UC Riverside

Global change has severely impacted the flow regimes of river systems across the globe, and the species inhabiting those systems now find themselves adapted to hydrographs that no longer exist. This includes many of the flow regimes in California, as streams are experiencing an altered precipitation cycle and a system of dams, culverts, and wastewater treatment plants that have transformed rivers that were once naturally flowing into occasionally concrete-lined channels. In the following chapters, I report the results of several studies aimed at understanding the relationship between flow variability and the persistence of ecological communities in river systems, particularly diatoms and benthic macroinvertebrates. In chapter one, I utilize a set of high-elevation experimental stream channels to subject the stream benthic community to a gradient of drought treatments. In this study, I found that pool habitat serves as a better refuge to benthic macroinvertebrates than riffles, contingent on the presence of hyporheic flow. Additionally, perennial flow–adapted communities appear to resist flow reduction, up to the point when surface flow is lost. My second chapter examines the relationship between benthic macroinvertebrate community composition and flow regimes, spatial connectivity, and environmental variables at large spatial scales and long temporal scales. I found that spatial processes, such as drainage density and upstream area, and flow metrics, such as flow variability and zero-flow days, best explain temporal beta diversity in macroinvertebrate communities at the multi-catchment scale. Chapter three explores the impact of urban flow disturbance, in the form of effluent discharge from wastewater treatment plants and storm runoff, on benthic diatom and macroinvertebrate communities through space and time. I found that macroinvertebrates in such systems are resistant to flow perturbations, compositionally reflected in a relatively disturbance-tolerant community, while the diatoms reflected fast-paced resilience strategies. While erratic flow shutdowns from wastewater treatment plants result in losses of taxa and lower densities of individuals, large storm events further compound flow regime disturbance in the urban stream system. Overall in this dissertation, I demonstrate that flow disturbance is a strong predictor of benthic freshwater communities, and that determining the extent of hydrological disturbance at the correct timescale is critical to the conservation of freshwater biodiversity.