Heterotrophy and Lipids as Indicators of Resilience to Climate Change Stress in Scleractinian Corals

Coral reefs are biodiverse ecosystems with high biological, cultural, economic, and recreational value that are facing multiple anthropogenic stressors, the greatest of which is global climate change via warming and ocean acidification (OA). Increased warming throughout the century may reach a point where frequent bleaching, the expulsion of corals’ symbiotic algae that can provide the coral with over 90% of its daily metabolic requirements, may cause widespread mortality. OA, a result of increased carbon dioxide dissolving into seawater, changes the chemistry of seawater such that the pH of the ocean becomes more acidic. This decrease in pH is accompanied by a decline in the saturation state of calcium carbonate, which impairs the ability of corals to build their skeletons by increasing the energetic cost of calcification. Research within the last decade has demonstrated that coral heterotrophy, the ability of the coral animal to feed on plankton in the water column, and coral energy reserves (lipid stores) may be good indicators of resilience to both warming and acidification. The gaps in our knowledge regarding heterotrophy and lipids are which coral species are capable of increasing their feeding effort, and what conditions drive this capability. To evaluate resilience capability of common corals from the Florida Reerf Tract (FRT), field and lab studies were completed to address the following questions: i.) Do in-situ calcification rates and lipid content vary by species, season, and/or site in the FRT? ii.) Are feeding rate and lipid content plastic responses during climate change stress? iii.) Are calcification and feeding rate variable within the same species from different source locations during climate change stress? iv.) Does preconditioning to elevated CO2 prior to thermal stress affect bleaching susceptibility? Results showed that i.) there may be metabolic tradeoffs between calcifying and storing lipids, and certain species can possibly mediate that tradeoff using heterotrophy. ii.) Past history of nutritional repletion may be an important factor in predicting resilience to climate change stress. iii.) An endangered species can increase its feeding rate and lipid stores to mitigate reductions in calcification under CO2 stress, underscoring the importance of heterotrophy and lipids in future conservation science. iv.) Preconditioning to high CO2 (a globally relevant scenario post 2040) will likely worsen the effects of thermal stress on calcification and feeding rate, underscoring the importance of reducing CO2 emissions on a global scale. This dissertation demonstrates that the resilience of reef-building corals of the FRT in future warmer, more acidic oceans is affected by heterotrophy and lipids, but this response will be dependent on reefs with naturally high abundance of heterotrophic food sources. This dissertation has implications for potential feeding protocols in coral gardening and nursery programs and the planning and placement of marine protected areas