Paper Reviewed
Ern, R., Johansen, J.L., Rummer, J.L. and Esbaugh, A.J. 2017. Effects of hypoxia and ocean acidification on the upper thermal niche boundaries of coral reef fishes. Biology Letters 13: 20170135.

Writing as background for their work, Ern et al. (2017) say that "a prevailing theory suggests that the upper thermal limits of fishes are constrained by hypoxia and ocean acidification." Thus, it became their objective to test this hypothesis for three coral reef fish species: black-axil chromis (Chromis atripectoralis), five-lined cardinalfish (Cheilodipterus quinquelineatus) and spiny chromis damselfish (Acanthochromis polyacanthus).

In a controlled laboratory setting they first tested the impact of hypoxia on each species' critical thermal maximum (CTmax, the temperature at which a loss of equilibrium is exhibited in each fish due to a temperature-induced collapse of vital physiological functions). This was accomplished by exposing the fish to temperature increases of 2°C per hour above the average summer temperature of their inhabiting region, whereupon they made various observations with each temperature increase. Results of this phase of their analysis revealed that the CTmax of all three of the coral reef fish species was unaffected by hypoxic vs. normoxic conditions, leading the scientists to conclude that the CTmax of all three species may be classified as oxygen-independent.

In the second phase of Ern et al.'s work, they repeated the protocol of their first experiment after acclimating C. atripectoralis for two weeks in an ocean acidification scenario where the pCO2 of the seawater corresponded to an atmospheric CO2 concentration of 1000 ppm. Results of that analysis revealed there was no significant difference in CTmax under normal or reduced seawater pH. Consequently, the researchers state that "the effect of elevated CO2 on oxygen supply capacity is either absent, or insufficient to cause a significant change in the oxygen limit for thermal tolerance," and they conclude that "ocean acidification is therefore unlikely to act as a significant synergistic stressor with hypoxia on the upper thermal limits of this species." What is more, they add that "the lack of significant change in CTmax of C. atripectoralis in normoxia suggests that the thermal tolerance of the physiological mechanisms responsible for setting CTmax is also not affected by elevated CO2."

With respect to the significance of their findgins, Ern et al. say that moderate environmental hypoxia and future ocean acidification "should have little impact on the upper thermal limits of these species and their resilience to transient heat waves." And they add that "to the degree these tropical, stenothermal fish species occupy the extent of latitudes tolerable within their thermal range boundaries, our findings suggest that moderate hypoxia and ocean acidification are unlikely to impact their latitudinal distribution ranges via direct limitations on their upper thermal limits."