Indicator: Aquatic Macroinvertebrates

Summary

Although affected by extreme events such as bushfires and droughts, the health of rivers in the ACT remained fairly static or improved slightly in the reporting period. River health in the ACT was largely determined by human activities, such as land use and river regulation. Removal of willow trees along Ginninderra Creek will affect the health of that water course, but the full effects are not yet apparent. The ACT’s watercourses are vulnerable, will remain so, and will need constant monitoring.

What the results tell us about the ACT

There were seasonal patterns of decline and recovery during the reporting period, particularly at urban sites, with autumn monitoring showing improvement over the previous spring period.

Sampling at the test sites indicated either relatively static conditions or some improvement, although conditions still ranged from severely impaired to similar to reference sites. Major impacts on river health at the test sites included nutrient enrichment from urban and rural runoff, sedimentation, habitat degradation and river regulation. All of these factors, with the exception of river regulation, relate directly to land use practices.

For example, the river health at sites in Tuggeranong Creek (58), Ginninderra Creek (64, 195, 196), Yarralumla Creek (189) and the Queanbeyan River (235), where the dominant land use above the catchment is urban, can be attributed to nutrient, sediment and pollutant-rich urban runoff. Furthermore, low dissolved oxygen levels reported at Ginninderra Creek (64, 195, 196) and the Molonglo River (242) can be attributed to decomposition of leaf fall and detritus.

Lack of shading by river bank vegetation and the shallow, warmer waters that are associated with prolonged drought contribute to algal growth and consequent changes in the abundance of some macroinvertebrate species. Removal of willow trees from Ginninderra Creek and Yarralumla Creek in 2000 will alter the creeks’ physical and chemical attributes and the composition of aquatic biota, but these changes will take some years to be fully expressed. Revegetation programs using native species will mitigate erosion and provide habitat.

In 2003, serious levels of river health impairment were identified at Ginninderra and Jerrabomberra Creeks and Paddys, Gudgenby, Queanbeyan and Molonglo Rivers. Urban sites experiencing low flow conditions were also seriously impaired. These conditions are considered to be a function of:

  • delivery of detritus by rainfall after the extensive bushfires
  • concentration of nutrients, chemicals and toxins by drought conditions
  • ‘toxic’ storm runoff events associated with infrequent rainfall during prolonged drought
  • changed rates of organic decomposition due to drought imposed low flow conditions
  • choking of habitat and sensitive biota by fine sediment deposition.

These impacts are the result of extreme events, but the vulnerable condition of parts of the landscape due to landuse also need to be acknowledged. Thus the impact of agricultural practices, residential development, and modified flow regime are compounding the problems associated with post-fire and continuing drought conditions. Assessments indicate that Yarralumla Creek, Ginninderra Creek, Queanbeyan River and Molonglo River are the sites most severely affected by human activities (Dickson et al. 2003).

The overall changes in stream health over the reporting period are shown in Tables 1 and 2 (in the downloadable pdf version). While it is clear that conditions in 2003 reflect the combined impacts of human activities and extremeevents, current system dynamics and the continuing drought suggest the ACT’s rivers are very vulnerable, and will remain so in future. While they remain vulnerable, rivers may either recover, adapt, or suffer further impairment. Continued monitoring will be necessary to establish their resilience and recovery.

Data sources and references

ACT Water Quality Reports (1999–2000, 2000–2001, 2001–2002) prepared and published by Environment ACT, Canberra. The reports can be viewed at the Territory and Municipal Services website

Dickson, A., Lemann, C. and Norris, R. 2003 ACT Water Quality Monitoring Program Macroinvertebrate Component. April 2003. Unpublished Report to Environment ACT (Cooperative Research Centre for Freshwater Ecology, University of Canberra, ACT).

Lemann, C., Nichols, S. and Norris, R. 2002 ACT Water Quality Monitoring Program Macroinvertebrate Component. November 2002. Unpublished Report to Environment ACT (Cooperative Research Centre for Freshwater Ecology, University of Canberra, ACT).

Mawer, D., Nichols, S., Lemann, C. and Norris, R. 2002 ACT Water Quality Monitoring Program Macroinvertebrate Component. April 2002. Unpublished Report to Environment ACT (Cooperative Research Centre for Freshwater Ecology, University of Canberra, ACT).

Sims, N., Mawer,D., Lemann, C. and Norris, R. 2001 ACT Water Quality Monitoring Program Macroinvertebrate Component. Final Report. Spring 2001. Unpublished Report to Environment ACT (Cooperative Research Centre for Freshwater Ecology, University of Canberra, ACT).

Sloane, P. and Norris, R. 2001 ACT Water Quality Monitoring Program Macroinvertebrate Component. Final Report. Autumn 2001. Unpublished Report to Environment ACT (Cooperative Research Centre for Freshwater Ecology, University of Canberra, ACT).

Notes

River health in the ACT is assessed using the AUSRIVAS method which investigates invertebrate diversity in specified stream sections (test sites) in relation to reference sites with similar habitat attributes. Testing is undertaken twice a year during autumn and spring, and this picks up differences caused by temperature, flow, leaf fall plants and life cycle stages of biota.

Test sites were selected on the basis of potential or known impacts, such as rural degradation, sedimentation, river regulation, discharge of treated sewage effluent and urban runoff. Reference sites are representative of a range of environmental conditions.

Major land uses represented in the assessment include native, rural and urban. Minor land uses include forestry, rural residential and cropping.

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