Ecological Impacts of Artificial Flows in an Effluent-Dependent Aridland River

In arid and semi-arid environments where surface water resources are scarce, the discharge of treated wastewater into streams often supports or creates aquatic habitat and in many cases is actively managed for environmental benefits. Although common across the world, there is a lack of research on these novel effluent-fed systems and their impacts on aquatic ecology. This dissertation presents a detailed case study of the lower Santa Cruz River near Tucson, Arizona, USA, which is dependent upon effluent discharge for perennial baseflow. In this dissertation, I examine 1) the general suitability of effluent to serve as habitat for aquatic invertebrates, 2) the impacts of an artificial flow regime on fish stranding, 3) the prevalence and drivers of microplastic pollution in the river, and 4) the role of natural and artificial disturbance in shaping aquatic invertebrate communities at several locations along the river. The lower Santa Cruz River supports a diverse aquatic invertebrate community (156 taxa) that is shaped by water quality gradients, as well as artificial disturbance from daily flow intermittence and natural disturbance from seasonal floods. Daily drying also impacts aquatic vertebrates: stranding of mosquitofish is common in certain reaches of the river and is governed primarily by flow recession rates during periods of reduced effluent input. Also, microplastic pollution from point and non-point sources is ubiquitous throughout the river, and the abundances of this pollution in the water column and riverbed sediment are affected by flooding. Despite the ubiquity of microplastics in the river, only six percent of sampled mosquitofish had consumed microplastics and no obvious health impacts were observed in those fish. This dissertation makes a novel and timely contribution to urban stream ecology research with findings relevant to effluent-fed rivers across arid and semi-arid regions of the globe. This research addresses many of the unique benefits and challenges of effluent discharge for aquatic ecosystem support, analyzes previously unstudied phenomenon, and identifies knowledge gaps that require future research.