Figure 1. Interactive effects of oxygen and temperature in aquatic environments. High temperature increases the likelihood of aquatic hypoxia by decreasing gas solubility, altering mixing, and increasing the metabolic activity of microorganisms. Particularly in areas affected by eutrophication, this can cause large declines in water oxygenation, resulting in hypoxia or anoxia. High temperature and hypoxia also have interacting effects on fish through their joint effects on energy supply and demand, because increasing temperature increases demand while hypoxia reduces energy supply. These effects at the biochemical level cascade up to affect processes at the physiological level, such as cardiovascular and muscle function, which in turn affect whole-organism growth and performance and tolerance. ATP, adenosine triphosphate. Illustrated by Rashpal S. Dhillon
Abstract
Determining the resilience of a species or population to climate change stressors is an important but difficult task because resilience can be affected both by genetically based variation and by various types of phenotypic plasticity. In addition, most of what is known about organismal responses is for single stressors in isolation, but environmental change involves multiple environmental factors acting in combination. Here, our goal is to summarize what is known about phenotypic plasticity in fishes in response to high temperature and low oxygen (hypoxia) in combination across multiple timescales, to ask how much resilience plasticity may provide in the face of climate change. There are relatively few studies investigating plasticity in response to these environmental stressors in combination; but the available data suggest that although fish have some capacity to adjust their phenotype and compensate for the negative effects of acute exposure to high temperature and hypoxia through acclimation or developmental plasticity, compensation is generally only partial. There is very little known about intergenerational and transgenerational effects, although studies on each stressor in isolation suggest that both positive and negative impacts may occur. Overall, the capacity for phenotypic plasticity in response to these two stressors is highly variable among species and extremely dependent on the specific context of the experiment, including the extent and timing of stressor exposure. This variability in the nature and extent of plasticity suggests that existing phenotypic plasticity is unlikely to adequately buffer fishes against the combined stressors of high temperature and hypoxia as our climate warms.