Human impacts on wetlands

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Many Queensland river systems have been impounded as water supplies for agricultural, industrial and urban needs, for flood mitigation, and to limit up-stream tidal (saltwater) ingress. The impoundment of water has led to a variety of impacts on fish and benthic invertebrate populations.

Alterations to flow regimes

Flow regime is the major factor that determines the physical characteristics and therefore habitat in waterways which in turn influences the local aquatic species composition. Australian native aquatic species have evolved to cope with the highly variably natural flow regimes of our waterways. Natural patterns of longitudinal (along a waterway) and lateral (across waterways onto floodplains) connectivity are essential for native fish to maintain healthy populations. Changes to the natural flow regimes through the use of waterway barriers such as dams and weirs reduces the natural flow variations, disadvantaging native fish species and favouring introduced exotic fish species.

Most Queensland catchments have large seasonal fluctuations in their natural flow regimes. This is likely to increase into the future due to climate change. Waterway barriers reduce the seasonal variations in the natural flow regimes.

Flood flows are decreased or depressed by waterway barriers which in turn change the natural physical characteristics of the waterway. Controlled releases throughout the drier months equally reduce the flow regime variability by limiting the low flows, important for shallow water habitats such as riffles, rapids and stream beds.

Diurnal or daily flows may also be altered by major daily peaks associated with the demand for irrigation water. Downstream demand for irrigation water means that many waterways must accommodate constant unseasonal high flows. As water flows for irrigation are tied to agricultural requirements and not the biological requirements of native fish species, changed timing of flows may impact on fish populations in the following ways:

  • lack of flows during seasonal fish migration periods
  • lack of access along waterways and out onto floodplains for breeding fish to locate mates or spawning grounds
  • a seasonal reduction of habitat availability and diversity for fish and benthic invertebrates
  • eggs and fish fry being stranded above the level of flowing water
  • the highly productive flood plain areas being inaccessible to fry during rearing seasons
  • increased siltation causing a loss of interstitial spaces in the substrate for benthic invertebrates.

The timing of fish movement for pre-spawning migrations and spawning is thought to be related to environmental cues or a combination of cues triggered by flooding, water temperature and water levels. Overland water flow, through the floodplains, is thought to be vital in adding the necessary environmental clues to trigger fish migration in many species. Therefore changes to flow regimes may also cause undesirable delays in migration and spawning or even prevent them altogether.

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Barriers to fish movement

At least one third of Queensland's native freshwater fish species are known to migrate significant distances during some stage of their life cycle. Most native fish undergo at least small scale movements related to feeding, habitat availability and reproduction. Many estuarine fish species seek freshwater reaches to reduce parasite loads. Some native freshwater fish species like barramundi and Australian bass are required to migrate to estuaries to breed. The construction of waterway barriers such as dams, weirs, barrages and stream crossings on waterways are listed as a serious threat to fish movement within the waterway.

Waterway barriers have had major implications for native fish species that need to migrate for breeding purposes. When fish spawning migration is delayed or prevented, spawning may not be possible. As a result of reduced or no recruitment of juveniles to fish populations, waterway barriers have caused significant declines in native fish poulations and threaten their long-term survival.

Waterway barrier removal

The removal of unused and obsolete waterway barriers is the most effective and preferred method to restore fish passage and water quality. However there has been little support in Queensland communities for this. Weirs often have social, aesthetic and even heritage values to the local community. In addition, weir removal can be expensive, to the point where the cost of a fishway is similar to the cost of removal on medium sized structures. Attempts to instigate weir removal programs by fisheries agencies elsewhere in Australia have met with limited success.

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Changes in water chemistry

The impoundment of water in reservoirs leads to a number of changes in water quality. Water released from the bottom or deep layer of an impoundment is characterised by colder temperatures, lower levels of dissolved oxygen and a build up of toxic substances such as hydrogen sulphide, iron and manganese. Within the impoundment these deeper waters are unsuitable for fish. Furthermore, the release of this poor quality water affects the survival of fish and benthic invertebrates downstream.

Installation and employment of multi level offtakes, which can source water from the full range of headwater levels, address the issue of poor quality water releases.

Newly constructed impoundments can cause prolonged reductions in the quality of downstream water releases. The breakdown of organic matter in the impoundment can lead to low oxgygen levels and high levels of sulphides. These in turn deter fish from moving through fishways and into the upstream impounded waters.

While the duration of low water quality can be managed to an extent, by clearing as much vegetation as possible, the organic matter in the soil cannot be so easily dispensed with. All impoundments will have a period of poor quality water releases.

Impoundments also trap sediment eroded from upstream, potentially resulting in downstream erosion and stream bed lowering reducing the habitat variability downstream.

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Pollution of freshwater wetlands

Many wetlands, particularly those near cities, have been polluted by human activities. Waterways often carry toxic loads of nutrients, heavy metals, pesticides and contaminants from previous activities that involved sewage plants, chemical factories, refineries and industry. Increased environmental awareness of the impacts of point source (single location) pollution has reduced their occurrence in Queensland´s wetlands today.

