Extreme Hydrological Drought Threatens the Endemic Freshwater Mussel, Alathyria jacksoni Iredale, 1934, Northern Murray–Darling Basin, Australia

Freshwater mussels are ecosystem engineers: They modify substrates through burrowing, mediate water quality through filtration, provide food and habitat for other organisms and play a substantial role in the biogeochemical cycling of nutrients; they are also seriously threatened globally. This study used a combination of predictive modelling, hydrological analysis and field surveys to explore the extent of subpopulation decimation of the endemic hyriid freshwater mussel, Alathyria jacksoni , in Australia's northern Murray–Darling Basin (nMDB) after one of the most severe hydrological droughts on record. Predictive species distribution modelling suggested A. jacksoni should occur in the mid to lowland reaches of most rivers of the nMDB but be absent from the westerly Warrego and Paroo Rivers. Field surveys during and after the severe drought of 2017–2020 found that mussels did occur in the reaches as predicted, but in a large number of sites surveyed, mussel mortality was extreme at 97–100%. Hydrological analysis showed the duration of cease to flow (CTF) events during this drought was substantially longer than the maximum CTF duration for the preceding 37 years. This dramatic increase in the duration of dry conditions has had a devastating impact on mussel subpopulations and highlights the risk to northern mussel subpopulations of the combined impact of anthropogenic water resource use and increasing hydroclimatic variability related to climate change. Given the importance of freshwater mussels in riverine ecosystems, the loss of this large benthic invertebrate over an extensive area of river system is likely to have profound ecological and ecosystem impacts.