Separating Thermal and Viscous Effects of Temperature on Copepod Respiration and Energy Budget

AbstractChanges in temperature alter the viscosity of fluids, which impacts the force needed to move and the diffusion rates of gases. This is particularly salient for organisms that operate at mid to low Reynolds numbers. In this study, we investigated the independent effects of changes in temperature and viscosity on oxygen consumption rates of two coastal copepods (Acartia tonsa and Parvocalanus crassirostris) and used bioenergetic models to predict how these patterns could influence copepods in the natural environment. We found that only temperature influenced copepod oxygen consumption rates, indicating that copepods were not impacted by reduced oxygen diffusivity or increased energetics of movement resulting from higher seawater viscosity. We developed energy budgets based on novel respiration experiments in conjunction with data from the literature and found that cold temperatures do not result in higher scope for growth, because decreased metabolic costs are offset by reduced feeding capability. Our energy budgets imply that observed copepod temperature ranges in natural waters match theoretical ranges of optimal net carbon assimilation. At cold temperatures, feeding on motile prey yielded higher net carbon assimilation compared to feeding on non-motile prey, implying that motile prey are more favorable and may be actively selected for at cold temperatures. Finally, our models predicted that A. tonsa had a higher maximum net carbon assimilation as a percentage of body mass, indicating that copepods that use a similar sink-and-wait feeding strategy may be better able to exploit ephemeral food sources compared to continuous-swimming copepods such as P. crassirostris.