State of the Environment

2001

Inland Waters Theme Report

Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Jonas Ball, Sinclair Knight Merz Pty Limited, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06750 7

Water quality and sources of pollution (continued)

Eutrophication and algal blooms

Eutrophication occurs when the major plant nutrients - nitrogen and phosphorus - accumulate in water (or sediments). Given the right conditions, elevated concentrations of nutrients stimulate the growth of aquatic flora to nuisance levels. Examples include microscopic algae in the water column which may result in algal blooms as well as vascular aquatic plants such as water hyacinth (Eichhornia crassipes). In freshwaters, phosphorus is considered to be the limiting nutrient for algal growth. There is, however, evidence that nitrogen (or the ratio of nitrogen to phosphorus) may also play an important role in triggering algal blooms (Harris 2001). Other factors such as water flow, turbidity and the condition of riparian vegetation are also important in the development of algal blooms.

Algal blooms are a natural occurrence, however, due to human activities (such as land clearing, destruction of riparian vegetation, water extraction, decreased flow and flow variability associated with weirs and dams, discharge of sewage and intensive agriculture), higher quantities of nitrogen and phosphorus have been reaching inland waters. Periods of low or no flow in many rivers have also increased due to high water extraction and river regulation. The combination of high nutrient levels and long periods of low or no flow provide ideal conditions for algal blooms to develop.

Blue-green algal blooms are of most concern in inland waters as certain species produce toxins that may cause skin irritations, gastrointestinal disorders, influenza-like symptoms and, in extreme cases, permanent organ damage and death (ANZECC/AWRC 1992). Many of the toxins produced by blue-green algae can affect people, livestock, birds and fish. Although most other algae do not produce toxins, all algal blooms can depress dissolved oxygen levels (possibly resulting in fish kills), reduce light penetration causing shading of other aquatic species, and/or alter water chemistry.

Blue-green algae have competitive advantages over other algae that increase the likelihood and persistence of blooms. Between 1928 and 1994, 84% of the reported algal blooms in Victoria were blue-green algae, with 75% of blooms being potentially toxic Microcystis and Anabeena species (Cottingham et al. 1995).

For human uses, blue-green algal blooms in drinking water resources is the most serious issue. Traditional water treatment methods are unable to remove the algal toxins from algae-contaminated water, while other alternative water treatment methods are expensive. Blue-green algal blooms may affect the recreational use of a waterway by decreasing its aesthetic amenity and posing a health risk to individuals who have direct contact with the algae.

Pressure: Contributing factors to eutrophication and algal blooms

Point sources of nitrogen and phosphorus [IW Indicator 3.3]

Point sources of nitrogen and phosphorus include sewage treatment plants, intensive agriculture (such as cattle feedlots and piggeries) and industry. Although in most river systems, point sources only contribute to 5% to 35% of the total amount of nutrients entering the waterway (Environment Australia 1996; NPI 2000), their impact can be proportionally greater. Point-source discharges are usually continuous and often contain high levels of nitrate and phosphate, forms of nitrogen and phosphorus that can be readily used by algae. In dry weather, diffuse source nutrient pollution is generally low and point sources are the largest source of nutrients. The greater stability of the water column in dry weather is generally more favourable to the development of algal blooms (SKM 2001) and there is also less flow in river systems to dilute point-source discharges.

Although there is information on the quantity of nutrients discharged from sewage treatment plants (NPI 2000), there is no comprehensive information on other point sources. Overall, the contribution of mining and industrial discharges to nutrient load is relatively small, primarily because most industry is located in the major urban centres and discharges to the sewer system. Also, industry and mining discharges are heavily regulated in most states. The contribution to nutrient loads from intensive livestock enterprises is potentially considerable as these facilities are widespread, often poorly regulated and generate wastes that are high in nutrients (e.g. manure).

Figures 13 and 14 show the quantities of phosphorus and nitrogen discharged to inland waters by sewage treatment plants each year. As New South Wales has the highest inland population, its sewage treatment plants also discharge the highest quantity of nutrients. The four river systems that receive the highest loads of nitrogen (greater than 100 tonnes per year) and phosphorus (greater than 30 tonnes per year) from sewage treatment plants are the Murrumbidgee, Hawkesbury-Nepean, Namoi and Hunter.

Figure 13: Tonnes of phosphorus discharged by inland sewage treatment plants each year.

Figure 13: Tonnes of phosphorus discharged by inland sewage treatment plants each year.