Non-point source pollution resulting from broad-scale land-use practices such as land clearing, urbanisation, cropping and grazing is a more widespread issue. Changed land use has led to the eutrophication of wetlands due to increased nutrients in the form of sedimentation, fertiliser run-off and organic wastes entering wetlands. These nutrients cause rapid increases in phytoplankton and aquatic plant growth. The high levels of organic matter may cause massive deoxygenation of the wetland resulting in a decline in water quality and a severe loss of aquatic life.

The effects of non-point source pollution are less easily and less quickly reversed than are those of point source pollution. Most industries including agriculture have and are continuing to improving their best management practices to reduce the effects of non-point source pollution on wetlands and the surrounding environment.

The protection and re-establishment of riparian vegetation to act as buffers is a good step in limiting the impacts of non-point source pollution.

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Habitat alterations

Snags (fallen timber), rocks and submerged aquatic vegetation are important in stream habitats, providing sheltered areas, spawning sites and habitat diversity. Instream habitat alteration including de-snagging (removing submerged or emergent logs or branches from watercourses) and channelisation (artificial modification of watercourses to ensure maximum flow and minimum flooding) of waterways have reduced the diversity and availability of aquatic habitat. Changes to the natural flow regime variation have reduced aquatic habitat diversity and availability.

The clearing of riparian vegetation leads to habitat loss, as this vegetation contributes directly to in-stream habitat through fallen trees, leaf litter and branches. Clearing of riparian vegetation is one of the major causes of habitat loss in Queensland's freshwater wetlands. Submerged roots and logs provide important shelter and spawning areas for native fish. Loss of aquatic habitat creates a shortage of feeding, hiding and spawning sites, and results in a subsequent loss of diversity of fish fauna present.

Impoundments act as sediment traps and can trap over 95% of the sediment load transported by a river, with the water passing over the impoundment relatively free of sediment. This can lead to a significant re-adjustment of the downstream channel morphology and substrate composition. If the flow velocities leaving the impoundment are sufficient, then considerable scouring and subsequent entrainment (washing away) of the downstream bed sediment can result. This can cause considerable erosion and downstream channel migration as well as the destruction of important habitat for in-stream animal and plant life.

The re-planting of riparian vegetation with native species is the easiest way to increase habitat diversity and improve native fish populations.

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Introduced fish

There are at least 43 introduced fish species recorded in Australia. Of these 15 occur in Queensland waters. These include:

  • the noxious gambusia (Gambusia holbrooki) (often referred to as mosquitofish)
  • tilapia (Oreochromis mossambicus and Tilapia mariae)
  • carp (Cyprinus carpio).

Introduced species threaten the existence of native species through competition for food and space, predation and the introduction of exotic parasites and diseases. For example, the swordtail (Xiphophorus helleri) and gambusia (Gambusia holbrooki) are two introduced species that appear to depress native fish populations through the occupation of space and competition for food.

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Introduced plants

Introduced aquatic plants include the water-fern salvinia (Salvinia molesta) and water hyacinth (Eichhornia crassipes). These are two of the world's most successful floating aquatic weeds. For example, three water hyacinth plants may produce 3000 new plants in 50 days.

These plants may colonise waterways to the extent of restricting the movement of boats, blocking pump suctions and irrigation structures and significantly altering water quality by reducing light penetration and dissolved oxygen levels beneath the water surface.

Fish moving along waterways and up through fishways are often unwilling to enter into water covered with water hyacinth mats, due to the low oxygen levels and low light levels.

Salvinia was first recorded for Queensland in 1953 from Bulimba Creek, Brisbane. A South American weevil (Cyrtobagous salviniae) was released in 1980. It now controls salvinia over most of its range in Australia.

Water hyacinth was first recorded (1895) from the Botanical Gardens in Brisbane. Successful control of water hyacinth has been achieved in parts of Australia with a South American weevil (Neochetina eichhorniae), while a second species (N. bruchi), released in 1990 to assist in control, is already established.

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Ponded pasture systems

Central Queensland and the Gulf of Carpentaria, like the rest of Northern Australia, experience distinct wet and dry seasons each year. During the dry season native pastures provide fewer nutrients for cattle. As a strategy to provide green forage for cattle during the dry season, ponded pastures of non-native water-tolerant grasses such as para grass (Brachiaria mutica) and aleman (Echinochloa polystachya) are grown in artificially ponded situations.

Ponded pasture grasses such as para grass is now widely spread throughout Queensland´s wetlands. This invasive grass smothers waterside vegetation such as native ferns and brush. Para grass also burns much hotter than native vegetation often resulting in hot fires that kill the native riparian vegetation including trees.

Ponded pastures are generally created by building low walls or banks at selected sites to trap available water from natural run-off during the wet season. Water for the pond is also obtained by diversion from adjacent water courses or from water-harvesting during flood flows in major streams.

More than 26,000 ha of ponded pasture has been established along the central Queensland coast. Ponded pasture systems benefit many grazing industries especially beef and dairy cattle.

Ponded pasture structures are potentially detrimental to aquatic species. Levee banks for ponded pasture prevent access by fish to and from floodplains and wetlands, for example barramundi, which spawn in inshore coastal waters in the mouths of estuaries. Barramundi juveniles use coastal swamps, marine plains and flood plains as nursery areas. The older juveniles use tidal and freshwater reaches of coastal waterways during their growth.

Ponded pastures may also result in fish kills. The ponded pastures may initially create habitat for fish but as this habitat dries out and water quality drops then fish may be trapped and die.

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