Coral reef ecosystems worldwide have been subjected to increasing natural and human-caused disturbances over the last few decades. One of the negative effects many of these stressors have on coral reefs is the breakdown of the symbiotic relationship between coral polyps and the photosynthetic algae, zooxanthellae, contained in the polyp tissues. This leads to loss of pigmentation, or bleaching, of the corals. Most bleaching events result from the loss of the zooxanthellae and/or the loss of the pigments of the zooxanthellae. If the magnitude and duration of the bleaching event are great enough, coral mortality will result, and lasting changes in the reef ecosystem may occur. This doctoral dissertation research project aims to assess whether coral bleaching events are increasing in frequency and to evaluate causative environmental factors for the bleaching events. The objective of this project is to develop and calibrate a methodology for identifying bleaching events using stable isotopic and trace element analysis in coral cores. To achieve this objective, a 15-year time series will be developed of oxygen and carbon stable isotopic measurements along with trace element measurements of strontium/calcium ratios on Porites coral cores obtained from Pandora Reef and the Keppel Islands on Great Barrier Reef (GBR). Large-scale bleaching events in 1991 and 1998 around both regions were probably caused by decreased salinity due to flooding, followed by elevated sea surface temperatures (SST). To calibrate methods, the student will compare the stable isotopic and trace element data from bleached and unbleached corals during the 1991 and 1998 events with in situ measurements and observations of SSTs and salinity on the GBR. The calibration effort will aid in the development and interpretation of continuous, long-term records of coral bleaching events for two regions of the Great Barrier Reef (Pandora Island and the Keppel Islands) as well as French Polynesia.
Coral reef ecosystems in the Pacific have been severely affected by mass bleaching events, and as a result, the mortality and changes in the reef ecosystems could have a dramatic effect on the people and countries in the region. The geochemical records generated as a result of this project will provide important information about the long-term history bleaching events on the GBR. These data will allow a more informed analysis of whether bleaching events are increasing in frequency as a result of human-caused climatic and environmental conditions or whether they are a natural phenomenon. The proposed research will provide scientists, resource managers, and policy makers with a better understanding of the long-term history of reefs and the potentially catastrophic impact of bleaching events. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.
