Freshwater ecosystems

2.3 Threatening processes

Australia is the world's driest inhabited continent (the driest being Antarctica), and rainfall over much of the land is highly variable. In the two centuries since European occupation, fresh water (both surface and ground) has often been a scarce commodity, and (generally speaking) supplies have been extensively harvested and allocated for human use. For example, if all existing water allocations in the Murray-Darling Basin (Australia's largest river basin, covering 14% of its total land mass – over a million square kilometres) were implemented, around 90% of the average natural stream flow would be diverted¹⁰. The lower Murray now experiences drought level flows three years out of every four, compared to one in twenty years under natural circumstances¹¹. The loss of biodiversity in the region and degradation of its rivers is well documented. In particular, the native fish species of the Murray-Darling Basin have suffered serious declines in both distribution and abundance resulting in the threatened status of one-quarter of the thirty-five species present (MDBC 2002).
In spite of gross over-allocation of the water resource, the Murray-Darling Basin Ministerial Council has had difficulty implementing a cap on water usage¹². The cap was set at 1994 extraction levels, and may not be sufficiently restrictive to protect the remaining biodiversity of many of the Basin's rivers and wetlands¹³. In the Queensland and Victorian sections of the basin, harvesting of surface flows with off-stream dams continues to be unregulated by State governments, although these flows should shortly come under State controls as new water legislation is implemented (see the discussion of State water programs below).
Wetlands have been extensively drained, cleared and grazed for agriculture. Overall, around 50% of Australia’s wetlands have been converted to other uses¹⁴. In some areas the situation is much worse: for example, less than 4% of wetlands in the south-east of South Australia remain, and about 1% in the Greater Adelaide Metropolitan Region. In New South Wales, the Macquarie Marshes, arguably one of the most important wetlands in the southern hemisphere for waterbirds, is among many major wetlands to be seriously degraded¹⁵. Because wetlands store and slowly release water over time, their loss has further accentuated the highly variable natural flows in unregulated rivers.
According to the 1996 State of the Environment Report (refer also to the Report's technical background papers) major problems affecting many remaining rivers, wetlands and aquifers of conservation value include (not in order of importance):

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  extraction of surface or groundwater flows;
  
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  alterations to flow volumes, timing, temperature and quality stemming from stream regulation (dams and weirs) and levee bank construction;
  
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  water pollution from agricultural, urban and industrial sources;
  
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  habitat degradation, including: (a) degradation of riparian vegetation by grazing and altered fire regimes, (b) weed infestation, (c) changed river morphology associated with erosion and sediment deposition, (d) recreational activity impacts, including destruction of vegetation, destructive litter such as nylon fishing lines, and poisoning of soils and biota with lead shot, (e) removal of large woody debris through river de-snagging programs, and (f) drainage, infilling or channelisation for agricultural purposes;
  
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  structures impeding the passage of fish, turtles and crayfish on life-cycle journeys;
  
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  introduced aquatic pests species (including plants, animals and micro-organisms);
  
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  disturbance of wildlife by recreational and urban encroachment, powerlines (causing birdstrike) and predation by feral animals such as cats, foxes and dogs; and
  
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  the direct and indirect effects of mining operations, including: (a) the mining of materials associated with water systems, such as peat, sphagnum moss, gold, certain gemstones, tin, sand and river gravel, (b) water pollution resulting from mining operations, especially sediment and acidic wastes, and (c) altered groundwater regimes resulting from mine dewatering or process water extraction.
  

Further information on these threats may be obtained from the wetland strategies developed by Western Australia, Victoria, New South Wales, Queensland, and the Northern Territory, and the draft wetland strategies now being developed by Tasmania and South Australia.
Prior to recent water reforms, groundwater and surface water flows were managed with little coordination, and the legacy of this mistake remains today¹⁶. See section 4.4 below. In some locations around Australia, groundwaters have been so heavily used that springs have dried up, along with their associated local ecosystems. A large proportion of groundwater harvested from the Great Artesian Basin is simply wasted¹⁷. Pollution with salt or chemical wastes, and changes to aquifer flow patterns, have destroyed or degraded substantial localised freshwater ecosystems¹⁸. Over-harvesting of fresh groundwater has, in places, resulted in degradation of the aquifer through the consequent intrusion of salt water.
Outstanding examples of sites which need urgent reservation to protect site-specific values are the artesian springs associated with the Great Artesian Basin (mound springs). These unique habitats contain a diverse endemic fauna (fishes, invertebrates). While some springs are now protected in South Australia, virtually none of those in Queensland are protected, including the group with the largest endemic fauna (Edgbaston Springs). Increasing evidence of extraordinary endemism in some groups (especially molluscs and crustaceans) shows that many taxa are confined to single streams or watersheds¹⁹. Reservation alone, of course, would be useless if developments in the region seriously deplete groundwater stocks and flows. The establishment of freshwater reserves needs to be accompanied by catchment and aquifer management programs.
An aquifer in Western Australia has the highest diversity of subterranean amphipods recorded anywhere in the world²⁰, highlighting stygofauna as an area of conservation significance and concern²¹.
The temporal and spatial mosaic of ecosystem types in the landscape are important for the protection of biodiversity. Freshwater biodiversity depends in part on the natural diversity of wetland types in the landscape, a pattern which is degraded by draining and damming and the replacement of a range of wetland types by a homogeneous landscape of farm dams²².
Water pollution may be categorised into seven broad classes (not in order of importance):

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  sand and silt, which smother aquatic micro-habitats;
  
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  suspended solids, or other pollutants causing increased turbidity, which inhibit light penetration and thus aquatic photosynthesis;
  
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  salt, which causes direct toxicity, and alters ecosystems by favouring salt-adapted species;
  
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  nutrients, which alter ecosystem balances, and can result in algal blooms with toxic effects;
  
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  industrial and agricultural chemicals, such as pesticides, which can cause acute or chronic toxicity; and
  
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  acid resulting from the disturbance of acid-sulphate soils, reasonably common along flat low-lying coastal areas of NSW and southern Queensland. Acid mine drainage may result from similar chemical processes where pyrites are exposed to gaseous or soluble oxygen. Australia does not suffer the acid precipitation problems of northern Europe and northern America.
  
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  thermal pollution, which is usually associated with dams or powerstations. Large dams fitted with low-level outlets discharge cold, anoxic water. Powerstations can discharge heated water from cooling systems. In both cases, temperature changes can have acute (eg: death) or chronic (eg: interference with breeding stimuli) effects.
  

Unconstrained grazing and trampling by introduced herbivores continues to degrade riparian zones over 70% of the continent²³. Introduced fish, such as carp, plague minnow (Gambusia) and trout have dominated ecosystems in many streams at the expense of native species. Exotic riparian vegetation with seasonal leaf-falls (eg: willow) has choked rural streams across southern Australia²⁴.

While all these issues are addressed, in one form or another, by government programs, many of these threatening processes (such as incremental development, and exotic pests) are by their nature extremely difficult to deal with. Freshwater ecosystems over much of the Australian continent continue to deteriorate.