Tasmanian marine habitats
Tasmania is situated between 39 and 43 degrees south, and between 143 and 148 degrees east, with a cool temperate maritime climate. The water temperatures vary from a low of around 10 degrees Celsius in the winter to around 24 degrees Celsius in late summer. Tasmania has 334 offshore islands ranging in size from wave-washed rocks in the Southern Ocean to the larger Flinders and King islands in Bass Strait. Mainland Tasmania has some 5000 kilometres of coastline, consisting of rocky headlands, sandy beaches and sea cliffs.
Major habitats
Pelagic and benthic environments
Estuaries
Intertidal zones
Major habitats
The marine environments vary greatly from the north to the south of our amazing island. Northern marine environments generally have higher temperatures, lower nutrient levels and are influenced by strong tidal currents. These conditions result in rich species diversity; however, few species are actually endemic to this region. The mix of species from other places is due to our close vicinity to mainland Australia and the advection (movement by water) of planktonic larvae in currents. The name plankton is derived from the Greek word planktos meaning to wander, and refers to the weak swimming movements of organisms in this category. Plankton can be subdivided into animals (zooplankton) and plants (phytoplankton).
The northern environments are considered to be relatively ‘young’ marine habitats as approximately 10,000 years ago. This area may have still been a land mass attached to (what is now) Victoria.The waters in the south are cooler and more nutrient rich. While there is lower species diversity in these waters, there is a high number of plants and animals found nowhere else in the world (endemic species).
Pelagic and benthic environments
Marine environments can be divided into pelagic (open ocean) and benthic (shallow coastal) ecosystems.
Pelagic or offshore oceanic systems in Tasmania include regions of deep water on the continental slope of the east and west coasts, seamounts, of which there are approximately 70 rising 200-250 metres above the sea floor and the subtropical convergence, which is the place where the tropical waters of the East Australian Current meet the sub-Antartic water from the south. If you want to see the Tasmanian sea floor up close - go for a virtual dive of the Tasmanian sea floor on the Australian Government Environment website it will show you that there is a whole dimension to the sea floor you may not have expected.
Benthic, the shallow coastal ecosystems of Tasmania can be divided into various sediment types and rocky reefs.
Images and a further explanation of each of these habitats are clearly shown in video links on the SEAMAP website
Sediment substrates
These are considered soft sediments and include gravel, sand, and silt substrates. They may be vegetated or non-vegetated. Silt substrates generally occur in sheltered estuaries and deeper water. Sand and gravel substrates are common in the shallow and exposed coastal regions where wave energy constantly turns the sediment over. Unconsolidated (loose) substrates support numerous bivalves, echinoderms and small crustaceans, as well as numerous fish species.
One important habitat associated with soft sediments is seagrass, which forms meadows in shallow sheltered environments. Several species of seagrass occur in Tasmania.
Rocky reefs
Rocky reefs, which occur along much of the Tasmanian coastline, are one of the most diverse and productive habitats in Tasmanian waters. Rocky reef structure can vary from a simple flat reef to a complex reef with large boulders and crevices. These regions are more highly vegetated by macroalgae (or kelp) than any other reefs in Australian waters.
Shallow exposed waters support a large number of algal species including bull kelp (Durvillaea potatorum), cray weed (Phyllospora comosa) and kelp (Ecklonia radiata) and more sheltered reefs support a mix of red and brown algae. A well known kelp species that forms dense forests is the giant string kelp (Macrocystis pyrifera).
Bull kelp (Durvillaea potorum) in the shallow coastal waters (image: J.Valentine)
The algal communities on Tasmanian reefs are threatened by the range extension of the long spined sea urchin (Centrostephanus rodgersii) down the east coast. Urchins can graze the algae down to bare rock creating large areas which are commonly referred to as algal barrens.
Centrostephanus rodgersii (image: G. Edgar)
Deeper reefs, where the light is limited, tend to be dominated by beautiful and colourful invertebrate life including sponges, bryozoans, gorgonians and sea whips. The colour and diversity of Tasmanian sponges would rival any Queensland reef. Rocky reefs are the basis of many of Tasmania’s commercial fishing enterprises, including the southern rock lobster and abalone fisheries.
Sponge reef environments of Tasmania (photo: TAFI)
Tasmanian estuaries
During a recent TAFI study 111 estuarine regions, where the river meets the sea, were recognised around Tasmania and associated Bass Strait islands. These sheltered environments are particularly important to many species as a habitat, shelter and food source or for providing a safe place for spawning and juvenile development.
