Award: OCE-1415268

Overview This project was designed to expand on an initial effort funded by NSF to explore the effects of ocean acidification on coral reefs. Having developed a foundational understanding of the organism-level effects of ocean acidification on corals and algae, in the present award we expanded understanding of ocean acidification on coral reefs through experiments with corals and algae grown in common gardens and with whole reef communities grown in the lab and on the reef. Growth in the common gardens revealed variation in the genetic capacity to respond to ocean acidification, and the experiments with reef communities addressed the complex response of diverse communities. Our analyses were conducted over temporal scales from weeks to a year, on both shallow back reef and deep fore reef deep habitats, and utilized autonomous experiments on undisturbed reef communities. At the conclusion of this award, our results reveal transformative discoveries related to a better understanding of the function of coral reefs in a more acidic future, the work has created an international experienced STEM workforce, and the scientific discoveries have been integrated with the curriculum of local schools. Intellectual merit Our research has answered important questions through experimentation with individual corals and algae as well as the reef communities they build in multiple habitats. At each scale of investigation, we have advanced conceptual understanding and technical skills to complete experiments extending from the laboratory to underwater on undisturbed reef communities. The underwater experiments employ a Free Ocean Carbon Enrichment (FOCE) approach to create a new standard for measurement of metabolism of reef communities under conditions employing a high degree of realism. Our research with corals and algae from shallow reefs suggests these organisms will not find refuges from ocean acidification by exploiting shaded microhabitats, but spatial refuges might be found in locations with high flow speeds and natural daily cycles in the concentration of seawater carbon dioxide. Coral and algae show variation in their sensitivity of calcification to ocean acidification among genotypes within each species, thereby revealing the potential for evolution to favor organisms with the capacity to resist future conditions. To enhance the ecological relevance of our experiments, our research expanded to large, outdoor flumes of seawater in which coral communities could be kept for many months and be exposed to ocean acidification conditions. These experiments revealed the subtle effects of ocean acidification on reef communities, notably by favoring the dissolution of the rock and sediments upon which the living reef community is located. Over at least years, the live cover of corals and algae on reefs may show only small effects of ocean acidification, and they may persist until the underlying framework weakens and crumbles. Nevertheless, the rate at which limestone is deposited by reef organisms will decline under acidic seawater conditions, with the effects of these reductions hidden by variation in ways by which the carbonate is deposited to produce new skeletons. Corals and algae may increase in length at similar rates under future conditions, but this extension might be achieved by compromising skeletal strength. The veracity of these predictions are consistent with the results of our FOCE experiments, which underscores the threats posed to coral reefs by ocean acidification. In one glimmer of hope, our FOCE experiments suggests that reef communities can attenuate their sensitivity to ocean acidification at night, thereby suggesting that mechanisms may be present to reduce the susceptibility of net calcification to ocean acidification. Broader impacts This research addressed the response of a beautiful and functionally important ecosystem to critical aspects of physical and chemical world that are changing in the 21st Century. By studying coral reefs and their response to ocean acidification, our work focuses on an ecosystem faced with functional extinction with dire consequences for humans that rely on them for income, protein, and coastal protection. Our results provide the most accurate and rigorous analyses of the ways in which reefs will respond to ocean acidification, thereby informing pro-active ecosystem management and societal decisions. The data from our analyses are freely available to inform these decisions. The research advances supported with this award have been accomplished by placing skilled teams of US researchers in a remote tropical location in the South Pacific. These teams have engaged diverse participants at multiple stages of career developed to work together to solve problems and advance science. These efforts have trained undergraduates, graduates, postdoctoral researchers, and technicians as globally-experienced members of the scientific workforce that can promote US leadership in science, technology, engineering, and math (STEM). To broaden the impacts of our work, we have integrated with high schools in California to advance scientific understanding and promote STEM careers through exciting discoveries and positive role models. In French Polynesia, this theme has been extended to local stakeholders and Village Elders, thus integrating our research with civic leadership decisions. Last Modified: 01/08/2020 Submitted by: Robert C Carpenter