Australian agriculture assessment 2001
National Land and Water Resources Audit, 2001
Nutrient loads to Australian rivers and estuaries
- Relative importance of different sources of nitrogen and phosphorus varies between river basins.
- The dominant sources of phosphorus (over 50%) are: hillslope erosion in Queensland and New South Wales; gully and river bank erosion, and dissolved phosphorus in run-off in coastal Victoria, South Australia, Western Australia and Tasmania; and in some basins urban point source discharges (e.g. 30% of the total load for Moreton Bay).
- Dissolved nitrogen in run-off makes up a greater proportion of the total load than dissolved phosphorus. Total nitrogen loads come mainly from hillslope erosion in Queensland and coastal New South Wales; contributions from hillslope erosion and dissolved nitrogen loads in run-off in the Murray-Darling Basin are comparable in magnitude; and over 60% of the total load occurs as dissolved run-off in coastal Victoria, South Australia, Tasmania and much of Western Australia.
- Management of nutrient exports will vary according to the relative dominance of nutrient sources.
Nutrient exports to receiving waters
- Total nutrient loads discharged from river basins are partly dictated by basin size-large basins export larger loads. Smaller basins can export large loads if they have high natural or induced export rates (e.g. due to steep slopes and intense rainfall; increases in population (sewage discharges) and changes in land use and management, such as intensive cropping on river flood plains).
- Efficiency of phosphorus delivery from rivers to the coast varies from as low as 3% in the Murray-Darling Basin to over 90% in Tasmania. Nitrogen deliveries vary from 14% for the Murray-Darling Basin to over 90% for Tasmania.
- The major sink for phosphorus and nitrogen is floodplain sedimentation, but reservoir sedimentation (for both nitrogen and phosphorus) and riverine denitrification (for nitrogen only) can account for significant proportions.
- Nearly 19,000 tonnes of total phosphorus and 141,000 tonnes of total nitrogen are predicted to be exported down rivers to the coast each year from areas of intensive agriculture: highest exports occur in the Far North, northern Queensland, Moreton Bay and coastal New South Wales.
Changes in river nutrient loads
- Changes in nutrient load can indicate where important changes in water quality may have taken place. However, as nutrient loads in many rivers are dominated by storm events, changes in nutrient loads do not give a complete picture of the nutrient aspects of water quality.
- Annual total phosphorus loads in river networks averaged nearly 3 times higher than estimates for pre-European settlement levels. Average annual total nitrogen loads were estimated to be more than double pre-European settlement levels.
- In over 100,000 km of river (60% of river length assessed), total phosphorus loads had increased by more than 3.5 times, possibly resulting in substantial ecological changes. Total nitrogen loads increased by this amount in about 30% of the assessed river length.
- The greatest nutrient load impacts arising from catchment disturbances were predicted to be in the Burdekin, Murray-Darling, Murchison and Greenough basins.
* Due to a lack of data, intensive rural industries (e.g. feedlots and piggeries) were not included. However it should be noted that these industries are subject to certification, specific industry guidelines, and are regulated and monitored through State government activities.
Increases in river nutrient loads generally lead to increases in the production of algae and aquatic plants, with follow-on effects up the aquatic food chain. Large nutrient increases typically favour a small number of species at the expense of others, and so while overall system productivity is increased, biodiversity is reduced. The reduced diversity of species is often associated with reduced system resilience, and catastrophic collapses are common. Such collapses may include the death and decay of large algal blooms, thereby increasing biological oxygen demand, lowering dissolved oxygen levels and leading to massive fish kills and high mortality amongst other river fauna (see Australian Catchment, River and Estuary Assessment 2001 for the Audit river and estuary assessments).
River nutrient budgets for phosphorus and nitrogen allow determination of:
- major sources of nutrients to rivers;
- major loss pathways for nutrients transported through river systems; and
- magnitude of nutrient exported to estuaries and the coast.
They are linked to landscape nutrient budgets, because erosion and surface run-off are important pathways for nutrient loss from the landscape. An understanding of the fate of nutrients lost from landscapes and ecological responses to nutrient loads in the receiving waters, can help guide land and water planning and management.
