| Rising atmospheric CO2 concentrations threaten coral reefs worldwide by causing ocean warming and acidification. When seawater temperatures are unusually high, corals lose a significant portion of their vital algal endosymbionts and/or photosynthetic pigments making them appear pale or white -- a process referred to as coral bleaching. As corals get most of their carbon from the algal endosymbionts, the breakdown of this symbiosis significantly weakens coral and can lead to widespread mortality if bleaching is severe. Bleaching events have been predicted to become annual events sometime later this century. Despite this knowledge, the impacts of annual bleaching on coral physiology, biogeochemistry, and overall resilience remain largely unknown. For the first time, annually recurring bleaching was simulated on ecologically relevant timescales by subjecting three Caribbean coral species ( Orbicella faveolata, Porites astreoides, and Porites divaricata ) to experimental coral bleaching (+1°C for 2.5 weeks) in two consecutive years. Impacts on their physiology and biogeochemistry were assessed in great detail immediately after repeat bleaching as well as after short and long term recovery. We show that repeat bleaching can dramatically alter thermal tolerance of the coral holobiont (i.e., animal host and endosymbiont). Species such as P. divaricata will be able to rapidly acclimate to frequent temperature stress and persist on future coral reefs, whereas others such as P. astreoides will show increasing bleaching susceptibility and may thus face significant decline.;Coral skeletal carbon isotopes are important paleo-climate proxies and have the potential to record past bleaching events. However, they are often confounded by strong kinetic isotope effects that can mask the bleaching signal in the skeleton and compromise overall accuracy of the proxy. A proposed data correction to remove kinetic isotope effects was tested for the first time using bleached corals. In addition, it was tested if photosynthesis to respiration (P/R) ratios can be reliably calculated from coral isotopes. We found that the data correction did not effectively remove kinetic isotope effects, and that isotope-based P/R ratios are in poor agreement with P/R ratios measured by respirometry. Therefore, the data correction should not be routinely applied to paleo-climate reconstruction, and P/R ratios should be obtained by respirometry only.;While the potential of some coral species to acclimate to frequent bleaching stress is encouraging, corals will also have to cope with increasingly more acidic seawater, which compromises calcification rates. Yet, the combined effects of ocean acidification (OA) and warming on coral physiology remain poorly understood. We therefore conducted a controlled tank experiment where four Pacific coral species (Acropora millepora, Pocillopora damicornis, Montipora monasteriata, and Turbinaria reniformis) were maintained at seawater pCO2 levels and temperatures expected by the end of this century (741 muatm, 29.0°C). Surprisingly, three of the four species studied were able to maintain calcification rates under these conditions. Further, we show for the first time that coral energy reserves were not catabolized to sustain calcification rates under OA conditions, which has important implications for coral health and bleaching resistance.;Overall, the findings from this dissertation research highlight that some coral species will be able to survive predicted near-future ocean acidification and warming. However, as seawater pCO2 increases beyond 700 muatm and coral bleaching becomes more frequent and intense, temperature- and CO 2-sensitive species will likely face significant demise. Since this is likely to have dramatic consequences for coral reef diversity and overall reef functioning, the future of coral reefs beyond the 21st century may be uncertain. |
