Effects of diel pCO₂ fluctuations on coral reef fishes now and into the future

The uptake of anthropogenic carbon dioxide from the atmosphere has increased the partial pressure of carbon dioxide (pCO2) and decreased the pH of the oceans, termed ocean acidification (OA). The majority of the models predicting future OA conditions are based on the open ocean, where pCO2 and pH are relatively stable. However, pCO2 fluctuates at a variety of temporal scales in coastal ecosystems. Coral reefs possess a daily cycle, where the pCO2 is elevated at night and lower during the day, mainly due to the photosynthesis, respiration, and calcification cycle of benthic reef organisms. Moreover, the magnitude of pCO2 fluctuations is projected to increase in the future as OA advances. To date, most studies on the effects of elevated pCO2 on reef fishes have used stable CO2 treatments, which may not accurately represent physiological responses under predicted future fluctuation in pCO2. In this thesis, I aim to examine fluctuations of pCO2 on coral reefs and how fishes will be affected in the future by these conditions.

While coral reef pCO2 is known to fluctuate on a daily cycle, much less is known about pCO2 variation at the microhabitat scale that animals occupy. Therefore, in Chapter 2 I investigate the pH/pCO2 variability of three different reefs and three coral reef microhabitats (hard coral, soft coral, and sand). I found typical diel variation in pCO2 associated with photosynthesis and respiration cycles. These fluctuations differed between reefs more than between the microhabitats within a reef. The variation was likely influenced by water flow and wind speed. These results fall within the normal range of coral reef diel variation and suggest that it is important to consider physical and hydrodynamic factors when projecting future pCO2 variation. Data from Chapter 2 on the current-day natural fluctuations of pCO2 in the environment can be used to inform ecologically relevant pCO2 treatments to measure how fishes and other coral reef organisms will be affected by OA in the future.

Elevated pCO2 has the potential to negatively impact fishes due to the increased costs of acid-base regulation. In Chapter 3, I explored the effects of an 8 h exposure to one of four pCO2 treatments, two stable (ambient and stable elevated) or two fluctuating levels of pCO2 (increasing, and decreasing) on two species of damselfishes, Acanthochromis polyacanthus and Amblyglyphidodon curacao. Acanthochromis polyacanthus required more energy upon exposure to both stable pCO2 treatments during the 8 h trial compared to fish exposed to the fluctuating pCO2 treatments. However, there was no effect of pCO2 treatment on the swimming or oxygen uptake rates of A. curacao. This suggests that, for some species of coral reef fishes, performing under fluctuating pCO2 conditions may be less energetically-costly than performing under stable pCO2 conditions. Moreover, these results highlight the importance of ecologically relevant pCO2 treatments when testing how fishes will perform in future OA conditions.

Hannan K. D., 2021. Effects of diel pCO₂ fluctuations on coral reef fishes now and into the future. PhD thesis, James Cook University, 201 p. Thesis.

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