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Award: OCE-1041270
Award Title: RUI: Ocean Acidification- Category 1- The effects of ocean acidification on the organismic biology and community ecology of corals, calcified algae, and coral reefs
This project focused on the effects of ocean acidification (OA) on coral reefs around Moorea, French Polynesia, where the United States operates the Moorea Coral Reef, Long-Term Ecological Research site (http://mcr.lternet.edu). OA describes the acidification of seawater by the dissolution of atmospheric carbon dioxide (pCO2) of human origin, and the changes in seawater chemistry threaten the ability of calcified organism to mineralize. These threats affect marine organisms throughout the world, but the challenges to tropical coral reefs are acute as the biological engineers of these systems – corals and calcified algae – rely on high rates of mineralization to build massive, wave-resistant structures providing goods and services to countless organisms, including humans. Together with threats to coral reefs created by rising seawater temperature and local-scale disturbances, some researchers have suggested the future of coral reefs is threatened. To address these issues, we embarked on a 4-year program to answer four questions: (1) what are the shapes of the relationships between pCO2 and the ability of corals and calcified algae to calcify, (2) how are the shapes of these relationships affected by light and temperature, (3) are the effects of pCO2 on calcification by corals and calcified algae affected by food and nutrients, and (4) how can results from small experiments with individual corals and calcified algae be summed to understand how coral reef ecosystems will change in a more acidic future? In terms of intellectual merit, we have published > 15 research papers (http://www.bco-dmo.org/project/2242) and 9 graduate theses in the course of addressing the project goals. Together, these papers contribute to the field in three important ways. First, our work has confirmed the magnitude of the threat posed to corals and calcified algae by OA, but we have consistently found that at least a few species are resistant to the extent of OA that is expected to occur on a centurial scale, potentially because they are capable of utilizing bicarbonate to support calcification in the light. Resistance to OA appears to be an intrinsic feature of these taxa, as we obtained similar results in Hawaii and Okinawa; in contrast, susceptibility to OA is slightly accentuated in fast growing corals and calcified algae. Together, these results provide a basis to understand which corals and calcified algae are likely to populate reefs of the future (i.e., the "winners"), and which are likely to become rare (i.e., the losers). Second, for corals and calcified algae that are negatively affected by OA, we have shown that their responses are linear without "tipping points". This is an important discovery, for while it shows that OA will depress calcification, the effects are likely predictable and gradual. Finally, as our experiments matured, we progressed from organism-scale analyses in indoor tanks, to community-scale analyses in large outdoor flumes exposed to natural sunlight and realistic flow speeds. We created replicas of reef communities in these flumes, and matched the communities to those occurring in the lagoon and at 17 m depth, and incubated them at elevated pCO2. The results of these experiments underscore the negative effects of OA on calcification of whole reef communities, but they show that the most serious effects will be caused by dissolution of sediments and the reef framework itself. In terms of broader outreach, our project has contributed to answering a question of broad societal value: will coral reefs endure in a more acidic future? Our work suggests that coral reefs will persist on a centurial scale, although the species of corals and algae building the reefs will likely change, and dissolution of foundational reef rock will progressively weaken the underlying framework. In the course of answering this question, we have provided unique opportunities for US students (12 graduates, and 7 unde...