Professor Graham Harris, ESE Systems
prepared for the 2006 Australian State of the Environment Committee, 2006
The larger-scale changes to land use, water allocation and flow regulation, which occur across the continent, have led to smaller-scale changes in habitat in inland waters. Within wetlands and in rivers reaches, habitats are modified by reduced flows, increased sedimentation of fine materials and removal of riparian vegetation. Thus the larger scale modification of the landscape has local effects that strongly influence biodiversity and ecosystem function.
Although many Australian rivers and storages are naturally turbid, soil erosion has significantly increased sediment loads to rivers and estuaries. Because of the altered disturbance regimes, it is presumed that suspended sediment concentrations are higher now than they would have been before European settlement of Australia. The highly-erodable clay soils, combined with the dilute and largely sodium-dominated nature of surface waters in Australia, means that clay and other particles stay in suspension in the water column and settle only very slowly. Inland waters flowing from catchments with large areas of intact native vegetation tend to be clearer than those originating in agricultural and urban catchments—there is a close link between soil type, topography, climate, land use and levels of suspended material in inland waters. The NLWRA showed that high levels of sediment and turbidity were widespread in inland Australia in 2000, particularly in areas of the wheat and sheep zone, where land clearing has been extensive. Increased sedimentation smothers species that require clean water and rocky substrates and interferes with gill function in invertebrates and fish. Fine sediments can become devoid of oxygen, which kills susceptible species. Turbidity can limit light penetration and growth of native aquatic plant communities.
In the assessments done for the NLWRA in 2000, nutrient levels in excess of the water quality guidelines were found in approximately two-thirds of catchments assessed. (The NLWRA focused its effort in the intensive land use zone.) The trends over time were mixed, with increases of nitrogen and phosphorus being approximately equal to decreases. The Australian landscape is naturally stripped of nitrogen through denitrification and low levels of nitrogen deposition, whereas phosphorus levels in inland waters tend to be high because of the large amounts of eroded and suspended materials in the water. Ecological processes in rivers, therefore, adapt to the scarcity of nitrogen relative to phosphorus.
Land use change in Australia has increased the export of nitrogen and phosphorus from catchments, and made these two nutrients more available for plant growth in rivers and storages; agricultural and urban catchments show higher levels of dissolved inorganic nitrogen and phosphorus in the water than pristine forested catchments, where organic forms predominate (Harris 2001). High concentrations of nitrogen and phosphorus in inland waters can lead to algal blooms under certain circumstances, oxygen depletion and, possibly, fish kills.
Freshwater wetland and lake systems can exist in two states—one with clear water, populated by a diverse assemblage of water plants; the other turbid and dominated by planktonic algae and cyanobacterial blooms. Once flipped from clear to turbid, it is often very difficult to reverse the process. It does seem that as a result of land use change, urban development, flow regulation and increased sediment and nutrient loads from the catchments, many (if not most) standing and slowly flowing Australian surface waters have been flipped into the turbid state; examples include lakes Mokoan and Wellington in Victoria. Both lakes are now highly turbid and they are frequently subject to major algal blooms, often of toxic cyanobacteria. Populations of water plants in most Australian rivers are also much reduced—through increased sediments, nutrients and rapid changes in water levels—so that what we now see are turbid rivers flowing through bare channels with few water plants.
