| Summary: | Global climate change induces multiple stressors on tropical coral reefs that threaten their persistence. Ocean acidification decreases calcification in most dominant reef builders, such as crustose coralline algae (CCA). Climate change also has the potential to increase the biomass of fleshy macroalgae and filamentous turf in coral reef ecosystems. While fleshy macroalgae and turf may shade, abrade, and have otherwise negative consequences on CCA metabolism, their high rates of photosynthesis may mitigate OA locally through carbon uptake, resulting in a local increase in pH. This thesis explored the effects of OA, combined with the presence of either fleshy macroalgae or algal turfs, on Lithophyllum kotschyanum, an abundant species of CCA in Moorea, French Polynesia. In a mesocosm study, three canopy types, clear mimics, dark mimics, and S. pacificum, were crossed with two CO2 levels, ambient (400 μatm) and elevated (1000 μatm). The clear, dark, and S. pacificum canopies resulted in stepwise decreases in calcification of L. kotschyanum. This response suggests that shading and likely flow moderation decrease CCA calcification. To separate the effects of fleshy macroalgal metabolism from the effects of its physical structure, a subsequent mesocosm and field experiment were performed. In the mesocosm study, a header tank that provided S. pacificum-treated seawater to treatment tanks was used to determine the metabolic effect of S. pacificum on L. kotschyanum. In the field, S. pacificum canopies were attached to 20 30 cm grids, upstream from CCA samples. Data from the mesocosm study support a positive effect of carbon uptake by S. pacificum, but the metabolic effect did not translate into the field. Because S. pacificum was placed in closer proximity to CCA samples in the field than in lab, the difference in L. kotschyanum calcification between the mesocosm and field experiment may be due to physical effects of the canopy in the field, such as shading. The combined results of these two studies suggest that ...
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