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Supervising Scientist Division

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Explanatory notes on ecosystem level responses

• Macroinvertebrate community structure
• Fish community structure

Monitoring using fish community structure

Assessment of fish communities in billabongs is conducted between late April and July each sampling year using non-destructive sampling methods applied in ‘exposed’ and ‘control’ locations. Two billabong types are sampled: deep channel billabongs studied every year, and shallow lowland billabongs dominated by aquatic plants which are studied every two years. Details of the sampling methods and sites were provided in the 2003–04 Supervising Scientist Annual Report (pp 35-41, chapter 2). These programs were reviewed in October 2006 and the refinements to their design detailed in the 2006–07 (pp 32-34, chapter 2) and 2007–08 (pp 32-35, chapter 2) Supervising Scientist Annual Reports (Shallow and channel billabong fish communities respectively).

For both deep channel and shallow lowland billabongs, comparisons are made between a directly-exposed billabong in the Magela Creek catchment downstream of Ranger Mine versus control billabongs from an independent catchment (Nourlangie Creek and Wirnmuyurr Creek). The similarity of fish communities in exposed sites to those in control sites is determined using multivariate dissimilarity indices, calculated for each sampling occasion. These indices are a measure of the extent to which fish communities of the two sites differ from one another. A value of zero percent indicates fish communities are identical in structure while a value of 100 percent indicates totally dissimilar communities, sharing no common species. A significant change or trend in the dissimilarity values over time could imply mining impact.

Channel billabongs

The similarity of fish communities in Mudginberri Billabong (exposed site) and Sandy Billabong was determined using multivariate dissimilarity indices calculated for each annual sampling occasion. A plot of the dissimilarity values from 1994 to 2011 is shown in Figure 1.

In previous reports, possible causes of trends in the annual paired-site dissimilarity measure over time have been advanced and assessed. Because the dissimilarity measure is most influenced by numerically-abundant fish species, it was possible to demonstrate that fluctuations in the measure over time were directly associated with longer-term changes in abundance in Magela Creek of the chequered rainbowfish (Melanotaenia splendida inornata), the most common fish species in this creek system Supervising Scientist Annual Report for 2003–04 (pp 35-38, chapter 2). Thus, effort has been directed at understanding the possible causes of interannual variations in the abundance of this fish species in Magela Creek.

In the Supervising Scientist Annual Report for 2008–09 (pp 31-35, chapter 2), a negative correlation between annual rainbowfish abundance in Mudginberri Billabong and the magnitude of wet season discharge (total for the wet season, January total and February total) was observed in Magela Creek. The negative relationship between rainbowfish in Mudginberri Billabong and wet season discharge identified in 2008–09 has been tested and remains significant (total for the wet season p=0.014, January total p=0.009 and February total p=0.014). This is supported by an examination of Figure 2 which shows the relatively low abundances of rainbowfish in Mudginberri Billabong in 2011 in relation to well-above average annual discharge in Magela Creek for that wet season. The reduced rainbowfish abundances after larger wet season flows may indicate greater upstream migration of rainbowfish past Mudginberri Billabong, thereby reducing the concentration of fish in Mudginberri Billabong during the recessional flow period.

The paired-billabong dissimilarity value for 2011 is consistent with the range of values reported since 2001, a period over which there has been no evidence of mine-associated changes to fish communities in Mudginberri Billabong downstream of Ranger (Supervising Scientist Annual Report 2008–09 (pp 31-35, chapter 2)).

Shallow lowland billabongs

Monitoring of fish communities in shallow billabongs has usually been conducted every other year (see Supervising Scientist Annual Report for 2006–07 (pp 32-34, chapter 2)). The last assessment of fish communities in shallow lowland billabongs was conducted in May 2009 with the results reported below. The scheduled sampling of fish communities in 2011 was postponed to enable staff resources to be dedicated towards an intensive sampling of other biota (phytoplankton, zooplankton and macroinvertebrate communities) in these shallow billabong habitats.

The monitoring program for fish communities in shallow billabongs is conducted in six billabongs, comprising three ‘control’ versus ‘exposed’ billabong pairs. In a similar manner to fish communities in channel billabongs (discussed above), the similarity of fish communities in the exposed billabong sites downstream of Ranger on Magela Creek (Georgetown, Coonjimba and Gulungul Billabongs) to those of the control sites (Sandy Shallow and Buba Billabongs on Nourlangie Creek and Wirnmuyurr Billabong – located on a Magela floodplain tributary) is determined using multivariate dissimilarity indices calculated for each sampling occasion. A plot of the dissimilarity values of the control-exposed site pairings – Coonjimba-Buba, Georgetown-Sandy Shallow and Gulungul- Wirnmuyurr Billabongs – from 1994 to the present, is shown in Figure 3.

The paired-site dissimilarities shown in Figure 3 range between 40 and 60% indicating fish communities in each of the billabongs comprising a site pairing are quite different from one another. In the Supervising Scientist Annual Report for 2006–07 (pp 32-34, chapter 2) it was identified that the particularly high dissimilarity values observed in the Coonjimba-Buba pairing for 2002 and 2007, and the Gulungul-Wirnmuyurr site pairing for 2002 (Figure 5), were attributable to high densities of particular aquatic plant types in one or both of the billabongs. Excessive plant densities are unfavourable for fish communities as fish movement, and hence residency, is physically prevented.

The influence of aquatic plants on fish community structure is further supported by the slightly increased dissimilarity observed in the Georgetown-Sandy Shallow pairing in 2009. The increased dissimilarity appears to be related to an increase in the density of the emergent aquatic plant, Eleocharis sp, in Georgetown Billabong, combined with reduced plant density (dominated by emergent lilies), in Sandy Billabong. The divergence in aquatic plant habitats between the two billabongs appears to have resulted in reduced similarity (increased dissimilarity) in fish community structures (Figure 3).

In the Supervising Scientist Annual Report for 2006–07 (pp 32-34, chapter 2), an increase over time was noted for the paired Coonjimba-Buba billabong dissimilarity values, irrespective of the removal of years 2002 and 2007, for which high values are associated with unusually high aquatic vegetation density in one or other of the billabongs (discussed above). The reduced dissimilarity reported for 2009 has allayed concerns of the possibility for increasing dissimilarity over time, as a weak relationship, only, is now present when the years 2002 and 2007 are included in the data analysis (p = 0.03).

Further information about ecosystem monitoring at Ranger Mine can be found in the Supervising Scientist Annual Reports.

• Supervising Scientist Annual Reports