A modelling approach was developed to combine outputs from erosion and river sediment transport modelling, with landscape-plant-soil-atmosphere-nutrient flux modelling and point source discharge data. River nutrient transport modelling considers dissolved nutrients that are associated with those bound to suspended sediments. Exchanges between these forms are modelled for phosphorus. Losses from transport include:
- fine sediment deposited in reservoirs and on floodplains; and
- denitrification of dissolved nitrogen to nitrogen gas.
Modelling river nutrient transport
Agricultural and urban disturbance within a catchment leads to increases in nutrients exported to the river systems. These increased nutrient loads affect river ecosystems, usually in undesirable ways. Assessing changes in nutrient loadings is therefore an important aspect for assessing river condition, and one that highlights the linkages between a river and its catchment.
Assessing river nutrient load is complex-either using measured data or by modelling-because of complex processes involved in nutrient sourcing and transport, and the highly time dependent variability of river flow. Process modelling is usually carried out in conjunction with detailed daily hydrology modelling. However, this is not required for broad-scale assessments of changes, and in any case sufficient data are often not available.
A model of river nutrient transport (Annual Network Nutrient Export-see box) was developed to predict current and pre-European nutrient loads in Australian rivers.
Losses of sediment-bound nutrients occur due to fine sediment deposition and long-term storage on floodplains and in reservoirs. These terms were estimated using the Sediment river Network (SedNet) model (Prosser et al. 2001) and assumed that:
- floodplain deposition accords to settling velocity theory; and
- reservoir deposition uses a modified 'Brune rule' (Brune 1953) that estimates the trap efficiency of a reservoir as a function of the storage capacity and the annual flow volume.
Nutrients not lost from transport by fine sediment deposition or denitrification are exported from the river network. As the Annual Network Nutrient Export model does not represent estuarine nutrient transport and transformations, 'exports' from the river network combines nutrient delivery to estuarine and near-shore marine environments.
ANNUAL NETWORK NUTRIENT EXPORT (ANNEX)
- Sums nutrient sources delivered to each link* of a river network, and accumulates the consequent loads to determine average annual exports.
- Combines soil nutrient concentrations from Australian Soil Resources Information System (see Appendix 2) with estimates of average annual sediment loads from SedNet modelling (see Water-borne erosion section) to estimate the average annual nutrient loads to rivers associated with water erosion.
- Combines estimates of average annual nutrient loads for surface run-off from BIOS modelling (see Landscape balances section), with point source data from the National Pollutant Inventory (www.environment.gov.au/epg/npi/database/database.html) to estimate the average annual loads of dissolved nutrient to rivers.
Annual Network Nutrient Export considers the following nutrient source terms at each network link (Figure 6.1):
- sediment-attached nutrients from hillslope erosion (from SedNet);
- sediment-attached nutrients from gully erosion (from SedNet);
- sediment-attached nutrients from river channel bank erosion (from SedNet);
- dissolved nutrients in surface run-off and subsurface drainage (from BIOS/Equil); and
- point source nutrient discharges (from National Pollutant Inventory database).
Nutrients are transported in both dissolved and sediment-attached forms. The model assumes that the:
- sediment-attached nutrient load is associated with the clay fraction of the sediment being transported entirely in suspension; and
- capacity for transport of nutrients both in dissolved forms and associated with suspended sediments is unlimited.
Annual Network Nutrient Export also models the exchange of phosphorus between the suspended sediment and dissolved forms. This process is described by an adsorption coefficient (Ks) that expresses changes in particulate phosphorus concentrations as a ratio of the resulting proportional change in dissolved phosphorus concentrations. Ks is largely determined by sediment particle size and mineralogy. The model assumes that:
- the concentration of phosphorus adsorbed to the suspended sediment is in equilibrium with the dissolved phosphorus concentration;
- the system is in steady-state;
- phosphorus transport associated with phytoplankton is a small component of the total budget; and
- there is no exchange between dissolved and sediment-bound nitrogen.
