Compounding effects of agricultural land use and water use in free-flowing rivers: confounding issues for water management

Defining the ecological impacts of water extraction from free-flowing river systems in altered landscapes is challenging as multiple stressors (e.g. flow regime alteration, increased sedimentation) may have simultaneous effects and attributing causality is problematic. Furthermore, patterns of water extraction (e.g. pumping timing and rates) may govern the characteristics of hydrologic impacts and ecological responses. We examined the impacts of land and water use on rivers in the upper Ringarooma River catchment in Tasmania (south-east Australia), which contains intensively-irrigated agriculture, to support implementation of a Water Management Plan (WMP). Temporal trends in river condition were assessed using a 19-year dataset of macroinvertebrate community monitoring. Macroinvertebrates were also sampled at 19 sites before and after a dry irrigation season in 2012/13, while spatial land and water use datasets were used to characterize study sites as having upstream sub-catchments with: (1) low agricultural land use and low water use, (2) high agricultural land use and low water use, or (3) high agricultural land use and high water use. In addition, the response of macroinvertebrate communities in edge water and thalweg microhabitats to diel flow variability was investigated at sites with stable and variable baseflows. Water extraction-related stressors were found to exacerbate impairment associated with agricultural land use (e.g. reduced macroinvertebrate density, more flow avoiding taxa). Increased diel flow variability relating to water extraction appeared to contribute to the degradation by making edge water habitat uninhabitable for many taxa. These findings support the need to implement the Ringarooma WMP in accordance with its environmental flow thresholds, allocation limits and adaptive management provisions. To inform an adaptive management process, further ecological surveillance monitoring and targeted research to test the effectiveness of flow management strategies is being be conducted.