- Greater use of fertilisers results in increased nutrient concentration in waterways.
- High nutrient concentrations in northwest Tasmanian estuaries are rapidly flushed from the estuaries due to the large tidal range and high river flows in winter.
- Nutrient management interventions should focus on the way farmers manage and apply fertilisers over whole farms with special attention to risk factors on individual farms.
for Farming in Tasmania - Bill Cotching
13. NUTRIENTS IN TASMANIAN WATERWAYS
Impacts and drivers
Water quality is an inherent part of a healthy environment and is also important when surface waters are used as sources of community drinking water, swimming and water sports. Nitrogen and phosphorus are two of the main factors, along with light (turbidity), that control the growth of aquatic plants and animals. When these nutrients enter a waterway at greater than background rates, they can become an environmental problem resulting in excessive aquatic plant growth and reduced levels of dissolved oxygen or ‘eutrophication’, starving aquatic animals of oxygen.
Most farming operations in Tasmania add nutrients to the land as fertiliser and this increases the amount of nutrients that can get into aquatic ecosystems following heavy rainfall and heavy irrigation. Improved drainage and hill slope erosion can exacerbate this nutrient supply problem. Outflows from wastewater treatment plants and industrial sites are point sources of pollution that can also contain high levels of nutrients.
The intensive land uses of dairying and cropping are the major drivers of nutrient and sediments ending up in the rivers and estuaries in Tasmania. More fertilisers are applied to the land under dairying and cropping than other less intense land uses and this results in nutrient enrichment in waterways (Broad and Corkrey 2010). These intensive land uses occur in flatter landscapes with high rainfall and runoff and tend to be concentrated in north and northwest Tasmania where the highest catchment nutrient loads occur. East coast catchments with low rainfall and runoff, and less intense land use have the lowest nutrient loads. Nutrient generation rates of 27 kg/ha/yr of total nitrogen and 11 kg/ha/yr of total phosphorus from dairy pastures into surface waters are at the higher end of those reported in Australian studies (Table 13). The results for total nitrogen were found to be more consistent across all Tasmanian catchments than those of total phosphorus indicating that different processes of generation and attenuation were involved for each nutrient. Research found that dairy pastures in different parts of a catchment exhibited large variation in total phosphorus losses that depended on the rates of fertiliser applied and the connectedness of the landscape to the river systems via artificial drainage (Figure 10), (Broad and Cotching 2009, James and Cotching 2010). The high nutrient generation rates for urban and industrial land use should be noted as these can originate from point-based sources such as sewage treatment plants and industrial sites.
|Land use||TN generation rate (kg/ha/yr)||TP generation rate (kg/ha/yr)|
|Grazing modified pastures||1.0||0.2|
|Urban and industrial||14.0||2.2|
Table 13. Nutrient generation rates for total Nitrogen (TN) and total Phosphorus (TP) in Tasmania.
In Tasmania, areas used for pasture farming often have no native vegetation along the stream banks or the riparian vegetation is in poor condition which can result in bank erosion and decreased shading leading to high in-stream temperatures and high algal production. Poor quality riparian zones can also result in faster and more direct runoff to rivers with little chance for nutrient interception in the riparian zone. Research has found that when more than 40–50% of a catchment in Tasmania is used for grazing, significant negative impacts on the ecological condition of the waterways occur (Magierowski et al. 2010). These impacts may be caused directly by livestock entering the waterway for drinking water or by changes in the type and condition of riparian vegetation on farms used for animal grazing.
Tasmania contains some of the most pristine estuaries in Australia which come in a variety of types including coastal inlets, drowned river valleys, barrier estuaries, river estuaries and coastal lagoons (DPIW 2009). Estuaries on the north and west coasts have large tidal ranges (up to 3m) plus high rainfall and runoff, while estuaries on the east coast are relatively much drier and are subject to lower tidal ranges (<1m) (Edgar et al. 1999).
There has been much development around Tasmania’s estuaries with associated increased siltation, deterioration of water quality and loss of habitat (DPIW 2009). Research in Tasmania found that high nutrient concentrations in an estuary do not necessarily imply that an estuary is in poor condition, as these nutrients may be rapidly flushed from the estuary or may occur when other components essential for excess productivity, such as long hours of daylight and warm temperatures, do not occur (Ross et al. 2012).The Duck and Montagu estuaries in north-western Tasmania are good examples, as they have elevated nutrient concentrations, especially in winter, but indicators of algal growth (chlorophyll a) are still low. The reasons why algal blooms do not occur in these northwest estuaries are the high river flows in winter together with frequent and large tidal flushing resulting in short residence time of nutrients in the estuaries.
Figure 10. Artificial drainage provides strong connectedness of the landscape to the river systems resulting in a build-up in nutrients in waterways.
Using farm nutrient budgets and soil testing are key components in the management of nutrients on farms (see chapters 2 and 3) and if these nutrient management interventions are adopted across large proportions of individual catchments in Tasmania, then the load of nutrients reaching surface waters is likely to be reduced (Cotching et al. 2012).
Apart from good nutrient management, riparian zone management can be one of the most effective means of reducing non-point source pollution in farmed landscapes (Phillips 1989). This is because managed riparian zones function as barriers or filters to nutrient and sediment inputs from disturbance associated with agriculture and forestry. Using fencing to exclude livestock from streams can substantially improve water quality but the fencing needs to be installed over a large proportion of the minor streams in a catchment to be effective and even narrow buffers (10–30 m) can deliver substantial benefits (Smethurst and Petrone 2010). If livestock continue to have access to a stream and drainage from roads and tracks is not diverted across pasture or other vegetation before it reaches the stream, poor water quality will inevitably continue to be a problem. If grass and weed growth are problems in the fenced off riparian zones, then short term crash-grazing in dry weather can be adopted. Farmers wanting to explore tree farming options in riparian areas should seek advice from agro-forestry regulators and advisors.
The inherently poor drainage in some flatter and low-lying Tasmanian catchments is a significant factor limiting sustainable farming in this landscape. Production improvements have been derived from draining the swamp land, but the installation of man-made drains has provided a large degree of connection in the landscape that provides pathways for the easy and quick delivery of applied fertiliser nutrients to local rivers and streams (Cotching et al. 2010). Managing the rate and timing of fertiliser (Chapter 2) and effluent applications under dairying (Chapter 11) would provide more effective opportunities to improve water quality than the very difficult task of altering the drainage systems.
Management strategies under cropping that have shown to produce the lowest rates of nutrient leaching without compromising crop yield, are deficit irrigation (applying just enough water to make up for crop use plus evaporation), scheduling small irrigation applications at short intervals, and using lower rates and split applications of nitrogen fertiliser (Cotching and Lisson 2010).
Land management practices of nutrient budgeting, soil testing and riparian zone management are important to reduce the total nutrient delivery to rivers and estuaries, but even where best management practices are adopted, nutrients resulting from intensification of land use will still be delivered at higher than natural rates (Cotching et al. 2012). One of the consequences of more intense land uses producing greater nutrient enrichment in waterways, is that further intensification of land use in Tasmania associated with irrigation developments, is likely to result in greater surface water nutrient loads. This is more likely to be a problem in lower rainfall catchments with longer rivers, such as the Macquarie and South Esk, or in estuaries with restricted tidal flushing such as Boobyalla in the northeast.