UC San Diego

Understanding how relatively healthy coral communities respond to disturbance provides an important baseline to which the response and recovery of more degraded ecosystems can be compared. Here, Palmyra Atoll was used as a case study to examine the response of an intact coral reef community to biotic and abiotic stressors in the form of a species invasion and bleaching event. Techniques from the fields of spatial, chemical, and microbial ecology were used to investigate the response of benthic communities to these disturbances. A decade-long, spatially robust dataset found contrasting patterns in the invasion of a corallimorph, Rhodactis howesii, around the atoll. Decreasing cover of R. howesii was observed at the epicenter of the invasion, but contrasted sharply with increased spread and cover of the corallimorph at additional sites. Sites with high coral cover and low levels of disturbance experienced lower levels of invasion, while sites with high wave energy and lower coral cover experienced increased corallimorph growth. Competitive mechanisms of the corallimorph were examined using an assay that found evidence of bioactive chemical compounds present in R. howesii tissue that caused coral mortality. To further investigate the impact of invasion on coral health, 16S rRNA amplicon sequencing of coral-corallimorph interaction zones showed shifts in coral-associated microbial communities with increasing proximity to R. howesii. Overgrown coral tissue was dominated by bacterial families strongly associated with stress and disease, suggesting that the corallimorph can directly impact coral colonies through microbial community alteration. Evidence of acclimatization was found by studying the response and recovery of coral-associated bacterial communities to the global stressor of the 2015-16 bleaching event. Although microbial community composition shifted as a result of bleaching, healthy symbionts were maintained in coral tissue and pathogenic bacterial invasion was not observed. These data, in combination with the low mortality at Palmyra, support the hypothesis that natural temperature variability may lead to a more resilient community in the face of thermal stress. Collectively, the results of my dissertation provide valuable insight into the response of coral communities to invasion and their ability to adapt to increasing thermal stress events over time. This dissertation also highlights the importance of long-term and spatially robust data sets that allow for accurate quantification of reef trajectories in response to disturbance events.