Australia-Wide Assessment of River Health: Northern Territory Bioassessment Report (Final Report)
Department of Infrastructure, Planning and Environment
Monitoring River Health Initiative Technical Report Number 8
Environment Australia, 2002
ISBN 0 642 54840 4
About this report
The National River Health Program (NRHP) commenced in 1994 with the aim of improving management of Australia's rivers and streams. One of the programs with the NRHP was the Monitoring River Health Initiative (MRHI), later known as the Australia-wide Assessment of River Health (AWARH). The Department of Lands, Planning and Environment was the lead agency for the MRHI in the Northern Territory.
The MRHI aimed to develop predictive models for the assessment of the ecological condition of running water sites in each state and territory. Models were derived from data on the biota and environmental variables at sites considered to be in reference condition. Model outputs include a relative measure of taxa richness, the Observed to Expected ratio (O/E) based on the detection of aquatic macroinvertebrate families using standardised methods for sample collection and habitat measurement.
In the Northern Territory 199 reference and test sites were sampled between 1995 and 1999. Reference sites were distributed in catchments throughout the Top End of the Northern Territory. Most sampling effort, especially in the period 1998-99, was confined to the Adelaide, Daly, Finniss/Reynolds catchments and catchments draining the Darwin region. Assessments of stream health in this report are confined to these catchments and regions.
Thus far six predictive models have been assembled for assessment of stream condition based on the macroinvertebrate fauna of Edge and Sandbed habitats in the Early Dry, Late Dry and combined Early/Late Dry seasons. Classification of reference site data identified from two to four groups for each habitat/season model based on the similarity of the macroinvertebrate fauna at each site. In all cases differences between site groups for each model were relatively small, and for both Early Dry season models these differences may be confounded by seasonal effects. There was no clear geographic patterning in the distribution of site groups, and group membership frequently varied between years. Consequently, the models are not strongly underpinned by unambiguous ecological differences among stream types.
Sample sizes for most classes of disturbance were insufficient for rigorous analysis of model performance. However, some models were successful in detecting a gradient of mining pollution in one stream. Sites throughout the Darwin suburban and rural area rated consistently poorly, suggesting that there may be land management issues in these areas which need to be addressed.
The O/E SIGNAL index, based on the pollution tolerance of taxa, was found to provide an insensitive measure of water quality and habitat degradation and needs to be redeveloped prior to application in streams of northern Australia.
The current models may be improved following an iterative process of adjustment of the set of reference sites and predictor variables for each model. At present they should be applied, and the results interpreted, with caution and preferably within a sampling design which includes spatial replication and adjacent control sites.