Paper Reviewed
Jarrold, M.D., Humphrey, C., McCormick, M.I. and Munday, P.L. 2017. Diel CO2 cycles reduce severity of behavioral abnormalities in coral reef fish under ocean acidification. Scientific Reports 7: 10153, DOI:10.1038/s41598-017-10378-y.

Observations show that pCO2 can vary 50-600 µatm from the mean in shallow reef areas over the course of a day (Kayanne et al., 1995; Shaw et al., 2013; Albright et al., 2013; Kline et al., 2015), which oscillations are primarily driven by the biological process of photosynthesis/respiration and calcification/dissolution over a day-night cycle. Yet despite such natural diel variability in pCO2, nearly all ocean acidification (OA) experiments are performed at static pCO2 treatment levels, which clearly do not represent real-world pCO2 conditions. Consequently, OA experiments that fail to incorporate realistic diel pCO2 fluctuations must be treated with a healthy dose of skepticism.

The most recent example from the literature exploring the problematic design of static OA pCO2 treatments comes from Jarrold et al. (2017). Publishing their work in the journal Scientific Reports, the four-member scientific team examined the behavioral responses of two reef fishes at stable and diel cycling pCO2 treatments.

In the first of the two experiments they performed, Jarrold et al. exposed juvenile damselfish (Acanthochromis polyacanthus) and juvenile clownfish (Amphiprion percula) to two stable (unchanging) CO2 levels (480 or 1000 µatm, corresponding to respective current and future pCO2 levels frequently examined in OA experiments) and two fluctuating high CO2 treatments that oscillated ± 300 and ± 500 µatm from a 1000 µatm mean (with the high pCO2 peaks lasting approximately 3 hours in the 24 hour cycle). Under these conditions, behavioral lateralization analyses were performed on the damselfish, whereas responses to a predator cue were analyzed for the clownfish following one week of exposure to the seawater pCO2 treatments.

In the second experiment, three stable CO2 levels were utilized (480, 750 and 1000 µatm) and two fluctuating high CO2 treatments (750 or 1000 µatm) that oscillated ± 300 µatm from the mean, with the high pCO2 peaks lasting approximately eight hours, or one-third, of the 24 hour cycle. As with the first experiment, behavioral lateralization was examined in damselfish, while the predator cue response in the clownfish was examined after one week of seawater pCO2 treatment exposure.

So, what did their experiments reveal?

First, a noted shortcoming inherent to the design of both experiments, was the authors' failure to include a control treatment (i.e., 480 µatm) with oscillating pCO2 levels (i.e., ± 300 or ± 500 µatm from the mean), which omission may well have skewed their findings. Nevertheless, Jarrold et al. report that behavioral lateralization of damselfish and response to a predator cue of clownfish were negatively impacted in fish subjected to stable, elevated pCO2 conditions in both experiments. However, they also report that "the severity of two behavioral abnormalities commonly observed under elevated CO2 are reduced when fish experience a diel cycling pCO2 regime." Indeed, in some instances, diel cycling of pCO2 under OA conditions was able to fully restore the behavioral abnormalities observed under static OA pCO2 conditions to normal levels observed under the control pCO2 treatment. Consequently, in light of such findings, Jarrold et al. say their work indicates that "previous studies have probably over-estimated the behavioral impacts of OA on coral reef fishes once they have settled to reef habitats where diel CO2 cycles are prevalent." And when one considers the potential for adaptation to occur (recall that the fish utilized in this study had been exposed to their pCO2 treatment levels for only one week), the minimal impacts of OA that were observed in this study could quite likely entirely vanish!

References
Albright, R., Langdon, C. and Anthony, K.R.N. 2013. Dynamics of seawater carbonate chemistry, production, and calcification of a coral reef flat, Central Great Barrier Reef. Biogeosciences 10: 6747-6758.

Kayanne, H., Suzuki, A. and Saito, H. 1995. Diurnal changes in the partial pressure of carbon dioxide in coral reef water. Science 269: 214-216.

Kline, D.I., Teneva, L., Hauri, C., Schneider, K., Miard, T., Chai, A., Marker, M., Dunbar, R., Caldeira, K., Lazar, B., Rivlin, T., Mitchell, B.G., Dove, S. and Hoegh-Guldberg, O. 2015. Six month in situ high-resolution carbonate chemistry and temperature study on a coral reef flat reveals asynchronous pH and temperature anomalies. PLoS One 10: e0127648.

Shaw, E.C., McNeil, B.I. and Tilbrook, B. 2012. Impacts of ocean acidification in naturally variable coral reef flat ecosystems. Journal of Geophysical Research 117: C03038.