Table 2.9.1 Resource status and trend summary of nutrient export (P) in selected catchments number Catchment/system Summary Hazard Trend Confidence in risk impact of agriculture Confidence in trend 1 Swan–Canning System The main sources of nutrients are agriculture, predominantly from cattle grazing in the Ellenbrook sub-catchment, and urbanisation. High Variable Adequate evidence and high level of consensus Limited evidence or limited consensus 2 Peel–Harvey System The main source of P is agriculture, particularly from broadacre dryland grazing of cattle. The estuary condition has improved with the construction of the Dawesville Channel; however, blooms and fish kills now occur in the lower reaches of major rivers. Very high Variable Adequate evidence and high of consensus Limited evidence or limited consensus 3 Leschenault Catchment The main nutrient losses originate from diffuse sources, including broadacre beef and dairy grazing and dairy sheds near streams. All monitored sites in the estuary and catchment exceed target levels. High Variable Adequate evidence and high level of consensus Limited evidence or limited consensus 4 Vasse–Geographe Catchment Most of the nutrient loads come from broadacre grazing of beef and dairy cattle. Upstream tributaries and wetlands have poor water quality. High Variable Adequate evidence and high of consensus Limited evidence or limited consensus 5 Scott River Catchment Agricultural land use is changing from dryland grazing to more intense irrigated enterprises, resulting in an increasing nutrient impact. Moderate Variable Adequate evidence and high level of consensus Limited evidence or limited consensus 6 Wilson Inlet Catchment The main diffuse sources of nutrients are beef and sheep grazing with broadacre cropping. Phosphorus concentrations in tributaries are increasing. Insufficient data for full assessment Variable Evidence and consensus too low to make an assessment Evidence and consensus too low to make an assessment 7 Oyster Harbour Catchment Most of the nutrient come from diffuse agricultural sources in higher rainfall areas supporting grazing enterprises. Insufficient data for full assessment Variable Evidence and consensus too low to make an assessment Evidence and consensus too low to make an assessment quality quality quality quality quality trial system trial
8 Bremer Catchment Most of the nutrient loads come from broadacre grazing and cropping. Moderate Variable Adequate evidence and high of consensus Limited evidence or limited consensus 9 Lake Warden Catchment Most of the nutrient loads come from diffuse agricultural sources, particularly broadacre grazing and cropping enterprises, and some farm forestry practices. High Variable Adequate evidence and high level of consensus Limited evidence or limited consensus Condition grades Very high High Moderate Low Very low Nutrient export hazard grades matrix Agricultural contribution to P load Significance of phosphorous load (P load above target) Very low (30%) Low (30–60%) Moderate (60–90%) High (90–120% Very high (>120%) >75% Moderate Moderate High Very High Very High 50–75% Low Moderate Moderate High High 25–50% Low Low Moderate High High <25% Very Low Low Low Moderate Moderate quality quality
Discussion and implications Phosphorus is an essential element for plant and animal growth and many of WA’s agricultural soils were originally amongst the most P deficient in the world (see Chapter 2.8). While P is critical for profitable agriculture, its export through surface water run-off and leaching increases the chance of algal blooms in receiving water bodies. In addition, organic matter flowing from the landscape to receiving water bodies depletes dissolved oxygen during decomposition. These two factors increase the likelihood of water problems. The degree of impact depends on characteristics of both the source and transport. Some of these source and transport factors can be managed to reduce the risk of export (Figure 2.9.4), while others are inherent to the landscape or the environment and are beyond the control of land managers.
Table 2.9.3 Phosphorus significance (loads) and agricultural P source Catchment Catchment area (km2) Current P load (t/year) Target P (t/year) % P over target* Agricultural P source (%) in beef/ P source (%) in cropping P source (%) in dairy P source (%) in P source (%) in other P source total (%) Swan–Canning System 2 090 26 12 117 33 – – 11 1 45 Peel–Harvey System 11 930 145 75 93 50 2.7 10.1 13.6 6.8 82.7 Leschenault Catchment 1 981 27.8 10.7 160 43 – 31 0.7 – 74.7 Vasse–Geographe Catchment 2 000 53 20 165 32 – 25.4 5.7 <1 63 River Catchment 700 11 8 38 33 – 45 – <1 84 Wilson Inlet Catchment 2 379 13 – – 60 – 11 10 2 82 Oyster Harbour Catchment 3 780 23 – – 67 2 4 – 11 84 Bremer Catchment 680 1.2 0.95 26 94.4 – – – – 94.4 Lake Warden Catchment 2 132 2.6 1.4 86 49 51 – – – 99 Data for the Swan–Canning and Peel–Harvey systems, and Leschenault, Scott River and Wilson Inlet catchments were obtained from their current water improvement plans. Data for the Oyster Harbour Catchment was obtained from Master (2007). Data for the Lake Warden Catchment was obtained from DAFWA (2009). Data for the Bremer Catchment was obtained from DAFWA (2008).