Loss of dissolved nitrogen by denitrification is modelled as an exponential decay process dependent on the residence time of flow in the network link, water temperature, and a rate constant that varies according to river substrate type
* A link is the stretch of river between tributaries.
The processes that control movement of nutrients and sediments in Australia's estuaries vary, but can be grouped into six broad categories (Figure 6.2). Tidal-dominated estuaries move suspended sediments and associated bound and dissolved nutrients relatively efficiently from the estuary to the near shore and marine environments. Table 6.1 outlines the major process for deposition and movement to sediments by estuary types.
* Details of the assessment of Australia's estuaries are reported in Australian Catchment, River and Estuary Assessment 2001 and the Australian Natural Resources Atlas.
Assessment area and scope
The assessment area defined by the Working Group on Land Resource Assessment (as part of the Australian Soil Resources Information System project) includes:
- all river basins that contain areas of intensive land use;
- selected river basins (in the Northern Territory and the western division of the Murray-Darling Basin) that do not contain intensive land use, but for which resource data were available-whole river basins were used so that processes such as hydrology and sediment and nutrient movement could be modelled and balanced over entire river basins.
The Annual Network Nutrient Export model was run for each of the 18 regions (see Figure 5.1, p. 163) used by SedNet to provide prediction of nutrient budget terms link-by-link* for river networks across the assessment area. The model was calibrated using estimates of nutrient loads from field measurements from 93 sites across Australia. Detailed descriptions of the model and the calibration process are provided in a technical report available on the Australian Natural Resources Atlas.
Biological nutrient stores in rivers are ignored by Annual Network Nutrient Export, but predictions of increased nutrient loads indicate the potential for changed biological response:
- increases in nutrient loads can lead to increases in the frequency and size of algal blooms; and
- increases in nutrient loads have been shown to increase the biomass of aquatic plants and higher trophic levels, and to change the composition of aquatic communities often leading to reduced biodiversity (e.g. Schindler et al. 1971, Cole 1973, Edmonson & Lehman 1981, Vollenweider 1992).
Aquatic organisms respond to nutrient concentrations rather than to nutrient loads. Nutrient concentrations will generally:
- increase with increasing nutrient loads; and
- vary with flow and other aspects of water quality.
Predicting actual ecological response to increased nutrient loads is difficult - a large proportion of the load is moved by floods and may pass through the river network with little ecological impact (e.g. rivers with high efficiency of sediment and nutrient delivery). Reservoirs and estuaries represent nutrient stores, and because all of the nutrients stored in these water bodies can be considered as ultimately bio-available, ecological responses are more closely related to total nutrient loads.
* A link is the stretch of river between tributaries.
RIVER NUTRIENT BUDGETS AND NET EXPORTS
Constructing nutrient budgets
Budgets of average annual total nutrient loads were constructed for each major region and for each river basin (Tables 6.2, 6.3, 6.4, 6.5 and Appendix 1); only nutrient budgets were compiled for the connected drainage network for the Murray-Darling Basin region (i.e. those streams and rivers that are connected through the network to the basin outlet). Three river basins (Paroo River, Lake George, Wimmera-Avon Rivers) do not contribute to the basin outlet in this network. Estimates of net exports for river basins include contributions from upstream basins, to cope with the subdivision of the Murray-Darling Basin and the Avon (Western Australia) basin into multiple river basins. In the Murray-Darling it was assumed that:
- the Murray-Riverina receives the outputs of the Upper Murray, Kiewa, Ovens, Broken, Goulburn, Campaspe, Loddon;
- the Murray then flows along the border of Benanee (where the outputs from the Lachlan and Murrumbidgee are added);
- the Darling receives the outputs of the Border Rivers, Moonie, Condamine-Culgoa, Warrego, Gwydir, Namoi, Castlereagh, and Macquarie-Bogan; and
- the Lower Murray receives the outputs from the Darling basin and the Murray River in the Benanee basin, as well as small inputs from the Mallee.
