Influence of temperature, precipitation, and cloud cover on diel dissolved oxygen ranges among headwater streams with variable watershed size and land use attributes

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Dissolved oxygen (DO) concentrations in streams are driven by multiple, interacting biotic and abiotic processes. While DO variability largely reflect cyclic patterns of respiration and photosynthesis coupled to diel cycles, physical processes such as floods that disturb biofilms and variation in temperature disrupt such cycles. In urban settings, DO cycles are typically greatly altered by elevating nutrient concentrations and reducing light-shielding riparian vegetation. We analyzed diel variations in DO from sensors distributed throughout six headwater streams to quantify (1) diel DO range patterns among watersheds of varying size and urbanization intensity, (2) the conditions that lead to abrupt declines in diel DO ranges, and (3) the amount of time needed for diel DO ranges to recover post-disturbance. In very small streams, disruptions to diel DO ranges appear to occur following severe fluctuations in atmospheric temperatures while precipitation events were primarily related to diel DO disruptions in larger streams. Precipitation events ≥ 1.5 cm over a 1-day period or ≥ 2.5 over a 2-day period consistently resulted in abrupt depressions of diel DO variations. While we primarily analyzed abiotic variables, we acknowledge that photosynthetic activity producing DO was also an important variable as shown by an analysis of how cloud cover influenced DO variations. Recovery of diel DO ranges to pre-disturbance conditions varied among sites, with the smallest watershed site reaching 50% pre-disturbance ranges in an average of 4.5 days and the largest and most urban sites reaching the same threshold over an average of 2.1 days. Urban sites typically exhibited greater diel DO ranges but did not exhibit lower precipitation thresholds for resetting diel DO ranges. DO ranges were more likely to be disrupted by precipitation events when water temperatures were cooler, which suggested different impacts of hydrologic controls on DO variations across seasons. Our findings suggest that streams consistently possess discharge thresholds that, if exceeded, lead to abrupt declines in the magnitude of the diel change in DO, but urban streams may show greater variation in diel DO concentrations with implications for fish habitat, redox-sensitive microbial processes, and contaminant transport and transformation.