Source: Data for New South Wales, Victoria, Queensland and Tasmania were obtained from licensing databases supplied by state regulatory agencies. Data for Northern Territory, Western Australia, Australian Capital Territory and South Australia were obtained from the National Pollutant Inventory 2000.

Figure 14: Tonnes of nitrogen discharged by inland sewage treatment plants each year.

Figure 14: Tonnes of nitrogen discharged by inland sewage treatment plants each year.

Source: Data for New South Wales, Victoria, Queensland and Tasmania were obtained from licensing databases supplied by state regulatory agencies. Data for Northern Territory, Western Australia, Australian Capital Territory and South Australia were obtained from the National Pollutant Inventory 2000.

Despite discharging similar volumes of wastewater, Queensland sewage treatment plants discharge approximately twice as much nitrogen and three times as much phosphorus as sewage treatment plants in Victoria. In Victoria, 75% of sewage treatment plant discharges are tertiary treated (i.e. discharges undergo a nutrient-stripping process), whereas in Queensland less than 5% of discharges are tertiary treated. Tasmania, South Australia, Northern Territory and Western Australia discharge low quantities of nitrogen and phosphorus directly into inland waters because of their smaller inland populations compared to other states.

Diffuse sources of nitrogen and phosphorus [IW Indicator 3.3]

Diffuse sources generally contribute 65-95% of nutrients entering inland waters (State of the Environment Advisory Council 1996; NPI 2000). The major diffuse nutrient source is catchment and streambank soil erosion, although there are other sources such as fertilisers, groundwater inflow, sediment exchange and direct atmospheric deposition. In recognition of the importance of diffuse source contributions to nutrient loads, the NSW EPA has recently embarked on a major program to define and manage diffuse source pollution (T. Church, pers. com.).

Run-off from agricultural and urban land may contain higher levels of nutrients compared with undeveloped land due to:

Nutrients from diffuse sources are more fully discussed in the Land Theme Report.

The quantities of nitrogen and phosphorus exported from catchments into inland waters have been shown to increase with the catchment clearance and population density (Harris 2001; McIvor 1995). More importantly, dissolved inorganic nitrogen exports have been shown to increase substantially (Harris 2001). This is of major concern as this form of nitrogen is more bioavailable to algae and may increase the risk of eutrophication and algal blooms.

Soil erosion

For a complete discussion of soil erosion see the Land Theme Report. A summary of the findings and their relevance to inland waters is presented below.

The clearing or degradation of vegetation (e.g. through grazing) leads to a greater risk of wind and water erosion. Accelerated erosion results in soils which are often high in nutrients being washed into inland waters and increasing the risk of eutrophication and algal blooms, as well as increasing the turbidity of waters. Approximately 1.6 billion tonnes of soil are lost to wind and water erosion every year in Australia. The land use with the highest rate of soil erosion is intensive cropping such as vegetables, flowers and tobacco cultivation. However, they only contribute less than 5% of total soil loss. Native pasture lands are the source of 60% of the total soil loss.

Gully erosion is the major source of sedimentation in rivers. Land clearing and removal of riparian vegetation in the upper reaches of small tributaries and streams have led to these waterways becoming incised. Gradually the incision moves further upstream, causing gully erosion in the upper parts of the catchment. There is evidence to show that most of the sediments in inland rivers are from recent subsoil deposition and that physical and chemical changes in the sediments and overlaying water can leach nutrients from the sediments into the water column (Donnelly et al. 1998). Soil erosion and sedimentation in inland waters can have other significant impacts such as siltation of river channels, infilling of wetlands, smothering of aquatic flora and fauna, and reduced light penetration.

Gully erosion along a creek in Bathurst, NSW

Gully erosion along a creek in Bathurst, NSW

Source: JE Williams.

Turbidity is an indicator of the amount of soil and organic matter suspended in water. Many of Australia's inland rivers and wetlands have a high natural turbidity, whereas the coastal systems generally have a lower turbidity (NSW EPA 1997). To determine where high turbidity was a major environmental issue, the turbidity of rivers and streams between 1995 and 1999 was compared with state and territory water quality objectives. Where at least 7-10 years of flow-weighted turbidity data were available, an assessment of trends in turbidity was also undertaken. Detailed information on catchment and individual site turbidities and trends in turbidity can found at the NLWRA website (http://www.nlwra.gov.au/ ).