Nutrient budget values for basins along the Murray River are less accurate than statistics for other basins, because the Murray River forms the boundary of some basins. Moreover, while the river is nominally defined to be in New South Wales, the location of the river in the 9 second digital elevation model (used to define the river network for this assessment) falls alternately in adjoining basins, depending on the mapped location of the basin boundary. This puts different sections of the Murray River into each of these adjoining basins.View reporting regions map
Table 6.2 Total phosphorus budgets (t/yr) by major region.
|Far North Qld||2 942||106||144||0||114||1 184||0||2 122||11||2.6|
|North Qld||3 966||129||187||155||158||1 292||140||3 163||17||2.3|
|Burdekin||11 909||1 271||309||0||178||8 185||2 960||2 522||13||5.9|
|Fitzroy||9 526||1 008||519||0||248||8 377||919||2 006||11||7.0|
|Moreton Bay||971||93||209||597||73||538||254||1 153||6||4.4|
|Qld South||2 426||336||277||53||164||1 748||362||1 146||6||4.4|
|Murray-Darling Basin||10 719||4 387||4 434||124||1 306||16 295||3 977||699||4||2.6|
|NSW North||987||277||396||5||199||572||10||1 282||7||3.6|
|NSW South||1 902||667||428||101||283||1 314||279||1 788||9||2.8|
|WA South||53||1 009||299||146||569||1 254||17||805||4||2.6|
|Totals/averages||46 086||10 312||8 138||1 331||4 240||42 194||8 992||18 921||100||2.8|
TP is total phosphorus, PP is particulate (sediment-bound) phosphorus, DP is dissolved phosphorus. All loads are in tonnes per year (t/yr), and the export percent is the region export as a percentage of the assessment area total. Times natural is the network link-averaged increase in multiples of the pre-European load.
Table 6.3 Relative (%) magnitude of phosphorus source and sink terms for the major regions and for the assessment area.
|Far North Queensland||89||3||4||0||3||36||0||64|
|New South Wales North||53||15||21||0||11||31||1||69|
|New South Wales South||56||20||13||3||8||39||8||53|
|South Australia Gulf||25||35||13||2||26||52||3||44|
|Western Australia South||3||49||14||7||27||60||1||39|
Table 6.4 Nitrogen budgets (t/yr) by major region.
|Far North Qld||15 457||425||575||0||2 487||5 826||0||190||12 928||5||1.9|
|North Qld||15 789||515||746||175||3 486||5 123||559||174||14 854||6||2.2|
|Burdekin||33 615||5 084||1 234||0||3 915||23 528||7 695||2 359||10 266||12||3.6|
|Fitzroy||29 108||4 033||2 077||0||5 692||27 086||3 112||1 877||8 834||11||3.3|
|Moreton Bay||5 205||373||837||1 348||1 616||2 035||1 747||244||5 353||3||2.6|
|Qld South||9 179||1 346||1 108||49||3 646||6 501||1 290||714||6 822||4||2.4|
|Murray-Darling Basin||36 352||17 556||17 736||1 208||33 126||53 309||15 952||22 388||14 330||29||2.1|
|NSW North||7 759||1 108||1 583||0||4 511||3 682||38||427||10 815||4||2.2|
|NSW South||8 926||2 666||1 710||663||6 442||5 364||1 590||804||12 650||6||1.8|
|Vic East||1 500||731||865||253||5 897||1 222||39||699||7 285||3||1.3|
|Vic West||527||851||694||0||3 318||1 167||78||537||3 609||1||1.7|
|SA Gulf||905||815||313||31||3 975||1 294||65||1 349||3 332||2||2.6|
|WA South||982||4 038||1 196||694||15 282||4 481||57||5 250||12 404||6||2.5|
|Indian||445||1 368||444||0||2 032||1 972||0||1 016||1 301||1||2.7|
|Tasmania||1 536||348||1 432||518||6 774||369||255||170||9 815||3||1.2|
|Totals/averages||170 264||43 999||34 130||4 938||108 792||148 605||32 559||39 699||141 258||100||2.1|
TN is total nitrogen, PN is particulate (sediment-bound) nitrogen, DN is dissolved nitrogen and all loads are in t/yr. Times natural is the network link-averaged increase in multiples of the natural load.
