Using Causal Modelling Principles to Integrate Long-term Macroinvertebrate Monitoring Data for the Murray River with an Existing Hydroclimate-Salinity Model for the Murray-Darling Basin to Predict the Ecohydrological Effects of Future Climate Scenarios

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the Department of Ecology, Environment and Evolution, College of Science, Health and Engineering, La Trobe University, Victoria, Australia.

Balancing the needs of the environment and agriculture for water in the Murray-Darling Basin is challenging given the extreme variability in inflows and the likelihood that surface water availability will decrease over coming decades as a consequence of climate change. The distribution of water resources is further complicated by the need to manage salinity in the Basin’s rivers and wetlands, and an incomplete understanding of how catchment-scale hydrology is linked to ecological outcomes. Both policy makers and managers would benefit from ecohydrological models that could be used to evaluate policy choices and management actions under different climate and development scenarios. With this end in mind, statistical models were developed that link biotic variables to river flow, salinity, and other environmental drivers, and these models were then integrated using the principles of causal modelling with an existing hydroclimate-salinity model for the Murray-Darling Basin. The data used for model building stemmed primarily from the Murray-Darling Basin Authority’s River Murray Water Quality and Biological (Aquatic Macroinvertebrate) Monitoring Programs, spanning 2,300 km of the Murray River and a period of 33 years (1980–2012) that encompassed major floods and droughts. The hydroclimate-salinity model was derived from the Sustainable Yields Project, conducted by the Commonwealth Scientific and Industrial Research Organisation. Statistical models developed in this study revealed that major floods initially depress macroinvertebrate abundance and richness, but then lead to a sustained increase and a shift in community structure lasting up to 32 years, which may be related to an influx of coarse and large woody debris during floods, whereas flows of a lesser magnitude have little or no discernible direct effect. Predictions from the integrated causal model indicate that future climate scenarios would result in appreciable changes in environmental variables, particularly flow, water temperature, and salinity, and shifts in macroinvertebrate abundance, richness, and community structure.