River Health in the New South Wales Lower North Coast, Hunter and Central Coast Catchments

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A report of AusRivAS assessments 1994 - 1999
Grant Hose and Eren Turak
Environment Protection Agency, 2004
ISBN 0 642 55099 9 ISSN 1447-1280

3. Australian River Assessment System (AusRivAS) Explained (continued)

3.4 Interpretation of AusRivAS Results

The AusRivAS program provides a number of different outputs that can be used in assessing the condition of a river site. To gain the maximum information from these outputs it is important that a thorough understanding of the science behind the program is obtained.

In their simplest form the AusRivAS outputs do not require expert scientific interpretation. As a data user however, it is always important to be aware of the assumptions and limitations of data before any interpretations can be made. Further discussion on the interpretation of AusRivAS outputs can also be found in Coysh et al. (2000) and Barmuta et al. 2002

3.4.1 Interpreting O/E results

3.4.1.1. O/E results indicate taxa (fauna) richness

Where an O/E result classifies a site as either band X or Band A this result in its simplest form indicates that the assessed site was fauna rich. This result indicating that the integrity of the macroinvertebrate community was high. This may be the case even for sites that are aesthetically unpleasing as it is based upon a count of bugs present and not any visual interpretation of site condition!

While specific information about a single site may be of use, many users of AusRivAS methodology have a further interest in knowing what the results mean in the context of a whole river system or broader ecosystem health assessment and the subsequent management implications. It has generally been found that if the integrity of the macroinvertebrate community is high then the following is likely to be true:

  • The site has good water quality.
  • The water quality at the site over the past few months has been comparable to those of undisturbed rivers. More specifically key physico-chemical parameters such as water temperature, pH, dissolved oxygen concentrations and conductivity/salinity in the water are maintained within natural ranges. This would most likely mean that no serious pollution incidents or persistent release of contaminants have occurred recently.
  • Good Habitat diversity.
  • Many of the natural habitats of invertebrates in these rivers are still present or have been replaced by new, or equivalent habitats. Examples of these habitats are e.g. cobbles, boulders, leaf packs, logs, ribbon weed (Valisneria sp.) or water milfoil (Myriophyllum sp.) beds, overhanging banks, watergum or casuarina roots, trailing Lomandra sp., ferns or bottlebrushes. Examples of substitute habitats include bricks, timber from old buildings, willows (Salix sp.), and Egeria sp. beds.
  • Plentiful food sources.
  • A diverse range food sources support diversity in macroinvertebrates. These include leaves and twigs from the riparian vegetation, particulate and dissolved organic matter, or suitable prey species.

Whilst the above generalities may apply, it is always important to be aware of confounding factors that could result in incorrect site assessments. As always there is a possibility that incorrect site results were obtained. It is important to check results and interpretations. Some common reasons for poor results are:

  • Incorrect environmental information was entered.
  • Sampling was not carried out according to the protocol. For example, excessive effort was used.
  • The site is on an unusual type of stream and AusRivAS is not able to assess this site reliably.

From a management point of view, a good AusRivAS result may be interpreted in the following way:

  • Current management practices are good enough to maintain at least one of the significant components of the river ecosystem (macroinvertebrates).
  • The river still contains significant ecological values and biodiversity. This makes it worth investing resources into the maintenance or rehabilitation of the river.
3.4.1.2 Outputs indicate the taxa (fauna) were poor

A second straightforward scenario is if all results indicate the integrity of the macroinvertebrate community was significantly compromised (Band C or D). The following may cause these results:

  • There was a serious pollution incident prior to sampling.
  • Natural habitats in the stream have been lost.
  • The food sources of macroinvertebrates (such as leaves and twigs from the riparian vegetation, particulate and dissolved organic matter, or suitable prey species) were not available.
  • Although the conditions at the site are congenial, macroinvertebrates were unable to recolonise the site either because of a major barrier upstream that stops drift (e.g. dam) or because the macroinvertebrate assemblages in all the potential sources of colonisation had been impaired.
  • The stream system has experienced severe stress due to natural causes e.g. prolonged drought or floods.
  • The disturbances in the catchment have pushed key physical parameters (temperature, pH, dissolved oxygen, suspended solid concentrations, conductivity) outside the natural range, or beyond the range tolerated by those taxa.
  • Macroinvertebrate collection and picking was performed poorly or not according to the AusRivAS protocol (see Turak and Waddell 2001a).
  • The macroinvertebrate habitat available at the site is inadequate (see Turak and Waddell 2001a).
  • Incorrect environmental data were entered into AusRivAS.
  • The site is of an unusual type and AusRivAS is not providing reliable assessments for it. For example, samples collected from streams that are naturally very acidic (pHAusRivAS.

From a management point of view this result could mean any number of things and further information would be required before any meaningful management action can be taken. The first action should be to examine the above list and eliminate causes that are unlikely to be responsible for the poor macroinvertebrate population assessment score.

3.4.1.3. Interpreting the O/E score

Whilst the band of impairment will give a first indication as to whether or not the macroinvertebrate assemblages at the site are impaired, greater insight into the condition of the site can be gained from the O/E value itself. Where for example, an O/E-Taxa value of between 0.2 and 0.5 may both place the site in the same band (mostly C) the difference is a loss of 80 % and 50 % of the expected taxa respectively. This difference is very large. This is where the specific site characteristics need to be considered. For example, at a river site with a large dam upstream, which results in change in water flow and temperature regime, the loss of 50 % of the expected taxa is not surprising. However, losing as high as 80 % of the taxa would be unusual because there are quite a few invertebrate families that should be able to colonise and maintain populations below dams and these would invariably exceed 20 % of the expected taxa. Therefore an O/E value as low as 0.2 is likely to have been caused by factors additional to the dam.

3.4.1.4 Making Use of Taxa Lists

For further directions on the likely cause of poor macroinvertebrate fauna one would need to look at other AusRivAS outputs such as the list of taxa that were "expected and not collected" and those that were "not expected but collected". The "taxa expected and not collected" might help to pinpoint causes of ecological degradation. From understanding of the ecology of individual taxa, there is the opportunity to better understand physical conditions at specific sites. For example, at a site where episodic discharges of fine sediments from a construction activity has occurred, observations that a number of filter-feeding families are missing provides a step towards linking the construction activities to biological degradation. A resulting management priority should be controlling sediment input. For sites where most of the riparian vegetation has been lost, the absence of expected shredder taxa (ie those animals that feed on vegetation) might indicate that clearing has been too extensive and that the priority in management should be the re-establishment of the riparian vegetation. If, at a site where river flow has been markedly reduced, animals that live in fast flowing waters are expected but not collected, options for restoring natural flows must be considered.

The probability of individual taxa occurrences also provided useful information. Where low probability taxa are identified or absence this may provide further insight into the cause of ecological change. For example, if families that are known to have preference for warmer temperatures are collected from a small stream with cleared riparian vegetation this may indicate that the removal of vegetation has led to increased water temperatures and changes to the fauna

In summary, the knowledge the specific ecology of taxa groups can provide further information relating to possible causes of biological degradation and help in the defining of management priorities. AusRivAS provides a range of data and there will however be the potential for conflicts between the different assessment results This includes assessment based upon riffle and edge habitats, between O/E-SIGNAL and O/E-Taxa values, or between spring and autumn results. Frequently these conflicts together with the methods discussed above will help pinpoint the main cause of ecological degradation at the site.