Collaborative Research: Ocean Acidification and Coral Reefs: Scale Dependence and Adaptive Capacity K. Gross (NCSU, PI). R. C. Carpenter and P. J. Edmunds (CSUN), collaborators. Coral reefs support some of the globe’s most spectacular and biodiverse ecosystems. Yet, many of the stony coral species that populate these reefs are thought to reside in environments at or near the limits of what their physiology can tolerate, making those species and the reefs that they inhabit particularly vulnerable to ongoing environmental change. In particular, reef-forming corals, like many other marine invertebrates, rely on the chemical process of calcification to build their exoskeletons. As ocean waters acidify, the process of calcification becomes more challenging, and thus costlier for the coral individual to maintain. Moreover, ocean acidification can make coral skeletons more fragile and thus more susceptible to fragmentation and erosion from hydrodynamic stress. Thus, an important question facing marine biology today is to understand how the ongoing process of ocean acidification might alter the physiology and ecology of coral species, and how those changes might in turn impact the reef ecosystems that those coral species inhabit. In this research, we first developed a conceptual framework to understand how the impacts of ocean acidification on coral reefs are connected across different levels of biological organization. This framework identified the primary biological processes that link the effects of ocean acidification on individual coral colonies to the populations, communities, and ecosystems that those corals inhabit. This research provides a roadmap for marine biologists to integrate research across multiple, nested levels of biological organization, thus enabling a more synthetic understanding of how the full impact of ocean acidification is experienced on contemporary reefs. We also identified reefs across the globe that might serve as "oases" of resilient coral in otherwise degraded seascapes. We then developed and analyzed a mathematical model of the population dynamics of a common coral species in shallow reefs of the Pacific Ocean. We used that model to investigate how changes in the physiology of individual coral colonies caused by chronic environmental stressors like ocean acidification might impact the reefs in which those colonies are found. We found that changes in the physiology of individual colonies have a bigger impact on whole-reef coral abundance when reefs simultaneously experience intermittent die-offs, such as those that might be caused by coral bleaching, by tropical cyclones, or by outbreaks of coral predators such as the Crown-of-Thorns starfish. Thus, we expect slowly changing features of the environment --- like ocean acidification or increasing sea temperature --- to disrupt coral reefs more profoundly in habitats where occasional die-offs from tropical cyclones, predators, etc. are more intense and/or more severe. This phenomenon arises because die-offs shift the reef to a regime of continual recovery and replenishment. In such a regime, changes in the physiology of individual colonies have a bigger effect on coral cover than they do on more stable and fully developed reefs, where open substratum is more quickly occupied by existing coral colonies. Finally, we also showed that under a variety of scenarios, coral populations tend to wax and wane on their own, as cohorts of colonies establish, grow, die, and are replaced by subsequent cohorts. This finding suggests that not every reduction in coral cover can be automatically attributed to environmental degradation, although it is still fully consistent with the suggestion that environmental degradation is driving much of the decline in coral abundance observed on today’s reefs. Altogether, our research complements ongoing laboratory and field work by extending our understanding of how ocean acidification and other slowly changing environmental variables impact coral populations to scales of time, space, and biological organization that often lie beyond the reach of direct experimentation. This understanding sharpens our ability to project the fate of coral reefs and the ecosystems that they support in the future marine environments that likely await. Last Modified: 02/19/2020 Submitted by: Kevin Gross
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