Estuaries are places where rivers meet the sea. Semi-enclosed bodies of coastal water where fresh and saltwater meet, they can encompass bays, inlets, river mouths, mudflats, wetlands, mangroves, salt marshes and reed beds.
Estuaries are highly productive coastal environments, providing habitat for a wide variety of fish and invertebrates of ecological, commercial and recreational importance (such as crabs, prawns and oysters). They also supply food to many resident and migrant birds.
Estuaries may be permanently or periodically open to the sea, with salinity that varies from almost fresh to very saline. Environmental conditions may be stable over long periods of time or change frequently or rapidly. Estuaries are therefore complex and highly variable environments that often appear to be unpredictable. The coastline of Victoria has some 123 bays, inlets and estuaries, varying in water area from about 2,000 km² to 1 km². Victoria’s estuaries contain a wide variety of sheltered habitats, including intertidal and subtidal reef, channels, seagrass, Ruppia (Sea Tassel), mangroves and saltmarshes. They are dominated by intertidal sandflats and mudflats, and subtidal sediment beds
In marine embayments, environmental conditions are marine and relatively stable. The communities are made up of inshore coastal species. Many estuaries flow into marine embayments. In some estuarine environments environmental conditions can vary on an hourly time-scale with the tides, and seasonally with rainfall and long-term changes associated with unusual rainfall patterns. Unique communities that can survive in these highly variable environments have developed.
Discharges from upstream catchment areas meet the ebb and flood of tidal discharges in estuaries, and the aquatic flora and fauna of freshwater regimes also meet their marine counterparts there. Hence estuary habitats are subject to influences from both marine and riverine environments. These include saltwater and freshwater input, sedimentation, tides and periodic flooding.
The varied habitats of Victoria’s estuaries are associated with diverse and productive communities of aquatic invertebrates that live buried in the sand. Estuary mud and sand flats are important feeding grounds for local and migratory shorebirds, and nurseries for ecologically, recreationally, and commercially important fish such as Australian Salmon (Arripis trutta), King George Whiting (Sillaginodes punctata), and Black Bream (Acanthopagrus butcheri).
Unusual features of south-eastern Australian estuaries
Worldwide, there is generally freshwater input at the head of an estuary. As this water flows through the estuary, it is mixed with seawater carried into the mouth with the incoming tide. The tidal outflow exceeds the inflow so that there is a net movement of water through the estuary.
However, most south-eastern Australian estuaries do not fit this model. Many have sand barriers across their mouths, which cut off or restrict the inflow of seawater for periods of months to several years. Also, under conditions of hot weather and low rainfall, in south-eastern Australia evaporation of water from the estuary may be similar to the freshwater input. Under these conditions, salt concentrations in parts of the estuary are similar to that of seawater, or even slightly higher. Evaporation rates in south-eastern Australian estuaries never greatly exceed the freshwater input – the situation which produces the hypersaline estuaries of north-western Australia.
Special estuarine species – benthic burrowers
Estuaries are home to a variety of animals and plants. The benthic (sea-floor) community is made up of animals living in the sediment. The diversity and abundance of this benthic community indicates the overall health of the estuarine ecosystem.
Benthic animals are those associated with the bottom of seas, rivers, or lakes. A large proportion of the biodiversity of estuarine habitats is found in the benthic community. Many of the worms, shrimps, snails and bivalves that live there are important food sources for fish and birds.
Unlike fish and plankton which can move up and down in the water column, benthic animals live in what is essentially a two-dimensional environment. Concentrations of fish and plankton form and disperse in response to tides and weather. But because of their reduced mobility, benthic communities do not change very much in response to changing conditions.
Benthic species live in an environment where concentrations of pollutants are likely to occur. Many benthic animals can only recolonise an area by larvae settling, so they cannot recolonise until the next breeding season. Any mobile animals will often move back into the area slowly. Short-term pollution events are therefore detectable in the benthic community for a long time.
Chemical changes associated with the change from freshwater to saltwater in estuaries cause dissolved materials transported down rivers into the estuary to collect together. The flow of water in estuaries is slower than in rivers, so this material and other suspended particles can settle out of the water into the sediments. These processes allow pollutants to reach greater concentrations in the sediments than in the water. Because of their close association with the sediment, benthic organisms are affected by pollutants before animals in the water column. The benthic community can therefore be the first organisms to show weakening environmental health in an estuary.
Estuaries are used by the Victorian population for transport, commercial and recreational pursuits. Many of our major estuaries have cities located on or near them. Many Victorians participate in fishing and other water sports based on estuaries. As a result, the health of our estuaries has a high profile in Victoria. However, we currently have no way of assessing whether the overall health of our estuaries is improving, being maintained or deteriorating. Few estuaries are considered as being in a natural or near-natural condition.
Since estuaries are at the bottom end of catchments, they are subject to all the impacts on the catchment and are especially vulnerable to the cumulative effect of human activities. Estuaries are often the most popular and heavily used portions of a river reach, and can be used for boating access, fresh water, fertile land for agriculture, and fish and shellfish catch. Consequently, Victorian estuaries are under pressure from urbanisation, farming, modified stream flows and drought. As many catchments in Victoria are degraded, most estuaries are at risk. There is probably no other natural ecosystem as vulnerable to upstream human activities than an estuary, so integrated management along the entire watercourse is extremely important.