Inland waters
Theme commentary
Professor Graham Harris, ESE Systems
prepared for the 2006 Australian State of the Environment Committee, 2006
Response of animals and plants to catchment disturbance and the alteration of flow regimes
Data from the recent Millennium Ecosystem Assessments clearly shows that freshwater aquatic biodiversity is declining rapidly around the world. This is a result of land use change, flow regulation, water extraction, draining of wetlands, and habitat modification. Australia is no exception to this global picture. The situation is further complicated by the general lack of awareness and knowledge about the biodiversity and importance of aquatic organisms. Compared with terrestrial species, there are few threatened species listed from inland waters, few nominations for the threatened species list, and the general taxonomic knowledge is also poorer. Other than for a few groups of ‘iconic’ species—for example, fish, frogs and waterbirds—there is little knowledge or appreciation of the urgency of the situation.
Bacteria and algae
Cyanobacteria (or blue-green algae) are natural components of Australian inland waters, occurring frequently during low flow periods and in billabongs. There is evidence that cyanobacteria have become more widespread as river modification and the construction of storages has proceeded. Bloom frequencies fluctuate from year to year as both climate variability and flow regulation alters flow regimes in Australia’s inland waters. Cyanobacterial blooms currently account for about 10 per cent of the total cost of environmental protection in Australia. These blooms are frequently highly toxic to people, stock and pets, so they interfere with many beneficial uses of water. During an algal bloom, people and stock must be kept away from the water, fisheries are closed, and water treatment costs rise markedly. Warm temperatures, long water residence times and stratified water columns, high levels of light, and high phosphorus concentrations favour cyanobacterial blooms. Furthermore, many cyanobacteria species control their buoyancy, so they are able to float to the surface and out-compete other species in turbid waters. Some key toxic species are also able to fix nitrogen from the atmosphere. So an ability to both float and fix nitrogen gives them an advantage under conditions of turbid waters with low nitrogen and high phosphorus—conditions that are found in billabongs, storages, and weir pools during low flow periods. Cyanobacterial blooms, therefore, tend to be more common in dry years and less common during normal flow events. (Indeed control measures employing flushing flows have proved to be successful in many cases.)
Macro-invertebrates
Macro-invertebrate populations are routinely used as indicators of river health. Our understanding of the habitat requirements of macro-invertebrates leads us to believe that there is a causal link between river modification, physical habitat features and river health as measured by this technique. As might be expected from the generally poor state of Australian catchments and rivers , invertebrate data showed that one-third of river length is impaired to some degree, having lost between 20 per cent and 100 per cent of the various kinds of aquatic invertebrates that should live there (Table 8). New South Wales has the poorest aquatic biota condition; approximately 50 per cent of river length has impaired aquatic biota. These data may be under-estimates of the actual condition of our rivers, particularly in the Murray-Darling Basin, because the assessments depend on the identification and sampling of unmodified reference sites. These may be difficult to find in some of the more modified regions. There is inconclusive evidence of change in macro-invertebrate populations in recent years.
| Period of assessment | Number of sites at each level of diversity compared with reference sites | |||||
|---|---|---|---|---|---|---|
| More diverse | Similar to reference condition | Significantly impaired | Substantially impaired | Severely impaired | Total number of test sites | |
| 1990–2004 | 195 | 2465 | 1 556 | 433 | 56 | 4 705 |
| 1994–1999 (in ASEC 2001) | 154 | 1 702 | 963 | 254 | 39 | 3 112 |
Source: Australian River Assessment System (AusRivAS): National River Health Database, with input from: Natural Heritage Trust; state and territory lead agencies for river health assessment; Australian Government Department of the Environment and Heritage; CRC for Freshwater Ecology; Land and Water Australia; and Freshwater Systems Pty Ltd.
Invertebrate communities depend on small-scale habitat preservation (both instream and in the riparian zone), catchment condition, flow patterns and connectivity for their life cycles and reproduction. As might be expected, macro-invertebrate communities show greatest evidence of impairment in inland and coastal regulated rivers, where land use change has been extensive and habitat modification has been extensive. Also, invertebrate communities show evidence of considerable alteration in urban catchments, where nutrient and sediment loads are increased, sewer overflows occur during storm events, runoff patterns are changed and stream channels are frequently physically modified to control flooding.
