Aquatic macroinvertebrate communities are routinely monitored around the Ranger uranium mine in the NT to assess if any change in community structure can be linked to the effects of minesite runoff. The Supervising Scientist Division has sampled macroinvertebrate communities in shallow waterbodies (artificial or natural billabongs) intermittently, in 5 years since 1995.
In each of the thirteen waterbodies studied, samples were collected from littoral macrophyte habitats at five locations. In 1995, 1996 and 2006 all of the samples were processed using only live sorting methods, with limited follow-up laboratory sample processing conducted in 1996. Due to time constraints associated with field sample processing, the processing method was changed in 2011. The live sort was removed and replaced with field preservation of samples and subsequent laboratory subsampling and processing under stereomicroscope.
Multivariate analysis of the community structure data indicate distinct differentiation by year, with the live-sort and laboratory-processing years separated from one another in MDS ordination. Such a distinct split between the years raises the question of whether there is an actual shift in the macroinvertebrate community structure over time, or whether this simply reflects the different sample processing methods. For impact assessment studies, sample processing differences will confound particular interannual comparisons but not necessarily those whose design incorporates a reference-to-exposed waterbody type comparison. Techniques for analysis are explored to further determine the validity of comparing data across multiple years where different processing methods have been adopted.