Six Month In Situ High-Resolution Carbonate Chemistry and Temperature Study on a Coral Reef Flat Reveals Asynchronous pH and Temperature Anomalies
David I. Kline, Lida Teneva, Claudine Hauri, Kenneth Schneider, Thomas Miard, Aaron Chai, Malcolm Marker, Rob Dunbar, Ken Caldeira, Boaz Lazar, Tanya Rivlin, Brian Gregory Mitchell, Sophie Dove, Ove Hoegh-Guldberg
Kline, D. I., and Coauthors, 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.
Understanding the temporal dynamics of present thermal and pH exposure on coral reefs is crucial for elucidating reef response to future global change. Diel ranges in temperature and carbonate chemistry parameters coupled with seasonal changes in the mean conditions define periods during the year when a reef habitat is exposed to anomalous thermal and/or pH exposure. Anomalous conditions are defined as values that exceed an empirically estimated threshold for each variable. We present a 200-day time series from June through December 2010 of carbonate chemistry and environmental parameters measured on the Heron Island reef flat. These data reveal that aragonite saturation state, pH, and pCO2 were primarily modulated by biologically-driven changes in dissolved organic carbon (DIC) and total alkalinity (TA), rather than salinity and temperature. The largest diel temperature ranges occurred in austral spring, in October (1.5 – 6.6°C) and lowest diel ranges (0.9 – 3.2°C) were observed in July, at the peak of winter. We observed large diel total pH variability, with a maximum range of 7.7 – 8.5 total pH units, with minimum diel average pH values occurring during spring and maximum during fall. 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 2100. DIC and TA both increased from June (end of Fall) to December (end of Spring). Using this high-resolution dataset, we developed exposure metrics of pH and temperature individually for intensity, duration, and severity of low pH and high temperature events, as well as a combined metric. Periods of anomalous temperature and pH exposure were asynchronous on the Heron Island reef flat, which underlines the importance of understanding the dynamics of co-occurrence of multiple stressors on coastal ecosystems.
Figure 1. Location of the Heron Island reef flat research site. (a) Map of Australia with a black rectangle representing the Capricorn Bunker group of reefs at the southernmost end of the Great Barrier Reef with a circle for Heron Island. (b) Aerial photograph of Heron Island with the reef flat study site shown as a star, the Heron Island Research Station aquarium system as a black rectangle and the channel cut in the reef flat as a white arrow.