Introduction As water nears the Earth's surface various mechanisms act to temporarily store and transport it from place to place on its journey back to the ocean or atmosphere. In Tasmania, the numerous inland waters and wetlands are part of this cycle, providing a valuable resource to the State. Tasmania's diverse topography, geology, climate, landscape history and vegetation contribute toward the considerable natural variation in the quality, quantity and aquatic ecosystems of our inland waters. Understanding the characteristics and processes of these water resources is essential for their sustainable management. The hydrological cycle Although the total quantity of water on earth is static there is a never-ending circulation of water between the land, the oceans and the atmosphere. This continuous movement of water is known as the hydrological cycle (see figure below). When precipitation falls on land, it either falls directly onto the ground or is intercepted by leaves, branches and trunks of vegetation. This intercepted water may eventually move to the ground or evaporate into the atmosphere. The water that does reach the ground is either absorbed by leaf litter, stored in depressions and cracks, or filters into the upper layer of soil at a rate that depends on the soil's characteristics and the amount of water already in the soil. If there is more water than the soil can absorb, it will run off the surface (overland flow). Overland flow is a very rapid pathway for water movement and is usually confined to times of intense rainfall, to rocky areas, or to swamps and depressions that are already saturated. Overland flow can form drainage lines, which, in turn, may develop into distinct streams and rivers. This is known as channelised flow. On entering the upper soil layers (unsaturated zone) water may move laterally in a process known as interflow. This movement is especially evident where some layers are more permeable than others, such as a sandy soil on a rocky substrate. Interflow may be the main process in producing freshes or floods in streams. Water also moves out of the unsaturated zone through transpiration by trees and other vegetation, or by percolation into even deeper layers. Water that percolates into groundwater reserves may take months, years, or even hundreds of years to pass back to the surface. Nevertheless, groundwater reserves play a major role in sustaining flows in streams between surface recharge events, and are the main source of streamflow during dry periods.
Streams and rivers There is significant variation in the types of rivers and streams across Tasmania as they are influenced by the diverse climate, topography, geology, landscape history and vegetation. For example, streams and rivers may have permanent, seasonal or ephemeral flow, depending on the rainfall and run-off characteristics of the catchment. Where high rainfall is more consistent throughout the year (on the west coast), the streams tend to have a permanent flow. Conversely, streams on the east coast, and particularly in the north-east, often have ephemeral and flashy flows, due to the highly variable rainfall. Topography influences the energy of rivers. Tasmania has large flat areas, at high and low altitude, where the rivers have relatively low energy and often meander (e.g. Meander River, downstream of Deloraine, and the meandering section of Little Pine River on the Central Plateau). In between these flat areas the topography can be very steep, and here there are high energy rivers that can shift boulders during floods (e.g. Dunnings Rivulet). Geology can have dramatic effects on rivers. Where the geology is very hard (e.g. dolerite) rivers can form gorges and waterfalls, while in softer more erodible geology (e.g. sandstone and mudstone) rivers can often develop wider flood plains. Landscape history is the legacy of different climates and processes such as the glaciers that shaped parts of central and western Tasmania, or the periglacial processes that formed the huge screes that dominate the sides of dolerite mountains. Rivers in today's climate often cannot modify these features. Vegetation is very important for all rivers, because it affects the hydrology of a catchment and increases the stability of river banks and floodplains. Removal of vegetation is often followed by erosion. Lakes and wetlands Tasmania has an abundance of lakes, including the continent's largest permanent natural freshwater lake (Great Lake) and its deepest (Lake St Clair). These lakes have been created and shaped by geological, geomorphic, and various other processes. In some districts, most or all of the lakes have a common origin, such as the 'Thousand Lakes' region of the Central Plateau. Glaciation created many of Tasmania's lakes. Glacially scoured bedrock troughs, and basins formed from glacial till, are widespread in the Tasmanian highlands and the Central Plateau. Lake St Clair's depth is due to both of these processes. In the Midlands and the east of the State arid, windy conditions during