Abundance of susceptible coral explains prevalence patterns during Guam’s 2013 bleaching event

Background/Question/Methods

In 2013 coral reefs around Guam experienced a prolonged coral bleaching event. The episode lasted for 12 degree heating weeks - a value typically indicating severe bleaching and widespread mortality. During the event, 48% of the shallow reef (3 – 5 m depth) bleached. This severity resulted from anomalously high sea surface temperatures and irradiance levels. To characterize coral bleaching impacts, we asked how bleaching prevalence and species susceptibility were distributed around Guam. Using long-term wave exposure and the island’s geomorphology, we categorized the shallow reef zone following cardinal direction. This led to a quadrant-based approach. Testing this, we used 0.25 m² photo quadrats to record coral composition, coral colony size, and bleaching prevalence across 47 sites around Guam.

Results/Conclusions

We found that long-term wave exposure was positively correlated with coral composition such that highly susceptible coral species were most abundant in easterly exposed reefs where bleaching prevalence was highest. Bleaching prevalence was highest in Guam’s southeastern reefs (79%) and northeastern reefs (59%). Using univariate and multivariate approaches, we found that within reef communities around Guam, several abundant genera were highly susceptible (Acropora, Montipora, and Pocillopora), while others, particularly massive forms (Porites, Dipsastrea, and Goniastrea), were less susceptible. Interestingly, there was a stark difference in bleaching susceptibility among Acropora morphologies. Within this genus, only 35% of corymbose Acropora bleached while 80% of caespitose Acropora bleached. Species susceptibility defined the bleaching response around Guam. Because Guam’s exposed reefs are dominated by an abundance of specious and highly susceptible coral families (i.e., Acroporidae and Pocilloporidae), site diversity had the highest correlation with bleaching prevalence (R-squared = 0.35). With rising sea temperatures, many coral reefs are predicted to experience repeated annual prolonged thermal anomalies over the next century. This research is the most comprehensive study on coral bleaching within the Marianas archipelago and the approach presented here can be applied to test reef resilience and adaptive bleaching hypotheses following future bleaching episodes.