Table 6.5 Relative (%) magnitude of nitrogen source and sink terms for the major regions and for the assessment area.
|Far North Qld||82||2||3||0||13||31||0||1||68|
|View reporting regions map|
A strong contrast exists between patterns of phosphorus and nitrogen export. This is mainly due to the greater proportion of the total nitrogen load being transported in dissolved form. The low sediment delivery ratio dominating total phosphorus delivery for the Murray-Darling is a lesser influence on total nitrogen delivery.
- On average, it is estimated that from the rivers assessed, nearly 19 000 tonnes of total phosphorus are currently exported to the Australian coast each year: the largest export is from North Queensland, and over half the total exported comes from just four regions: North Queensland, Far North Queensland, the Burdekin, and the Fitzroy. This represents an average of nearly 10 000 tonnes of phosphorus delivered to the Queensland estuaries and coast each year.
- The average total nitrogen export is estimated to be around 141 000 tonnes per year, with the largest export being from the Murray-Darling.
- Five river basins are predicted to be net sinks for phosphorus: Murray - Riverina, Benanee, Castlereagh*, Darling and Lower Murray.
- At the basin level, the Burdekin, followed by the Fitzroy, are the largest exporters of phosphorus because although their export rates are not excessive, they are both large basins. The pattern of phosphorus net exports by river basin (Figure 6.4) shows the predominance of these two basins, but also highlights large exports from the Normanby, Herbert, and Brisbane rivers in Queensland and the Clarence and Hunter rivers New South Wales, as well as the large Murray-Darling Basin.
- The pattern of nitrogen net exports from basins (Figure 4) shows large exports from the big Murray-Darling, Burdekin and Fitzroy basins. Substantial nitrogen loads are exported from several of the river basins within the Murray-Darling, in particular the Murray and Murrumbidgee rivers.
* The Castlereagh result is spurious, and is caused by the length of the Darling River that flows along its border in which significant deposition occurs. This is a result of the river and basin mapping, as outlined for the Murray River.
Relative sizes of sources and sinks
- Most phosphorus comes from hillslope erosion (65%) with loads as high as 85% in those regions contributing the largest loads.
- Gully erosion contributes a high proportion of total phosphorus load in some regions (e.g. gully erosion represents 59% of the load in the Indian Ocean region).
- Urban point source discharges represent 31% of the total phosphorus load in the Moreton Bay region. A large proportion of this enters the lower Brisbane River and can be expected to have significant ecological consequences for the lower river and Moreton Bay.
- 60% of the phosphorus load that reaches river networks is deposited on floodplains with fine sediment.
- 27% of the phosphorus load that reaches river networks is exported to the coast.
- 13% of the phosphorus load that reaches river networks is deposited in reservoirs with fine sediment. This particulate phosphorus should be considered in the long term as being bio-available for phytoplankton growth. The largest load of particulate phosphorus deposited in reservoirs occurs in the Murray-Darling, a region with a large number of reservoirs and a recent history of reservoir cyanobacteria bloom problems.
The efficiency of phosphorus delivery varies greatly between the regions.
- Small coastal catchments in Tasmania export over 90% of the phosphorus that reaches the drainage network.
- In the Murray-Darling only 3% of phosphorus is exported, but this applies only to the connected drainage network. This delivery ratio would be lower, if large areas of unconnected drainage had been assessed.
The sources of sediment for particulate nitrogen were assumed to be the same as those for particulate phosphorus.
- Most nitrogen comes from hillslope erosion (65%) with loads as high as 85% in those regions contributing the largest loads.
- Gully erosion contributes a high proportion of total nitrogen load in some regions.
Dissolved nitrogen contributes a greater proportion of the total nitrogen source than does dissolved phosphorus of the total phosphorus source.
- On average, from 30% to 69% of total nitrogen load exists as dissolved nitrogen in the Western Australia's southern region.
