The carbonate skeletons of stony corals provide the rigid framework for the most productive and diverse marine ecosystems - coral reefs. The underlying process of calcification - binding dissolved calcium and carbonate ions into solid aragonite - depends on seawater carbonate chemistry, particularly the aragonite saturation state. Corals grow in seawater oversaturated with regard to aragonite, but the global increase in carbon dioxide and ocean acidification are expected to lower aragonite saturation state and, hence, coral calcification and growth. Because aragonite saturation state varies with depth, deep-dwelling corals are believed to be most heavily affected by ocean acidification. The ocean is not static, however, and the areas where dynamic ocean processes lift under-saturated waters into the reach of corals provide an interesting setting to study coral calcification responses to natural variations in seawater chemistry exceeding the current IPCC scenario.
In the Andaman Sea, the sharp water density layer (pycnocline) separating aragonite-undersaturated deep from aragonite-oversaturated surface waters undergoes sudden jumps of more than 80 m. These solitary very large amplitude internal waves, or solitons, are generated by tidal streams across the shallow ridges of the Andaman-Nicobar island arc, displacing the pycnocline several times per tidal cycle.
Near the Andaman Sea margin along the Thai shelfbreak, the shoaling and breaking of these large amplitude internal waves cause intense mixing inducing frequent unperiodic changes in the physico-chemical properties of the seawater. The Similan Islands located 70 km off the Thai coast are located in the swash zone of breaking solitons. They feature flourishing reefs on their sheltered eastern margins, but only sparse reef development on their monsoon- and soliton-exposed west sides, where corals experience dramatic recurrent swings in temperature, nutrients and aragonite saturation state in the course of only minutes. How do corals cope with these harsh conditions? How is the benthic community affected? In cooperation with our Thai colleagues from the Phuket Marine Biological Centre, we address these question to elucidate the effects of ocean dynamics on coral reefs on the organism, community and ecosystem level - to understand the processes governing coral growth and reef development, and the resilience of corals and coral reefs to disturbance such as global warming.