loads resulting from sewage and agricultural use of fertilisers |
iii. |
Urban effluent |
iv. |
Foreshore development and dredging |
v. |
Marine farms |
vi. |
Modification to water flow through dams and weirs |
vii. |
Acidification of rivers and heavy metal pollution from mines |
viii. |
The spread of introduced pest species |
ix. |
Long term climate change |
Furthering the work of Edgar et. al. (1999), a more detailed analysis of the physical and chemical nature of 22 selected Tasmanian estuaries was conducted by Murphy et. al. (2003). This study described baseline levels and patterns of variation in commonly measured water quality parameters and provided a description of the relative state of 'health' of each estuary. A summary of the results of this study can be accessed on the State of Environment website: www.rpdc.tas.gov.au
Many estuaries on the north coast were considered as unhealthy, relative to other estuaries in the State, with elevated turbidity, nitrogen and phosphorous concentrations, particularly the Duck Bay and Don River estuaries. Estuaries in the north-east, such as Boobyalla Inlet, Little Musselroe River and Ansons Bay had high nitrogen or chlorophyll concentrations and may be susceptible to eutrophication. In comparison, estuaries in the other regions studied were generally healthy, with indicator levels in the low to medium range; the exceptions being Browns and Meredith Rivers and, on occasion, the Douglas River.
For information on other water monitoring and conservation programs being undertaken in Tasmania, see Pesticide Monitoring in Water Catchments and the Conservation of Freshwater Ecosystem Values Program.
Edgar, G.J., Barrett, N.S. and Graddon, D.J. (1999). A classification of Tasmanian estuaries and assessment of their conservation significance using ecological and physical attributes, population and land use. Tasmanian Aquaculture and Fisheries Institute Technical Series Report 2,. 205 pp.
Murphy, R.J., Crawford, C.M., and Barmuta, L. (2003) Estuarine Health in Tasmania, status and indicators: water quality. Tasmanian Aquaculture and Fisheries Institute Technical Report Series 16. 114pp.
Saenger, P. (1995). The status of Australian estuaries and enclosed marine waters. In Zann, L. and Kailola, P. (eds). State of the Marine Environment Report for Australia. Technical annex 1: The Marine Environment. p 53-60.Coastal CRC: www.ozcoasts.org.au/search_data/crc_rpts.jsp
Australian Natural Resources Atlas: www.anra.gov.au
OzCoasts and OzEstuaries: www.ozcoasts.org.au
Derwent Estuary Program: www.derwentestuary.org.au
CSIRO Huon Estuary Project: www.cmar.csiro.au/research/huonestuary
IMAS: www.imas.utas.edu.au/
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