Setting Salinity Water-Quality Standards Protective of Aquatic Life: Insights From Analyses of the Effects of Salinity on Stream Insects

Freshwater ecosystems are becoming saltier due to human activities like agriculture, urbanization, and resource extraction. This change in salinity is important because we know very little of how stream salinity affects aquatic insects at natural concentrations, much less at elevated concentrations. Increases in salinity may harm aquatic macroinvertebrates, which play important roles in stream food webs and are widely used as indicators of ecosystem health. Understanding how salinity affects aquatic insect growth, survival, and distribution is critical for developing effective criteria protective of aquatic life, but existing methods often rely on field surveys where multiple environmental factors covary, making it hard to isolate the effects of salinity from other pollutants. My research aimed to improve how we assess the effects of stream salinity on aquatic macroinvertebrate distributions and viability. In my second chapter, I conducted lab experiments to test how salinity and temperature affect the survival and growth of three stonefly species and compared these results to field-based estimates of thermal and salinity sensitivity. I found that temperature had a stronger overall effect on survival and growth than salinity, and that the only temperature results aligned in rank order with field-derived sensitivity estimates. In my third chapter, I developed models to assess whether variation in larval aquatic insect body size across different stream sites could be linked to salinity and other environmental conditions. I found that salinity and temperature were associated with large differences in body size for some, but not all, species. In my fourth chapter, I evaluated a common method used by water quality agencies to set salinity thresholds protective of aquatic life and identified several sources of bias: the varying number of individuals in subsamples used to inform the model, analytical methods chosen to estimate species' salinity sensitivities, and the presence of other stressors. Together, the findings from my research help clarify how salinity affects aquatic insects and how salinity standards can be developed to better protect freshwater ecosystems.