Asynchronous timing of hypoxia and thermal stress induces diel vertical movement

Abstract

In heterogeneous landscapes, mobile organisms can track habitat conditions across multiple spatial and temporal scales, but selecting one condition often comes at the expense of another. In aquatic ecosystems, trade‐offs between temperature and dissolved oxygen (DO) are ubiquitous, and diel cycles of solar heating and photosynthesis can make them dynamic. Heat stress and hypoxia often occur asynchronously because temperature maximums occur in the evening and oxygen minimums occur around dawn. A key question is whether animal behavior can soften temperature–oxygen trade‐offs by exploiting this asynchrony. We tested how fish respond to diel variation in DO and temperature within vertically stratified thermal refuges. We first monitored heterogeneity in habitat conditions to quantify diel variation in temperature–DO trade‐offs. We then simulated fish depth selection under three strategies: minimizing temperature, maximizing DO, or balancing both variables. Simulations used observed temperature and DO to model depth selection. We then evaluated fish behavior by tagging cutthroat trout with temperature loggers and monitoring their vertical habitat use within enclosures instrumented with temperature–DO sensors. Simulations showed that temperature‐minimizing or oxygen‐maximizing strategies resulted in constant depth selection, whereas fish that balanced temperature and oxygen exhibited diel vertical movement. Most fish exhibited diel vertical movement that matched simulation; however, the movement extent was smaller than expected as fish endured hypoxic conditions in exchange for colder temperatures. We show that diel vertical movement exploits temperature and oxygen asynchrony to soften the trade‐off between these variables. These findings likely apply to other aquatic systems where asynchronous stressors constrain habitat use.