| Summary: | Global climate change is not only the greatest threat to coral reefs, but also affects these tropical systems disproportionately, as shallow coastal environments tend to acidify and warm much faster than the open-ocean. Ocean acidification (OA) hinders the calcification of dominant reef builders, such as crustose coralline algae (CCA), and the concurrently warming ocean waters will likely exacerbate this OA perturbation effect. Natural heterogeneity of the physical reef environment may also impose additional stress on these sessile calcifiers, as CCA already experiencing high-frequency variability in seawater pH and temperature may not be able to tolerate further environmental changes that worsen any adverse effects. Because OA and warming are expected to intensify under climate change, and the scale and frequency of environmental variation is also expected to increase, I evaluated the interactive effects of variable seawater pH and temperature on the physiology of a dominant tropical coralline, Lithophyllum kotschyanum, in Moorea, French Polynesia. A 6-week field experiment demonstrated that L. kotschyanum photosynthesis declined in back reef environments with greater in situ variability, whereas calcification and respiration were unaffected by environmental differences. Net photosynthesis was 34% lower for algae in the downstream habitats where pH and temperature of seawater fluctuated at larger daily magnitudes of about 0.05 pH and 0.5 °C, respectively, than in upstream habitats. Net calcification and respiration also showed declined rates in downstream L. kotschyanum populations, although these trends were not statistically significant. The immediate photosynthetic response compared to calcification and respiration responses during this field study suggests CCA photosynthesis may be more sensitive to changes that amplify diel seawater variability occurring on reefs today. A subsequent 2-week mesocosm study was conducted to tease apart the direct effects of diel pH and temperature variation on L. kotschyanum ...
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