Information on the sinks for nitrogen provides valuable guidance for land and stream management. With less of the total nitrogen load transported in particulate forms, the percentage losses to floodplain and reservoir sedimentation are lower.
- Overall, export is 39% on average because of extra losses (11% on average) of nitrogen from the river system through denitrification. The percentage denitrification losses vary between regions from as high as 22% in Western Australia South and Indian Ocean regions, to as low as 1% in North and Far North Queensland. Denitrification losses are highest where the water residence times are longest in the lower gradient rivers with extended periods of low or very low flows.
- Percentage exports range from 93% in Tasmania where the dissolved nitrogen fraction is high and denitrification losses are low (down to 14% in the Murray-Darling where both sedimentation and denitrification losses are high).
Area-based nutrient export rates
Mapping total nutrient loads across drainage network links (Figures 6.6, 6.7) shows that the largest rivers generally carry the largest nutrient loads. Dividing these loads by the upstream catchment or basin area provides area-based nutrient export rates. Export rates indicate the differences in nutrient source intensity from:
- natural differences in topography and rainfall that give rise to natural differences in erosion rates; and
- differences due to intensity of resource use-more intensive resource use increases export rates by increasing soil erosion, loss of fertiliser nutrients, run-off of animal wastes and point source discharges.
The highest export rates are generally a result of high erosion rates driven by topography, rainfall and land use.
- Averaged across river basins, the total phosphorus export rate was predicted to be 0.11 kg/ha/yr and the total nitrogen export rate was estimated at 0.85 kg/ha/yr.
- Export rates ranged from zero to as high as of 2.4 kg/ha/yr for total phosphorus and to 10.5 kg/ha/yr for total nitrogen from the Mulgrave - Russell rivers in Far North Queensland (Figures 6.4, 6.5).
Changes in total nutrient loads
Increases in nutrient load
Across the assesed basins, the average total annual phosphorus load increased 2.8 times relative to pre-European levels.
- At the region scale, the river link-averged increases ranged from a seven times increase in the Fitzroy basin to a 30% increase in Tasmania (Table 6.2).
- At the river basin scale (Appendix 1, Figure 6.9), total phosporus loads have increased by more than 10 times in the river networks of the Proserpine, O'Connell and Styx basins in Queensland, all of which drain to the Great Barrier Reef lagoon.
Significant increases in total phosphorus of between 5 and 10 times have ocurred in the rivers of the large Burdekin, Fitzroy, Burnett, and Brisbane basins in Queensland; nine other smaller basins along the Queensland coast; the Border Rivers, Gwydir and Namoi basins in upper Murray-Darling Basin; and the Greenough and Murchison basins in Western Australia.
Across the assessed basins the average total annual nitrogen loads increased 2.1 times.
- At the region scale, the river link-averaged increase ranges from a 3.6 times increase in the Burdekin to a 20% increase in Tasmania (Table 6.4).
- At the river basin scale (Appendix 1, Figure 6.10), total nitrogen loads have increased by more than 4 times in the river networks of the Don, O'Connell, Styx and Proserpine basins in Queensland; and in the Wakefield basin in South Australia.
- Significant increases in total nitrogen loads of between 3 and 4 times have occurred in the rivers of the large Burdekin, Fitzroy, Burnett, and Brisbane basins in Queensland; five other smaller basins along the Queensland coast; the Gwydir and Namoi basins in upper Murray-Darling Basin; the Macleay River in coastal New South Wales; the Broughton and Lake Torrens basins in South Australia; and the Murchison, Blackwood, Frankland and Avon basins in Western Australia.
Basin averages must be interpreted with care, since increases in nutrient loads are not evenly distributed (Figures 6.11, 6.12). For example, several basins in the New South Wales section of Murray-Darling Basin showed much larger increases in total phosphorus in the upper reaches compared to the lower reaches, because of intense gully erosion along the western slopes of the Great Dividing Range.
Where nutrient loads have increased by over 3.5 times, it is likely that aspects of the rivers will be substantially degraded:
- In many basins (particularly in Queensland and Western Australia) over 75% of the stream length has been degraded by increases in total phosphorus loads (Figure 6.13).
