Local stressors reduce coral resilience to bleaching

Abstract

Coral bleaching, during which corals lose their symbiotic dinoflagellates, typically corresponds with periods of intense heat stress, and appears to be increasing in frequency and geographic extent as the climate warms. A fundamental question in coral reef ecology is whether chronic local stress reduces coral resistance and resilience from episodic stress such as bleaching, or alternatively promotes acclimatization, potentially increasing resistance and resilience. Here we show that following a major bleaching event, Montastraea faveolata coral growth rates at sites with higher local anthropogenic stressors remained suppressed for at least 8 years, while coral growth rates at sites with lower stress recovered in 2-3 years. Instead of promoting acclimatization, our data indicate that background stress reduces coral fitness and resilience to episodic events. We also suggest that reducing chronic stress through local coral reef management efforts may increase coral resilience to global climate change.

Figure 1. X-radiographs of various coral cores showing the different types of growth behavior discussed.

(A) Coral without the 1998 growth suppression, indicating resistance to bleaching in 1998. (B) Coral with the 1998 growth suppression, recognized by the bright high-density band, but with a quick return to pre-1998 extension rates, indicating resilience after bleaching. (C) Coral with the 1998 growth suppression and continuing depressed extension rates after 1998, indicating a lack of both resistance and resilience to bleaching. (D) A coral with relatively high average extension rate. (E) A coral with relatively low average extension rate. (F) A coral with a partial mortality scar on the left (noted by white arrow), coincident with the 1998 growth anomaly.

Figure 2. Map of the Mesoamerican Reef with locations of coral collections as black circles.

Dark grey denotes coral, light grey denotes land areas. T1, T2 = Turneffe Atoll (4 cores from T1, 13 from T2), S = Sapodilla Cayes (44 cores), U = Utila (17 cores), C = Cayos Cochinos (14 cores).

Figure 3. Bootstrapped means (solid lines) and 95% confidence intervals (shading) for extension rates after 1955.

Extension rates at Sapodilla and Utila remain suppressed after the 1998 bleaching event (p<0.001).