| Summary: | In the oceans, changes in seawater carbonate chemistry associated with increasing atmospheric concentrations of carbon dioxide (a process referred to as ocean acidification, or OA) is predicted to have significant effects on many marine species, particularly ones that calcify and/or photosynthesize. The effects of OA on these groups likely will be modulated by other environmental factors such as temperature and light. Understanding how these factors (which vary across different spatial scales within marine ecosystems), will interact with OA to influence calcifying autotrophs is important to inform predictions of how species distributions and community structure will shift under future OA and climate change. Thus, to explore the effects of temperature and light on the response of calcifying autotrophs to OA, this thesis investigated how temperature, light, and their interactions with OA affect the physiology and ecology of crustose coralline algae (CCA) found in Pacific coral reefs. In Chapter 2, surveys conducted in the backreef of Moorea, French Polynesia demonstrated that under contemporary conditions, microhabitats that differed strongly in their level of light exposure had significant heterogeneity in the relative abundances of different CCA species. Additionally, the competitive abilities of individual CCA species generally shifted across the light exposure gradient, sometimes leading to switches in competitive dominance within an interacting species pair. These shifts in competitive abilities could partially explain differences in the abundances of the CCA species across the microhabitat types. Thus, light appears to be important for species interactions and community structure of these calcifying autotrophs in a contemporary coral reef. Using this ecological context, in Chapter 3 experiments were conducted in Moorea using the most common species of CCA found in its backreef, Porolithon onkodes. These experiments demonstrated that OA has an inhibitory effect on some aspects of photophysiology (e.g., ...
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