Figure 15: Catchments where turbidity is considered an environmental issue

Figure 15: Catchments where turbidity is considered an environmental issue

Source: National Land and Water Resources Audit 2001a.

Turbidity in rivers, stream and dams regularly exceeded state and territory water quality objectives in all catchments of the Murray-Darling Basin except the Condamine in Queensland. Other river systems where turbidity is a major issue are in:

Increasing trends in turbidity were measured at many inland sites in New South Wales. Although water turbidity is an indicator of potential nutrient pollution from soil loss, it is also an important factor in the development of algal blooms. Highly turbid water limits light penetration of the water, and therefore there is less photosynthetic energy available for algae. In eastern waterways that are highly turbid with dispersed clays, light energy, rather than nutrients, has been identified as the major limiting factor to the development of algal blooms (NLWRA 2001a).

There is an increased risk of algal blooms if turbid waters become clearer due to a change in water chemistry or the settlement of suspended particles. A major cause of the massive blue-green algal bloom in the Darling River in 1991 was a sudden inflow of sulfate-rich water that increased the settlement of suspended sediment and the release of phosphorus from the bottom sediments (Roden & Edmonds 1997). The rate of settlement of suspended sediment is also greater in water reservoirs or impoundments because water turbulence is generally lower in these waters.

Loss of riparian vegetation

Riparian vegetation is critical for preventing stream-bank erosion and 'filtering' catchment run-off. In many areas, riparian vegetation has been removed or is seriously degraded. Intensive grazing pressure has been implicated in the destruction of riparian and catchment vegetation in some of Western Australia's northern rivers and this has, in turn, led to soil and gully erosion and water quality problems (WRC 1997). The destruction of riparian vegetation and stock access to creeks both contribute significantly to high nutrient levels in King Island streams (Bobbi et al. 1999b).

Riparian vegetation is also important in light limitation of algal blooms and as a source of carbon to streams. The condition and extent of riparian vegetation is further discussed in Condition of riparian vegetation in selected Queensland catchments later in this report.

Periods of low or no flow

There are a number of factors apart from nutrient enrichment that increase the likelihood of algal blooms. One important factor is the frequency and length of low or no flow periods, as during these periods there is a reduction in water movement and turbulence, a greater penetration of sunlight (due to reduced water turbidity), stratification of the water column and warmer water temperatures. Higher flows also 'flush' and dilute nutrients and algal blooms (Harris 2001).

A study of the relationship between river flow and the number of the blue-green algae cells (Anabeena spp.) in the Maude and Hay weir pools on the Murrumbidgee River found that river flow and algal cell numbers were inversely related (Jones 1994), that is, the lower the flow the higher the number of cells.

In many regulated rivers, the periods of very low or no flow have increased. For example, the frequency of no flow at the Murray River mouth in South Australia has increased from one year in 20 under natural conditions to one year in two under present conditions (Baker et al. 1998).

River systems at risk from increased periods of low or no flow have a high proportion of their divertible yield developed (see Surface water resources). The divertible yield is an indicator of low flow volumes in most catchments, as it generally excludes higher flows that are difficult and expensive to capture and includes baseflows and most substantial low flows that are easy to capture. Table 18 shows the drainage regions where more than 40% of the divertible yield is developed. Most river systems at greatest risk from increased periods of low or no flow are in the Murray-Darling Basin. Drinking water supplies for Melbourne, Sydney, Brisbane and Perth have also increased the risk of low or no flows in nearby river systems.

Table 18: Drainage regions with more than 40% of divertible yield developed
Drainage region Developed yield
(GL/yr)		 Divertible yield
(GL/yr)		 Percentage of divertible yield developed
Snowy-Shoalhaven 1460 3545 41
Border Rivers 150 342 44
Burnett 373 797 47
Perth-Mandurah 243 427 57
Sydney 581 1020 60
Macquarie 417 713 58
Condamine 172 286 60
Goulburn-Loddon 2205 2838 78
Hamilton 84 106 79
Namoi-Gwydir 627 777 81
Lower Murray 602 735 82
Lachlan 570 680 84
Murrumbidgee 2140 2505 85
Melbourne 558 653 85
Menindee Lakes 354 409 87
Brisbane 555 563 99
Wimmera-Mallee 98 98 100
Barwon-Darling 192 192 100
Mid-Murray 3553 2103 168

Source: Modified from NLWRA 2000a.

Messages about contributing factors to eutrophication and algal blooms