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Going with the flow – how a stream insect, Pteronarcys californica, exploits local flows to increase oxygen availability

For insects, aquatic life is challenging because oxygen supply is typically low compared to air. Although many insects rely on stream flows to augment oxygen supply, oxygen limitation may occur when oxygen levels or flows are low or when warm temperatures stimulate metabolic demand for oxygen. Behavior may allow insects to mitigate oxygen shortages – by moving to cooler, more oxygenated, or faster flowing microhabitats. However, whether stream insects can make meaningful choices depends on: i) how much temperature, oxygen, and flow vary at microspatial scales in streams and ii) the ability of insects to exploit that variation. We measured microspatial variation in temperature, oxygen saturation, and flow velocity within riffles of two streams in Montana, USA. Additionally, we examined the preferences of nymphs of the stonefly Pteronarcys californica to gradients of temperature, oxygen, and flow in lab choice experiments. Temperature and oxygen level varied modestly within stream riffles (∼ 1.8 °C, ∼ 8.0% of air saturation, respectively). By contrast, flow velocity was highly heterogeneous, often varying by more than 125 cm s-1 within riffles and 44 cm s-1 around individual cobbles. Exploiting micro-variation in flow may thus be the most reliable option for altering rates of oxygen transport. In alignment with this prediction, P. californica nymphs showed relatively little ability to exploit laboratory gradients in temperature and oxygen. By contrast, they readily exploited micro-variation in flow – consistently choosing higher flows when conditions were warm or hypoxic. These behaviors may help stream insects mitigate low-oxygen stress from climate change and other anthropogenic disturbances. Summary Statement Stonefly nymphs stressed by higher temperatures and lower oxygen availability often relocate to areas of higher flow. This behavior likely increases the ratio of oxygen supply to demand.