Fish
All the evidence points to reduced numbers and biodiversity of Australian native fish . In New South Wales, 22 per cent of the fish species that were expected to occur were not found in recent surveys, fish communities were frequently found to be in poor condition, and many species have reduced abundance or restricted distributions. Despite restocking programmes, 11 native fish species are listed as threatened in New South Wales. Many introduced fish species are now abundant and, indeed, recent evidence seems to show that regulation of Australian rivers favours introduced over native species. Recent work has shown that introduced fish abundance and biodiversity can be used as a measure of river health. Thirteen alien species were recorded from New South Wales freshwaters, with Carp (Cyprinus carpio), (Genus species Carassius auratus), Redfin Perch (Perca fluviatilis), Gambusia (Gambusia holbrooki), Rainbow Trout (Oncorhynchus mykiss) and Brown Trout (Salmo trutta) the most common. The greatest numbers of introduced species were recorded from rivers in the Murray region. Introduced fish alter habitats, and they compete with and prey on native fish, thus exacerbating the effects of flow regulation and land use change.
Recent trends in New South Wales fisheries statistics show a continued increase in introduced species, a decline of native species, and the continued spread of Carp, Gambusia and Oriental Weatherloach (Misgurnus anguillicaudatus). Recruitment of native fish is enhanced in wet years—which have been few since 2001—and the reintroduction of environmental flows and overbank flows is required to further enhance the success of native species.
Frogs
Frogs are very sensitive indicators of declining aquatic ecosystem health and extent, and many species have ‘gone missing’ around the world in recent years. Australia is no exception, ranking thirteenth in the world in terms of overall frog biodiversity and twelfth in terms of the number of threatened species. There is an increasing number of sites in Australia in which frogs are no longer found. Four species are listed as extinct under the Environment Protection and Biodiversity Conservation Act 1999, 15 are endangered and another 12 are listed as vulnerable; overall about 14 per cent of species are threatened. Frog populations have decreased markedly in the last decade or so, with several species thought extinct. This is thought to be due to the Chytrid fungus, which is a worldwide phenomenon. The extent to which this has been associated with anthropogenic impacts is unknown but it is suspected to be at least partly responsible for the decline. The change in species composition and abundance continues to be rapid. The situation is urgent.
Waterbirds
Australian waterbirds fall into two broad categories—migratory species that come to Australia from the northern hemisphere during the northern winter, and native species that breed in Australian wetlands and other aquatic habitats (these may also migrate between regions). Many migratory species are endangered or threatened by habitat loss either in their native habitats or along the migratory routes. Severe reductions in wetland extent in Australia have reduced the available habitat as well as the numbers and breeding success of Australian native waterbirds.
The distribution and abundance of waterbirds are closely connected to the occurrence of floods and wetland inundation during wet years. Many waterbirds are long lived and adapted to massive, but infrequent breeding events. Reduced flooding and strict river regulation—particularly during the recent dry period—has led to reduced breeding success in the southern and eastern parts of Australia. Over the last 50 years, the frequency of flooding flows in areas like the Barmah-Millewa Forest, Gwydir wetlands, Narran lakes and the Lowbidgee has declined because of river regulation and water extraction; waterbird breeding has also declined. Waterbird populations have declined by more than 80 per cent in the Lowbidgee wetlands in the last twenty years (Kingsford 2004). In the central-west of New South Wales, waterbird surveys show that out of 46 species studied, 29 species are declining and only five are increasing, while 12 showed no overall trend during the study (Reid et al. 2004). Environmental flow allocations for bird breeding, such as carried out in the Macquarie Marshes and the Barmah-Millewa wetlands, do improve breeding success and have been used in localised experimental allocations. Climate change—with its associated increase in the frequency of long dry periods—has the potential to impact strongly and negatively on all species.
Source: Kingsford and Porter (2005)
Wetland and floodplain communities
Wetland and floodplain communities have been degraded across large areas of the intensive land use zone through overgrazing, salinisation, lack of flooding, clearing and draining, and the introduction of weeds and introduced wild animals [link to Indicator LD_40]. Recent surveys of the floodplains of the lower Murray River have found that about 80 per cent of River Red Gums are stressed to some degree, with about 20–30 per cent of them severely stressed to the point of death. The Black Box woodlands of the terraces along the river are also severely stressed, and have experienced extensive historical contraction in range. Salinisation of floodplain soils and the lack of natural flooding events have seriously stressed the trees. If the riverine forests of the lower Murray River are seriously degraded, many other species will be adversely affected and river health will be degraded.
Exotic species
As noted above, it appears that changes in the flow regimes and ecology of Australian inland waters are favouring the spread of many introduced species. Furthermore, introduced species, like Carp , can change habitats and influence the ecology of other species. The continued introduction of more natural flow regimes is essential for the reestablishment of a more balanced fauna in our rivers.