Demographics of bleaching in a major Caribbean reef-building coral: Montastraea annularis

Mass bleaching events have become a major cause of coral decline at a global scale. In the summer/fall of 2005 the northeastern Caribbean experienced a record-breaking sea surface warming that resulted in a prolonged mass bleaching event and significant percent coral cover decline of the principal Caribbean reef-building coral Montastraea annularis. In this study, we measured changes in the vital rates of a M. annularis population before, during, and after the 2005 mass bleaching event; stochastically projected the population with different bleaching regimes using a 100 year horizon; and quantified the population level effect of the bleaching event using a life table response experiment. Size-based transition matrices from 2001-2009 were constructed following 399 colonies through time in 17 permanent photo-transects located in Culebra Island, Puerto Rico. Temporal variation in the population growth rate indicates the population (1) was in demographic equilibrium before the event (lambda approximate to 1.0), (2) suffered a significant decline in growth rate for two consecutive years after the event (lambda < 1.0), and (3) demographically recovered three years after the event (lambda approximate to 1.0). Partial tissue mortality due to bleaching caused dramatic colony fragmentation that resulted in a population made up almost entirely of small colonies by 2007 (97% were < 50 cm(2)). The stochastic simulation indicates that an annual probability of bleaching in excess of 6% would result in a decreasing population (lambda(s) < 1.0) with a reduction of more than 54% in colony abundance after 100 years of projection. The life table response experiments reveal that most of the effect that bleaching had on the population growth rate comes from changes in the survivorship of small colonies. Recent trends in population decline, as well as the life history traits of M. annularis, suggest that recovery of affected populations by sexual recruitment alone is unlikely. Our findings indicate that survival of small colonies will determine the viability of the M. annularis populations within the context of rising sea surface temperatures. We conclude that the demography of M. annularis is highly susceptible to bleaching and that its viability is seriously compromised under the predicted global warming scenarios.