glacial periods created many wind-scoured hollows, which became inland lakes and wetlands. Some of these include the saline wetlands of the Midlands, north-east, and in the Furneaux Group of islands (DPIWE 2000). Freshwater wetlands include swamp forests, dominated by either blackwood (Acacia melanoxylon) in the north-west, or paperbark (Melaleuca ericifolia) on King Island and other parts of the coast, which are both periodically inundated. Other freshwater wetlands include permanent lakes and deep or shallow freshwater marshes. The internationally accepted Ramsar definition of wetlands is necessarily broad, and also includes rivers. Deep flowing rivers, however, are not usually considered to be wetlands in Tasmania, unlike the marshy areas associated with rivers (DPIWE 2000). Tasmania's alpine and sub-alpine areas contain wetland types that are rare elsewhere, such as sphagnum bogs, string bogs and staircase ponds. Large areas of buttongrass moorland (with water often lying on the surface that periodically dries out) are located in the west and south of the State (DPIWE 2000). Extensive wetland systems have developed along the coast of Tasmania, especially along sandy shores (Kirkpatrick and Glasby 1981 and Kirkpatrick and Tyler 1988). As terrestrial influences are replaced by marine influences, there is a transition zone from inland aquatic systems to coastal systems. Water Quality Water bodies have individual physical and chemical properties, which are largely determined by the vegetation cover, the climatic, geomorphological and geochemical conditions in the catchment, and the underlying aquifer. Field-portable meters can measure the basic physico-chemical characteristics of water samples such as pH, temperature, turbidity and dissolved oxygen content. Chemical characteristics associated with dissolved and particulate matter in the water body usually require more detailed field or laboratory analysis. The surface water quality properties commonly assessed in Tasmania include:
Water Quantity Tasmania is part of the world's second driest continent (the driest being Antarctica). The State has 12% of Australia's fresh water resources, but only 0.9% of the land area and some 3% of the population. There is approximately 44 million Megalitres (a Megalitre, ML, is about 2/3 of an Olympic swimming pool) of average annual surface run-off (DPIWE 2001) and around 16 million ML of underground water (AWRC 1976) in the State. Surface water Given its large share of the Australian continent's fresh water resources, Tasmania's surface water resources- including lakes and wetlands, overland flow, streams, and rivers-may appear plentiful. However, an uneven distribution of rainfall across the State contributes to a higher run-off on the west coast compared to the east coast. The typical mean annual rainfall across the State is depicted in the image below. Temporal variations in rainfall also occur throughout the year, resulting in dry summer periods. This too is unevenly distributed across the State, with the west coast experiencing less rainfall variation than the east coast. The State is also subject to the effects of drought and flood events caused by the El Nino - Southern Oscillation phenomenon.
Groundwater Groundwater refers to all the water below the earth's surface. Tasmania has an extensive groundwater resource, trapped by igneous, sedimentary and metamorphic rocks. Although it occurs widely, Tasmania's total estimated groundwater resource (roughly 16 million ML) is not evenly distributed and its quality and yield can also be variable depending on the geology of the aquifer:
Aquatic Ecosystems The aquatic ecosystem may be defined as those natural systems that are either permanently or periodically under water, and combine physical, chemical, and biological elements within them. Water may be either running, e.g. a river or stream (lotic environments), or still e.g. a lake or wetland (lentic environments). Either environment can be considered in terms of three zones:
Individual biological communities have evolved and adapted to the natural physical habitat, water chemistry and flow regime of their particular location. These communities therefore depend upon the maintenance of natural conditions for survival and reproduction. Ensuring that aquatic ecosystems in our inland waters are cared for is a vital component of water-resource management. Biological processes in water are governed by a range of environmental conditions that determine the variety, composition and function of the various flora and fauna. In contrast with physical and chemical monitoring of individual parameters, the investigation of biological quality may be based on an assessment of the diversity and abundance of the species existing in an area. Streams and rivers Streams and rivers are populated by species, which have adapted to a dynamic flow regime. Flora and fauna must cope with highly varying flows ranging from stagnation, through to running water, and destructive floods. The landscape, climatic and vegetation characteristics and processes that have shaped the numerous stream and river types across the State also influence the associated flora and fauna. The distinct flora that grows in and around inland waters is known as riparian vegetation. Many species are commonly found, or attain dominance, only in riparian systems. The nature and condition of riparian vegetation vary with geographic location. Riparian plant communities usually form a narrow strip along the length of a river. Dominant plant species often change over relatively short distances. Riparian vegetation is vital for the wellbeing of our streams and rivers. Benefits include:
