THE IMPACTS OF ANTHROPOGENIC GLOBAL CHANGE AND LOCAL HUMAN ACTIVITIES ON REEF-BUILDING CORALS ON THE BELIZE MESOAMERICAN BARRIER REEF SYSTEM

Coral reefs are some of the most diverse and important ecosystems on earth, yet they are experiencing global scale declines in coral cover, diversity, and ecosystem health due to the impacts of climate change, ocean acidification, and local human impacts such as land-use change, overfishing, and pollution. This dissertation explores the impacts of thermal history on coral community composition (Chp 1), coral-associated Symbiodinium community structure (Chp 2), coral growth rates (Chp 3), and the acclimatization and/or local adaptation capacity of Sidereastrea siderea and Pseudodiploria strigosa corals (Chp 4) on the Belize Mesoamerican Barrier Reef System (MBRS). The Belize MBRS can be subdivided into three distinct thermal regimes following a nearshore-offshore gradient of warmer and more thermally variable to cooler and less thermally variable seawaters. Nearshore reefs (warmer and more thermally variable) experienced lower coral cover and diversity and that weedy and stress-tolerant coral species persisted on these reefs (Chp 1). Coral-associated Symbiodinium communities varied by thermal regime in one of the three study species, and that thermally tolerant Symbiodinium did not dominate in warmer nearshore reefs (Chp 2). This finding suggested that Symbiodinium likely did not play a large role in providing some corals with the capacity to sustain themselves in the warmest and most thermally variable thermal regimes. Nearshore corals grew faster than offshore conspecifics, yet suffered declining growth rates, while growth rates of offshore corals remained stable (Chp 3) suggesting that historically there has been a growth advantage to living nearshore. However, recent declines suggest that compounding negative impacts outweigh this growth advantage, leading to declining growth. In a follow-up reciprocal transplant experiment, native and transplant S. siderea and P. strigosa corals preferred the nearshore, indicating that nearshore species may not exhibit greater acclimatization ability when transplanted (Chp 4). Overall, low diversity nearshore reefs appear especially threatened by continued ocean warming, as corals on these reefs exhibit declining growth rates and are not better equipped to acclimatize to new conditions than do offshore corals. A swift and significant reduction in emissions combined with continued local scale mitigation would provide hope for the future survival of these corals.Doctor of Philosoph