Ecosystem Responses to Water Resource Developments in a Large Dryland River

Large floodplain rivers in dryland regions are becoming increasingly modified through water resource developments. Identifying ecosystem responses in these systems is challenging because of their natural variability, limited data, and the myriad of ways they are modified. This study used organic samples from snail, mussel, and fish specimens obtained from museum collections for the determination of carbon and nitrogen stable isotope ratios, for the Barwon-Darling River, Australia, between 1869 and 2005, a period of extensive water development. Three hypotheses were posed for this study: (1) The trophic status and food web character of the Barwon-Darling River will change in response to water resource developments; (2) responses in the trophic status and food web character will differ between different hydrogeomorphic zones identified along the river; and (3) food chain lengths will increase in response to water resource developments. Substantive changes in trophic status and components of the food web were detected between before and after water developments. Three lines of evidence support the conclusion of anthropogenically driven changes in the food web of this dryland floodplain-river system. Stable isotope ratios of fish, mussels, and snails differed between both hydrogeomorphic zones of the river and the before and after disturbance periods. Layman metrics, representing community niche space, differed between before and after disturbance periods. Also, both mean trophic position and food chain length differed between predisturbance and postdisturbance for the two functional process zones. The substantial shift in basal source contribution over time is a potential indicator of a state change and loss of resilience in this system. Plain Language Summary Resilience is the ability to absorb shocks and retain a normal functioning. This study indicates a loss of resilience in the aquatic ecosystem of the Barwon-Darling River as a result of water resource developments. A novel approach of using museum specimens of molluscs and fish enabled the trophic status and food web character over a 130-year period to be investigated, for the Barwon-Darling River, Australia, a system heavily impacted by water resource development. Marked differences in the trophic status and components of the food web were detected between before and after water resource development periods as indicated by the stable isotope ratios of the molluscs and fish. However, differences varied between two river zones. Food chain lengths increased but were greater in the upper river zone. Basal source contributions to the food web also changed substantially over time in association with water resource development. A flip to pelagic basal source dominance suggests the loss of resilience of this river system and potential regime shift.