Riparian communities are often the dominant feature of streams, from the headwaters down to the middle reaches. In the middle to lower reaches the pelagic component, such as zooplankton and fish, becomes more significant. The benthic fauna of rivers and streams typically adapts to changing flows by developing strong anchoring mechanisms, or escape behaviour. In Tasmanian streams benthic invertebrates are the primary source of food for many fish species. Lakes and wetlands Lakes and wetlands (see figure below) are generally low-energy depositional environments characterised by still water and soft, muddy bottoms. The physical structure of the water mass in lakes tends to show characteristic patterns, such as a yearly cycle of stratification and mixing. During stratification, the water mass separates into layers of different densities. Stratification is frequently accompanied by the development of biological, physical and chemical zonation. Some highland lakes in Tasmania are dimictic, stratifying in summer and winter and overturning in the spring and autumn. However, most Tasmanian lakes have irregular mixing cycles (polymixic), or are completely mixed all year round (holomixic). There are a few examples of permanently stratified (meromictic) lakes in Tasmania, such as three rare and significant lakes found along the lower Gordon River. Unfortunately, the hydro-chemical dynamics of these unusual lakes were upset by the Gordon Power Scheme and overturning has occurred in two of the lakes (Kirkpatrick and Tyler 1988). In the low-energy lentic environment, sediments and nutrients accumulate. This means that the flora and fauna throughout the aquatic and riparian environments tend to be more diverse and abundant in these systems than in fast flowing environments. For example, the phytoplankton population is usually large. However, the water depth of lakes and wetlands plays a significant role. If a lake has an extensive deep zone where the water may be dark, cold or oxygen deficient, the benthic (and fringing) components can be small. The opposite is usually true in shallow lakes and wetlands. Wetlands are distinctive areas with specialised flora and fauna. They are permanently or periodically inundated by water and can be fresh, saline or brackish. Permanent wetlands hold water all year round (although the level may drop during dry times). Seasonal wetlands hold water regularly at certain times of year, and ephemeral wetlands may dry out for years at a time, but reappear when enough rain has fallen (DPIWE 2000). Subterranean ecosystems Subterranean ecosystems are widely distributed throughout both karst and non-karst terrains. While this discussion is focused mainly on karst, a closely related, and no less significant fauna also dwells in the underground waters of non-karst regions. There is a reasonably good basic knowledge of the aquatic fauna of karst systems. In comparison, knowledge of the related fauna, which dwells in the underground waters of non-karst areas is poorly understood. Subterranean crustaceans have been found inside crayfish burrows (Lake and Coleman 1977) and springs in basalt (Knott and Lake 1980). Aquatic ecosystems in karst environments support a specialised fauna that is often distinct from that of surface waters. Habitats range from flowing underground streams to still lakes at the present water table (DPIWE 2000). Species that live solely in these environments have curious morphologies, including the degeneration or loss of eyes and body pigment, elongated antennae and legs, and enhanced sensory structures. Australia's most diverse temperate cave fauna associations are found in the cave systems of Tasmania (DPIWE 2000). The invertebrate groups that have been recorded in caves include roundworms (Nematoda); flatworms (Turbellaria); nemertine worms (Nemertea); segmented worms (Oligochaeta); water mites (Hydracarina); molluscs (Hydrobiidae); several orders of insects with aquatic larvae (caddis flies, may flies, stone flies and two-winged flies); and a rich assemblage of crustaceans (copepods, ostracods, syncarids, isopods, amphipods and decapods) (Eberhard et al. 1991). The mollusc and crustacean groups include many rare, endemic and cave-limited forms, although much of this material remains undescribed. These subterranean ecosystems are extremely fragile, from both a biological and geomorphological perspective. |
