Carbonate chemistry variability in the southern Great Barrier Reef: implications for future ocean acidification

• Main Author: Shaw, Emily Caitlin
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: UNSW, Sydney 2012
• Subjects: Natural variability; Ocean acidification; Coral reefs; Elliot
• Online Access: http://hdl.handle.net/1959.4/52490; https://unsworks.unsw.edu.au/bitstreams/21921e3d-52b8-45f5-a24b-4282fc5fdc89/download; https://doi.org/10.26190/unsworks/16021

Ocean acidification occurs as a consequence of the absorption of anthropogenic CO2 emissions by the ocean, which lowers sea surface pH and carbonate ion concentrations over time. Ocean acidification has the potential to alter marine biogeochemical cycling and biological processes. Coral reefs are believed to be particularly vulnerable to ocean acidification as the reef framework is built through calcification by marine organisms, where calcification is a key process affected by changing seawater chemistry. Despite the predicted sensitivity of coral reefs to ocean acidification, there are only limited measurements of carbonate chemistry on coral reefs, since the vast majority of carbonate chemistry measurements have been taken in open ocean environments. In this study the diurnal and seasonal carbonate chemistry variability was measured at Lady Elliot Island (LEI), southern Great Barrier Reef, Australia. Seasonal variability was observed in the waters offshore of LEI reef flat, similar to previous observations at subtropical time series locations, and driven primarily by seasonal temperature changes. On the reef flat, diurnal variability dominated, with the daily range of conditions exceeding those that are predicted to occur over the next century as a result of ocean acidification. Reef flat diurnal variability was driven primarily by biological metabolic processes, including community photosynthesis, respiration, calcification and dissolution. At low tide the reef flat was isolated from offshore waters allowing determination of net community calcification rates (Gnet). It was found that Gnet was directly related to the aragonite saturation state (Ωarag), and applying this relationship it was v predicted that end-century Gnet will be ~55% lower than the preindustrial value. For the first time, coral reef flat natural variability was used to predict future carbonate chemistry under a business-as-usual emissions scenario this century, showing that a decrease in seawater buffer capacity will lead to amplified ...