Summary PROJECT INACTIVE

Global climate models project increased rates of mass coral bleaching such that many of the world’s reefs will disappear by mid-century. Typically, models use a Degree Heating Week (DHW) – based bleaching threshold in combination with sea surface temperature (SST) output from global climate models. While these approaches are empirically justifiable, they are nonspecific in applying a generic bleaching threshold to all coral species and typically do not incorporate the potential for corals to adaptively respond to an increasingly warmer ocean. We are scaling up a mechanistic model of coral and symbiont ecological dynamics and symbiont evolutionary dynamics to the global scale, and estimate severe bleaching events based on declines in coral cover. Our model uses changes in SST predicted by the NOAA/GFDL Earth System Model 2 through 2100. We normalized the model to achieve a global bleaching frequency of 5% between 1985-2010 due to the lack of reliable global bleaching records. Coral bleaching responses to rising SSTs are tested using thermally susceptible versus tolerant coral types, with and without variation in symbiont thermal tolerance, and with or without assisted evolution. We found that without symbiont evolution, corals face mass mortality under even the lowest emission scenario (RCP2.6), but that symbiont evolution appears hypothetically capable to considerably mitigate projected global mortality except in the highest emissions case (RCP8.5). Introduction of a thermally tolerant “supersymbiont” appears to be a viable strategy to prevent widespread bleaching through 2100, at least hypothetically assuming that such a symbiont exists and can exchange freely among corals as it takes over. This global mechanistic model provides a valuable tool for assessing the potential effects of genetic or community-level variation in symbiont thermal tolerance on projected rates of mass coral bleaching.

Student Involvement

Undergraduate researcher: Marine Science major Steve Ryan has been working on this project since Fall 2016, and will continue on as a MS student in the Logan lab beginning in Fall 2017.

Broader Significance

The answer to the question of whether corals can adapt quickly enough is critically important for evaluating the merit of alternative conservation strategies. Whether the rates of evolution by natural selection will be fast enough for corals to keep up with the rate of current and future environmental change is unknown. Our model provides a valuable tool for assessing the potential effects of genetic or community-level variation in symbiont thermal tolerance on projected rates of mass coral bleaching.