- In a few basins (including the Avon, the Gwydir, the Broughton, the Wakefield and several coastal Queensland basins) over 75% of the stream length has been degraded by increases in total nitrogen loads (Figure 6.14).
Not all basins with large increases in river nutrient loading also export large loads. Comparison of river link-averaged values indicates potential impacts on river condition, but does not necessarily translate to equivalent increases in basin export (e.g. the Paroo basin shows a moderate increase in link-averaged phosphorus loading, but does not export phosphorus under the natural or the currently modelled scenario).
Nutrient exports need to be linked back to catchment land uses, because nutrient loss pathways for dissolved and sediment-bound nutrients differ and may require different control practices.
In the case of phosphorus, exchanges between dissolved (in water) and sediment-bound phosphorus pools occur during transport. The percentage increase in the load that is caused by each of four major source types has been determined (Appendix 1).
- phosphorus load increases are dominated by increases in channel (gully and stream bank) erosion in 61% of basins (or 45% by basin area); by increases in hillslope erosion in 36% of basins (or 55% by basin area), and by point sources in just four basins (Maroochy, the Queensland South Coast, the Onkaparinga and the Swan Coast-less than 1% by basin area).
- Nitrogen load increases are dominated by increases in channel erosion in 40% of basins (or 39% by basin area); by increases in hillslope erosion in 40% of basins (or 49% by basin area); by increases in surface run-off loads in 17% of basins (or 10% by basin area); and by point sources in four basins (Queensland South Coast, the Maribyrnong, the Onkaparinga and the Swan Coast-less than 1% by basin area).
- Nitrogen load increases in several basins are dominated by dissolved nitrogen in surface run-off. These increases reflect both increased nutrient inputs via fertiliser and dissolved organic losses associated with changes in vegetation type. Particular care with fertiliser management is required in those regions where increases in dissolved nutrient loads are a high percentage of the total increase.
NUTRIENT FORMS AND LIKELY ECOLOGICAL IMPACTS
Particulate phosphorus occurs in organic and inorganic forms (inorganic particulate phosphorus mainly occurs as phosphate ions chemically adsorbed to eroded clay mineral particles)
Dissolved phosphorus is immediately available to aquatic plants and algae, but particulate phosphorus can change into the dissolved form as local water quality conditions change.
Most or all of the phosphorus in transport has the potential to be bio-available in the long term. In the short term, the ability of the suspended sediment in a river to provide phosphorus for algal growth beyond that in the dissolved fraction is described by the relative buffering capacity (Froelich 1988), which is proportional to the inverse of the ratio of dissolved phosphorus to total phosphorus. That is, for low values of this ratio the buffering capacity is high, and for high values of this ratio the buffering capacity is low.
Variation in the ratio of dissolved to total phosphorus (Figures 6.15, 6.16) shows a spatial pattern in relative buffering capacity, and hence differences between rivers in their short-term ability to provide phosphorus from suspended sediments for algal growth.
- At the river basin scale, this ratio ranges from 0.01 in the Lake Torrens basin in South Australia to 0.97 in the Pieman River basin in Tasmania; with a basin average of 0.28. Generally, the highest values are for the rivers of Tasmania, east Victoria and parts of the south west of Western Australia. The low values for the major inland basins and the rivers of the Indian Ocean region, indicate the high relative buffering capacity of these rivers. In these basins, suspended sediments can be expected to act as a reasonably available source of phosphorus for algal growth should dissolved phosphorus become depleted.
Most of the particulate nitrogen is organic, although some exists as ammonium chemically bound to sediments.
Dissolved nitrogen is more available to most aquatic plants and algae than other forms, although ammonium is relatively easily stripped from sediments for biological uptake. However, ammonium is only a small fraction of total load and the ratio of the dissolved load to the total nitrogen load is a reasonable indicator of the proportion of the nitrogen load that is readily available to aquatic plants and algae (Figures 6.17, 6.18).