The NLWRA data for 2000 show that salinity is a problem in about one-third of catchments assessed, particularly in south-western Australia, and in the southern Murray-Darling Basin (see Table 6). These are all areas with a long history of clearing and intensive agricultural use. Reductions in the area of perennial native vegetation in these catchments have altered the surface and groundwater hydrology, increased groundwater recharge, and increased salt ingress to rivers and streams. The salinity trends in all regions were mixed, partly because of the short (8–10 year) data records analysed, but also because of climate and flow variability in these rivers. Instream and groundwater salt concentrations decrease during wet periods when dilution is greater, although falling groundwater tables (near the surface) may reduce salt ingress, so a clear distinction must be made between salinity exceedances (concentrations, which vary with flow) and loads (the amount of salt moving through a catchment system). It is dangerous to make definitive statements about salinity trends based on concentration data alone because of climate and flow variability, complex interactions between hydrology and salt stores, long groundwater residence times, and changing river management practices. The Murray-Darling Basin Commission salinity audit of 1999 (MDBC 1999) predicted rising salinity loads throughout the rivers of the southern Murray-Darling Basin for the following 100 years as groundwater salinity moves through these systems. These predictions have not transpired up to 2005 because the severe drought during the early 2000s caused regional lowering in groundwater levels and hence reduced salt discharge. This does not imply that a return to more normal rainfall patterns would not cause an increase in salt discharge.
|Water quality measure||Number of major exceedances||Number of significant exceedances||Number of basins assessed|
|Nutrient: total nitrogen||19||19||50|
|Nutrient: total phosphorus||40||20||75|
|Salinity: electrical conductivity||24||18||74|
Source: NLWRA 2000
Many regulatory and licensing programs have focused on the elimination of industrial, agricultural and other chemicals from the aquatic environment. While action is still required in some cases, it can be said that, overall, such programmes have been reasonably successful in the last decade and that instances of toxic concentrations of such chemicals are not frequent. For example, in all the recent major studies of the effects of urban development on the ecology of coastal waters and lagoons (Perth, Adelaide, Melbourne, Brisbane), toxic substances have been found to be a secondary issue behind biodiversity loss and nutrient loadings. Exceptions include streams downstream of some irrigation areas, some industrial and mining sites, disused sheep dip sites and old waste dumps, which continue to pollute groundwater. Innovative remediation techniques are now being developed and applied to many of these sites (for example, at the Kwinana oil refinery in Western Australia) including the development and use of specific microbes that have been selected to metabolise and eliminate industrial and other chemicals efficiently.
While cases of catchment-scale pollution from agricultural and industrial chemicals are rare, there are still recent examples of downstream impacts on ecosystems and on coastal fisheries (especially oysters). Because of the complexity of hydrological processes arising from altered flow regimes and of connectivity between events on land and in streams there is, as yet, considerable uncertainty around the evidence for the precise pathways and mechanisms of direct downstream effects (Harris and Heathwaite 2005). Nonetheless, there is enough evidence to be concerned that this is an area requiring more investigation (Table 7).
|From facilities 10–50 km from coast||From facilities >50 km from coast||Total for facilities > 10 km from coast||From facilities 10–50 km from coast||From facilities >50 km from coast||Total for facilities > 10 km from coast|
|Total nitrogen||36 462 128||1 950 869||38 412 997||2 493 071||1 643 984||4 137 056|
|Sulfuric acid||39 440||2 100||41 540||1 333 771||1 920||1 335 691|
|Ammonia (total)||15 450 626||125 044||15 575 670||915 136||318 054||1 233 189|
|Manganese and compounds||38 922||5 995||44 917||1 091 630||57 028||1 148 659|
|Total phosphorus||8 735 614||267 325||9 002 939||655 709||178 811||834 521|
|Total volatile organic compounds||2 111||167 213||169 324||295 065||746||295 811|
|Ethanol||57 224||2 000||59 224||290 313||118||290 431|
|Zinc and compounds||66 334||86 052||152 386||265 019||20 975||285 994|
|Chlorine||381 304||14 909||396 212||209 688||1 876||211 563|
|Copper and compounds||23 814||5 433||29 248||87 518||56 836||144 355|
|Fluoride compounds||1 276 162||22 656||1 298 818||90 183||22 165||112 349|
|Hydrogen sulfide||16 700||168||16 868||59 665||4 650||64 315|
|Cobalt and compounds||606||31 770||32 376||29 339||16 280||45 619|
|Nickel and compounds||5 835||754||6 590||13 173||19 161||32 334|
|Lead and compounds||23 067||21 288||44 355||16 015||6 120||22 134|
|Toluene (methylbenzene)||334||2 622||2 956||7 619||64||7 683|
|Xylenes (individual or mixed isomers)||330||1 529||1 860||5 666||34||5 700|
Source: National Pollutant Inventory database
Dams and large weirs placed across Australian rivers cause stratification of the water behind the dam walls, with water being warmer at the top and colder at the bottom. Because the water tends to be discharged at the base of the dam wall, these weirs and storages discharge colder water than would be ‘normal’ for the river. This impacts on the reproductive and migratory behaviour of downstream species. Because of the stratification cycle in the pool behind the wall, downstream discharges show quite different seasonal temperature patterns to those normally encountered in the river (Figure 5). Spring and summer temperature rises (which are often the spawning and migration cue for native animals) are delayed or eliminated. Data indicate that major dams can influence river water temperatures for hundreds of kilometres downstream from the wall and that temperatures may be reduced by as much as 10–12 °C. The precise pattern of behaviour will depend on the size and shape of each dam or weir, the pattern of seasonal discharge, and climate variability from year to year. Nonetheless, effects are evidently far-reaching and ecologically important. In some large dams, off-takes are now being constructed higher up the dam wall to reduce thermal pollution effects.
