Award: OCE-1514861

Elevated seawater temperatures are impacting reefs globally. When severe or prolonged warming persists, corals lose substantial numbers of their symbiotic algae, causing them to pale in a process known as coral bleaching, and depriving them of the food produced by their photosynthetic algal partners. Bleaching damages coral health, interrupts growth and reproduction, and can cause death. Ocean acidification (OA) – the decrease in ocean pH caused by increased carbon dioxide – can sometimes exacerbate the effects of coral bleaching. But we do not know if OA prolongs recovery time from bleaching or if coral feeding on zooplankton can help them to survive bleaching under OA conditions. If recovery takes longer under OA, there is concern that corals would not recover prior to next bleaching event, potentially leading to accelerated coral reef ecosystem failure. In the fall of 2014, a high temperature event resulted in massive coral bleaching across the Hawaiian Archipelago. This event allowed us to use corals that bleached or not under natural conditions to conduct a year-long recovery experiment to test the following three hypotheses: 1) OA increases recovery time from bleaching; 2) zooplankton feeding enhances rates of recovery from bleaching; and 3) corals pre-conditioned to warmer, more acidic conditions recover faster from bleaching under OA conditions than those not pre-conditioned. Six bleached and six non-bleached coral colonies of the coral species Montipora capitata and Porites compressa from two sites (24 colonies of each species total) were placed in the following treatments: 1) ambient pH + fed zooplankton, 2) ambient pH + unfed, 3) lower pH + fed zooplankton, 4) lower pH + unfed. The lower pH conditions were set to represent those expected under OA conditions at the end of this century. Corals were maintained under these conditions for a year, and we recorded calcification rate, bleaching status and survivorship monthly throughout the experiment. We also measured photosynthesis, respiration, and feeding rate for each fragment after 1, 7, and 12 months in the tanks and collected a sub-set of the corals for additional laboratory analyses including Chlorophyll a, biomass, surface area, and coral metabolism. Additional laboratory analyses (algal density, total lipid, lipid class, protein, and carbohydrate) are still in progress. Initial findings confirm the expectation that bleached corals had lower algal pigment concentrations (Chlorophyll a) than non-bleached corals for both species and sites. Recovery of coral pigmentation occurred within 7 months and was unaffected by OA or feeding rates for either species. Coral biomass was not affected by bleaching, OA, or feeding. Coral metabolism was compromised in M. capitata under OA conditions, but only if starved. Although calcification and survivorship were higher among unbleached than bleached corals, we saw no detectable effect of feeding on calcification or survivorship for either species over the year-long study, whereas OA led to a small reduction in calcification regardless of other factors. A second mass bleaching event in the fall of 2015 gave us the rare opportunity to observe how corals respond to repeat bleaching stress in the experimental tanks with or without acidification and feeding. The corals showed strikingly similar patterns of bleaching in 2015 as compared to 2014, with the previously bleached corals bleaching again while the previously unbleached corals largely avoided bleaching for a second time. The corals from the warmer site at Kane?ohe also showed lower susceptibility to bleaching and mortality than those from the cooler site at Waimanalo, but neither OA nor feeding affected those responses. Overall we find that corals living under naturally higher temperature and lower pH conditions (e.g., Kane?ohe Bay), are more tolerant of future ocean conditions. We also find that OA does not prolong recovery from bleaching, and that feeding (which would be natural on reefs) acts as a possible ?rescue effect? from OA stress for M. capitata. In addition, whether or not a coral bleached was the primary factor in determining performance, but the long-term consequences of thermal stress also appear to be site dependent and not completely ameliorated by feeding. Additional laboratory analyses are currently underway to confirm whether these preliminary results hold up and whether bleaching-resistant corals are pre-conditioned by the environment, or if certain reefs are populated by more resistant individuals that better survive these future ocean conditions. Broader Impacts: Two post-docs, 8 graduate and 8 undergraduate students were trained under this award, and findings constitute a portion of the dissertation of two graduate students and 4 senior theses for undergraduates. The research findings were presented at the 13th International Coral Reef Symposium, an undergraduate research forum, three public talks, and two departmental seminars to date. Two peer-reviewed publications are in preparation, and a third is expected. Finally, these data will undoubtedly play a role in future management decisions made by the State of Hawaii's Department of Aquatic Resources. All data from this research is deposited at http://www.bco-dmo.org/project/546319. Last Modified: 02/10/2017 Submitted by: Robert J Toonen