- At the river basin scale, the nitrogen ratio ranges from 0.08 in the Don River in Queensland to 0.98 in the Shannon River in the south-west of Western Australia. The Australian basin average was 0.61. Generally, the highest values are for the rivers of south-west Western Australia, north-west Tasmania and parts of Victoria. In these basins, a high proportion of the total nitrogen load will be readily available for algal growth.
The ratio of nitrogen to phosphorus in water indicates their relative availability to aquatic organisms. This ratio in algae approximates to the relative amounts of these two nutrients actually used by the algae.
An expression of the nitrogen to phosphorus ratio is known as the 'Redfield ratio' (Redfield 1958). The 'Redfield ratio' defines a threshold value (approximately 6.8, by weight) which can be used to evaluate the nutrient status of a water body.
Total nitrogen to total phosphorus ratios vary for pre-European conditions from slightly less than 6.8 to nearly 20 (Table 6.6). Where turbidity is not extreme and these ratios are high (e.g. south-west of Western Australia) growth of phytoplankton is likely to be phosphorus limited.
When the Redfield ratio is greater than 6.8 the water body is regarded as phosphorus deficient, and when it is less than 6.8 it is regarded as nitrogen deficient. Nitrogen deficient conditions favour those species of algae (including many blue-green algal species) that fix atmospheric nitrogen.
The Redfield ratio must be interpreted with caution because of the differing bio-availability of different nutrient forms under different river conditions. They are only a strong determinant of phytoplankton community structure in situations where population growth is primarily limited by nutrient supply. The ratio is less relevant in very turbid rivers, where phytoplankton growth is mainly limited by light.
Table 6.6 Total nitrogen to total phosphorus ratios for the current and natural modelled scenarios, and the ratio of total nitrogen to total phosphorus between current and natural for the major modelling regions.
|Region||Current TN/TP||Natural TN/TP||Current/natural TN/TP|
|Far North Queensland||6.3||8.8||0.77|
|New South Wales North||9.9||17.6||0.57|
|New South Wales South||10.0||16.3||0.61|
|South Australia Gulf||21.5||18.2||1.23|
|Western Australia South||24.7||22.4||1.11|
- Averaged across the entire river network, the pre-disturbance total nitrogen to total phosphorus ratio was close to 17, and is now currently 12.5. This reduction is generally caused by increased sediment loads, that in relative terms, increase total phosphorus loads more than total nitrogen.
- The ratio has increased in some areas (e.g. in parts of south Western Australia, South Australia, Victoria, Tasmania and the lowland rivers of the Murray-Darling Basin [Figures 6.19, 6.20]). However, in these areas the ratios were naturally high, and the change is unlikely to be ecologically important.
IMPLICATIONS FOR LAND AND WATER MANAGEMENT
- As a consequence of long residence times of fine sediment stores in parts of river systems, the long-term availability of nutrients to river ecosystems has increased in many areas. Dealing with nutrient sources is necessary, but management of these riverine nutrient stores is also required in the future, if adverse ecological responses are to be minimised.
- Priority areas for reducing river and estuarine nutrient loads are likely to differ. Large relative increases in river nutrient loads do not always coincide with large total exports, and estuaries differ in their sensitivity to increases in nutrient loading, particularly because of differences in residence times and tidal flushing.
- Erosion control and management will provide a significant benefit to managing supply of nutrients from increased sediment loads to most rivers.
- In areas where a large part of the increase is caused by surface run-off loads or point source discharges, close attention needs to be given to fertiliser applications, ensuring appropriate amounts are applied at appropriate times, and with application methods that minimise run-off losses.
- Point sources from intensive rural industries may be dominant nutrient sources in some catchments. They deserve close attention to ensure adequate retention of nutrients occur on-site.
- The dominant impact of storm events in determining total nutrient loads may mask spatial patterns in the changes in nutrient concentrations at low flows. This is important in determining ecological responses (e.g. the substantial increases in nutrient loads that are predicted to have occurred in some Far North Queensland basins do not match local knowledge of their ecological impact). Further work is required to improve the modelling of regional-scale water quality changes in rivers and land use mapping to include intensity and practice attributes.
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