Source: NSW Fisheries data 2002, cited in Department of Environment and Conservation, NSW (2003, Figure 5.3)
Instream habitats have been influenced by land use change in the catchments and gullying and other forms of erosion. Prosser et al. (in NLWRA 2001) were able to model the process of erosion, transport and deposition as a function of climate, slope, stream power, land cover and soil types; accounting for most of the observed patterns of erosion and deposition. As noted above, much of the eroded material has not reached the oceans, so large sections of the lower reaches of many Australian rivers now store these sediments and have slugs of sand that have smothered instream habitats of various kinds. This material is influencing ecological and biogeochemical connectivity within the channels and, particularly at low flows, has very much modified the habitat.
Instream habitat has been further modified by extensive ‘de-snagging’ of lowland rivers and by the decline of native vegetation and water plants. Very small scale aspects of instream habitat (such as sediment grain size and the existence of biofilms on snags and plant stems) are vitally important for the survival of the diverse flora and fauna of Australian rivers—especially invertebrates and juvenile fish. Therefore, the alteration of the physical habitat, through the replacement of a diverse set of structural habitats over a range of physical scales (cobbles, riffles, snags and a diverse range of aquatic plants) by a much more uniform habitat structure (sandy or muddy dispositional environments with few snags or plants) has led to reduced biodiversity. Re-snagging programmes —to replace ‘snags’ in lowland rivers—are an important way to replace habitat for fish, birds and other aquatic organisms, but the scale and scope of these programmes is small compared to the scale of the problem.
Riparian vegetation serves a number of critical ecological and biological functions. Riparian vegetation is a major habitat for many terrestrial plant and animal species, acts as a buffer for material moving off adjacent land, contributes detritus and other material to the instream ecosystem, acts as a nutrient filter, removes nutrients from the water, and shades the channel itself. Rivers with native riparian vegetation in better condition tend to have better water and instream habitat quality than those with vegetation in worse condition. Because of the key role of sites adjacent to the river channel in contributing material to the channel, measures of the intactness of riparian vegetation are, therefore, important components of measures of river health .
The proportion of river reaches with healthy riparian vegetation varies greatly between and within regions (Figure 6). As a general rule, the data seem to show a rough proportionality between habitat fragmentation, land clearance and the remaining riparian vegetation, so that there is less riparian vegetation in heavily cleared parts of the intensive land use zone (for example, Western Australia, the southern and central Murray-Darling Basin, and western and central Victoria) than in parts of northern Queensland, Tasmania and eastern Victoria. There appears to have been little change in the length of vegetated riparian zones in the major catchments between 1991 and 2004. Only minor increases in the condition of riparian zones have been recorded during that period.
