Revisiting inland hypoxia: diverse exceedances of dissolved oxygen thresholds for freshwater aquatic life.

Water resources in many regions are stressed by impairments resulting from climate change, population growth and urbanization. In the United States (US), water quality criteria (WQC) and standards (WQS) were established to protect surface waters and associated designated uses, including aquatic life. In inland waters of the south central US, for example, depressed dissolved oxygen (DO) consistently results in impaired aquatic systems due to noncompliance with DO WQC and WQS. In the present study, we systematically examined currently available DO threshold data for freshwater fish and invertebrates and performed probabilistic aquatic hazard assessments with low DO toxicity data that were used to derive the US Environmental Protection Agency's (EPA) Ambient Water Quality Criteria (AWQC) for DO and newly published information. Aquatic hazard assessments predicted acute invertebrate DO thresholds for Ephemeroptera, Plecoptera, or Trichoptera (EPT) taxa and species inhabiting lotic systems to be more sensitive than fish. For example, these organisms were predicted to have acute low DO toxicity thresholds exceeding the US EPA guidelines 17, 26, 31 and 38% and 13, 24, 30 and 39% of the time at 8.0, 5.0, 4.0 and 3.0 mg DO/L, respectively. Based on our analysis, it appears possible that low DO effects to freshwater organisms have been underestimated. We also identified influences of temperature on low DO thresholds and pronounced differences in implementation and assessment of the US EPA AWQC among habitats, seasons, and geographic regions. These results suggest some implemented DO guidelines may adversely affect the survival, growth, and reproduction of freshwater aquatic organisms in a region susceptible to climate change and rapid population growth. Given the global decline of species, particularly invertebrates, low DO threshold information, including sublethal (e.g., reproduction, behavior) responses, for additional species (e.g., mollusks, other invertebrates, warm water fish) across seasons, habitats, and life history stages using consistent experimental designs is needed to support more sustainable environmental assessment efforts and management of biodiversity protection goals in inland waters.