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Six Months of Natural pH Fluctuations on the Heron Island Reef Flat

Paper Reviewed
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, doi:10.1371/journal.pone.0127648.

So-called ocean acidification (OA) is defined as the future decline in seawater pH that is expected to result in consequence of rising atmospheric carbon dioxide concentrations. By the year 2100, for example, it is estimated that enough CO2 will have been absorbed by the world oceans to cause a 0.3 to 0.4 pH unit decline, which reduction has been projected to wreak havoc on marine life, including possibly driving some species to extinction. However, much remains to be learned about this topic, for in the words of Kline et al. (2015), "a significant knowledge gap in our understanding of OA impacts on coral reefs lies in understanding the exposure of reefs to natural variability in OA-relevant parameters on different time-scales."

In an effort to begin filling this critical void, the fourteen-member research team of Kline et al. undertook to intensely measure a number of important carbonate chemistry and environmental parameters on the Heron Island reef flat, near the southern end of the Great Barrier Reef (23.45°S, 151.92°E), over a period of 200 days from June through December of 2010. Among their findings, the researchers report observing large variability in daily pH values (see Table 1, pink shaded values), with differences often exceeding the magnitude of the predicted decline by 2100 (see Figure 1). What is more, they note that "as with many other reefs, the nighttime pH minima on the reef flat were far lower than pH values predicted for the open ocean" by the end of this century.

With regard to the cause of this variability, Kline et al. say it was "primarily modulated by biologically-driven changes in dissolved organic carbon and total alkalinity, rather than salinity and temperature." And in further explanation they write that "in essence, to a large extent reefs modulate their own exposure to pH, especially in back-reef zones that are sometimes isolated from open ocean water. Reefs are both experiencing pH diel cycles, but they are also contributing to the actual range of the diel cycle."

Such observations demonstrate the complexity of projecting both the degree of future pH decline and the response of marine life -- such as corals -- to that decline. Commenting on these problems, Kline et al. counsel that "natural environmental variability needs to be closely replicated in [OA experiments]" because such variability (e.g., daily fluctuation in pH) "will likely [1] impact coral reef physiological and ecological responses, [2] will ensure more biologically-relevant results, and [3] will be critical to understanding the impacts of local and global stressors in a high CO2 future."

Indeed, and until that natural variability is properly replicated in OA experiments, the response of marine life to future declines in oceanic pH must be taken with a large grain of salt.

Table 1. Monthly environmental data summary statistics including the average diel means, average diel minimum, average diel maximum and diel range for temperature, pH and pCO2 (± SD). See the source (Kline et al., 2015) for details on how these statistics were calculated.

Figure 1. High-resolution pH (blue line) and pCO2 (green circles) measurements for Heron Reef for two weeks in June (austral fall) and December (austral spring) of 2010. Adapted from Kline et al. (2015).

Posted 30 November 2016