Some Tasmanian estuarine environments contain species known nowhere else in the world; they are unique (endemic) to Tasmania; for example, the spotted handfish (Brachionichthys hirsutus), a protected species known only in the Derwent estuary and D'Entrecasteaux Channel. Other endemic species include the bare-backed three fin (Forsterygion gymnotum) and the seastar (Marginaster littoralis).
The salinity of an estuary depends on the mixture of sea water and freshwater and can vary greatly depending on how enclosed the estuary is, the size of the tides, the freshwater influx from surrounding land and the amount of mixing between sea and fresh waters. Estuaries may be stratified (little mixing), partially mixed or well mixed systems. The Derwent River is stratified; saline water has been recorded 30 kilometres upstream, as far as New Norfolk.
Estuarine environments are often defined by their shape or morphology, which is continually changing. Tasmania has all but one of the major types of estuaries which include:
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Drowned river valleys - estuaries with wide river mouths, rocky headlands and deep channels. There are five major estuaries of this type recorded in Tasmania, including Macquarie Harbour, Port Davey and the Derwent, and Huon estuaries. Tamar Estuary, as shown above, on Tasmanias north coast is a drowned river valley (Image: Google Earth).
River estuaries - estuaries where fast-flowing rivers discharge into the sea with little bar or lagoon development and poor water mixing (e.g. the Don and Pieman estuaries). Leven Estuary, as shown above, on Tasmanias north coast is a river estuary. (Image: Google Earth)
Barrier or bar estuaries - estuaries with sandbars across their mouths. These are generally associated with depositional coastlines and relatively high fluvial sediment loads. Examples of permanently open barrier estuaries are Prosser River and Ansons Bay. Wanderer River is an example of a seasonally-open barrier estuary, it is closed for part of the year when outflow seeps through beach sands. Great Swanport estuary, as shown above, on Tasmanias east coast is a bar estuary. (Image: Google Earth)
Saline coastal lakes and lagoons - flat swampy lagoons with irregular river input and infrequent openings to the sea. Incursion by seawater generally occurs only after extreme runoff events, when tides breach the sand barrier, or after artificial breaching (e.g. Big Lagoon and Cameron Inlet). Hibbs Lagoon, as shown above, on Tasmanias west coast has an associated coastal lake (Image: Google Earth).
Coastal inlets - enclosed marine embayment's with wide mouths. There is no large river system but freshwater comes from small creeks after heavy rainfall. West Inlet, on the north coast of Tasmania is a wide mouthed coastal inlet (Image: Gogle Earth)
Nearly half of Tasmania’s population lives in areas that drain directly into estuaries. With 84 per cent of Tasmanians living within estuarine catchment areas great pressure is placed on the ecosystems and the species within them through increased runoff, siltation, nutrient loading and effluent. In addition, indirect threats include foreshore development and dredging, marine farms, modification to water flow through dams and weirs, acidification of rivers and heavy metal pollution from mines, the spread of introduced pest species and long-term climate change. Intertidal zones, where the sea meets the land, are greatly influenced by the ebb and flow of ocean tides.
Intertidal zones
Rocky shores are a place of great interest and an excellent place to see many of the oceans creatures without getting too wet. The species on rocky shores usually can be found in particular habitats or zones defined by the exposure of that area to the sea water. They often appear as bands of different species along the rocks. The tidal areas are divided up according to whether they are exposed to sea water through wave or wind action or at low, mid or high tide. In Tasmania the conditions change considerably depending on where you are. The north coast is quite sheltered with little wave action, the west coast is quite exposed and battered by surf and in the east there are a variety of peninsulas sheltering many areas.
Beaches are also considered an intertidal zone.
Source information and acknowledgments:
A big thank you to Miles Lawler, Graham Edgar and Jeff Ross for their contributions to the Redmap Tasmanian Habitats Page.
Crawford, C.M, Edgar, G.J and Cresswell, G. (2000). The Tasmanian Region. Chapter 95. In Seas at the millennium: An Environmental Evaluation Volume 2. C.Sheppard (Ed.). Elisevier Science Ltd.
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 Report Series, 2, 210pp.
Edgar, G.J., Moverley, J., Peters, D. & Reed, C., 1994. Regional classification of Tasmanian coastal waters (and preliminary identification of representative marine protected areas). Ocean Rescue 2000 marine protected area program 1993/1994 project D705. Parks & Wildlife Service, Department of
