Macroinvertebrate response to streamflow and water quality stressors during the Millennium Drought.

Rivers are critical for freshwater biodiversity, yet they face significant challenges from natural and anthropogenic changes. The combined effects of these stressors on macroinvertebrate populations are less studied, particularly under severe drought conditions. This study investigates the combined effects of streamflow and water-quality variation on macroinvertebrate communities in the Goulburn River Catchment during the Millennium Drought, one of Australia's most severe climatic events. We analysed macroinvertebrate assemblages across five hydrologically distinct reaches of the Goulburn River, examining their responses to ecologically relevant streamflow indices and key water quality attributes. We adopted a comprehensive methodological approach, including redundancy analysis (RDA) and threshold indicator taxa analysis (TITAN), to identify critical environmental thresholds and influential variables. Results indicated significant spatial and temporal variations in taxa richness, abundance, diversity, and the EPT index between drought and post-drought periods. All reaches exhibited increased macroinvertebrate richness and abundance, with improved Shannon and EPT indices post-drought. In the unregulated Reach 1, taxa richness increased from 44 during the drought to 52 post-drought, with taxa abundance surging from 867 to 1902 individuals. Analysis of community composition (ANOSIM) indicated that taxonomic dissimilarities were greater during the drought than the post-drought period. Our study highlights the potential of effective management and restoration strategies, particularly their implications for environmental flow design and implementation.