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.

中文翻译：

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温度、降水和云量对流域大小和土地利用属性可变的源头水流中溶解氧范围的影响

溪流中的溶解氧 (DO) 浓度由多个相互作用的生物和非生物过程驱动。虽然 DO 的变化在很大程度上反映了呼吸和光合作用的循环模式与昼夜循环相结合，但洪水等物理过程会扰乱生物膜和温度变化会破坏这种循环。在城市环境中，溶解氧循环通常会通过提高养分浓度和减少遮光的河岸植被而大大改变。我们分析了分布在六个源头水流中的传感器的 DO 变化，以量化 (1) 不同规模和城市化强度的流域之间的 Diel DO 范围模式，(2) 导致 Diel DO 范围突然下降的条件，以及 (3) diel DO 恢复扰动后所需的时间范围。在非常小的溪流中，Diel DO 范围的中断似乎发生在大气温度剧烈波动之后，而降水事件主要与较大河流中的 diel DO 中断有关。1 天期间≥ 1.5 cm 或 2 天期间 ≥ 2.5 的降水事件始终导致 diel DO 变化的突然降低。虽然我们主要分析了非生物变量，但我们承认产生 DO 的光合活动也是一个重要变量，如云覆盖如何影响 DO 变化的分析所示。Diel DO 范围恢复到扰动前条件因站点而异，最小的流域站点在平均 4.5 天内达到扰乱前范围的 50%，最大和大多数城市站点在平均 2.1 天的时间内达到相同的阈值。城市地点通常表现出更大的日内溶解氧范围，但没有表现出用于重置日内溶解氧范围的较低降水阈值。当水温较低时，DO 范围更可能被降水事件破坏，这表明水文控制对跨季节 DO 变化的影响不同。我们的研究结果表明，河流始终具有排放阈值，如果超过该阈值，会导致 DO 的日变化幅度突然下降，但城市溪流可能会显示出日内溶解氧浓度的更大变化，从而对鱼类栖息地、氧化还原敏感微生物过程产生影响，以及污染物的迁移和转化。当水温较低时，DO 范围更可能被降水事件破坏，这表明水文控制对跨季节 DO 变化的影响不同。我们的研究结果表明，河流始终具有排放阈值，如果超过该阈值，会导致 DO 的日变化幅度突然下降，但城市溪流可能会显示出日内溶解氧浓度的更大变化，从而对鱼类栖息地、氧化还原敏感微生物过程产生影响，以及污染物的迁移和转化。当水温较低时，DO 范围更可能被降水事件破坏，这表明水文控制对跨季节 DO 变化的影响不同。我们的研究结果表明，河流始终具有排放阈值，如果超过该阈值，会导致 DO 的日变化幅度突然下降，但城市溪流可能会显示出日内溶解氧浓度的更大变化，从而对鱼类栖息地、氧化还原敏感微生物过程产生影响，以及污染物的迁移和转化。