Source: Environmental Resource Information Network (2004, unpublished data)
Wetlands are an important part of the aquatic waterscape because of high biodiversity and productivity, their potential role in improving water quality, and their key role in providing habitat for birds, amphibians and threatened species. Wetlands are structurally diverse and support a large range of biofilms and other important microbial populations. Unfortunately, wetlands have frequently been regarded as useless swamps to be drained, farmed or otherwise ‘reclaimed’ for agricultural and other uses. Wetlands also suffer when river flows are regulated because of either permanent inundation through their use as off-river storages or almost permanent desiccation because of the elimination of overbank flows. Most wetlands in south-eastern Australia consist of remnant fragments of their former extent.
It is now realised that the proper management of the remaining shallow floodplain lakes and wetlands along Australia’s major rivers requires the reinstatement of more natural flooding and drying regimes—something that is quite difficult to engineer in heavily regulated river systems. Irregular wetting and drying stabilises sediments, improves water quality, reduces introduced fish populations, and leads to a diverse population of waterplants and birds.
Ninety per cent of floodplain wetlands in the Murray-Darling Basin, 50 per cent of coastal wetlands in New South Wales and 75 per cent of wetlands on the Swan Coastal Plain in south-west Western Australia have been lost due to drainage and altered flow regimes . Weed invasion , conversion to agricultural use, and increased fire frequencies are ongoing and significant management issues for wetlands (Figure 7).
Source: Davis, et al. (2001, Figure 1)
The construction of dams, weirs, road crossings and other barriers to the migration of fish species has been extensive. Connectivity between upstream and downstream populations has been reduced or eliminated in many river drainages of the southern and eastern parts of Australia. Of the 53 native freshwater species in NSW, 28 undertake large-scale migrations, 16 migrate on local scales, and the status of the remaining nine species is unknown. At present in NSW, there are more than 4300 physical barriers but only 28 fishways that are effective for native fish . Construction of fishways is increasing, although inadequate, as the effects of migration barriers are becoming understood; but there appears to be a need to consider more carefully the biology and migratory behaviour of Australian native fish because not all native species appear to be able to pass through some of the devices that have been constructed.
The role that groundwater plays in controlling the health of ecosystems in Australia is often overlooked. Groundwater dependent ecosystems depend at least partially on groundwater to maintain their health. These ecosystems represent a diverse, yet distinct component of Australia’s biological diversity. Clifton and Evans (2001) identified six major types:
- terrestrial vegetation
- river baseflow systems
- aquifer and cave ecosystems
- terrestrial fauna
- estuarine and near shore marine ecosystems.
In practice, there is a continuum of ecosystem types with, for example, wetlands sharing many common features with riparian vegetation. Groundwater level, flux, pressure and quality (including temperature) can all control ecosystem health and location. There is also a continuum of groundwater dependence, ranging from those ecosystems that are entirely dependent on groundwater (for example, the mound springs of the Great Artesian Basin and most stygofauna) through to those that make limited or opportunistic use of groundwater (for example, many wetlands throughout Australia).
The first national assessment of groundwater dependent ecosystems was undertaken by Hatton and Evans (1998). Since then, many researchers have published on specific types of groundwater dependent ecosystems, with Hancock et al. (2005) providing an up-to-date assessment of baseflow and aquifer groundwater dependent ecosystems. At the national scale, relatively little is known about these ecosystems, but knowledge is rapidly improving with research. This especially applies to vegetation, wetland and river baseflow systems. The key challenge in this research is to relate the change in ecosystem health to the change in the groundwater regime. This ‘response function’ is slowly beginning to be understood for specific groundwater dependent ecosystems at individual sites.
The awareness in the scientific community of groundwater dependent ecosystems has increased greatly in recent years; however, the understanding of groundwater dependent ecosystems in the wider community remains poor. Nonetheless, the increased awareness of environmental flows for streams has caused surface water managers to start to consider baseflow for groundwater dependent ecosystems. As the baseflow component of many streams in Australia commonly represents about 50 per cent of the total streamflow, the need to manage groundwater and hence maintain streamflow, especially during the low flow dry season, has become obvious.