UC San Diego

Coral reefs are valuable ecosystems that provide billions of dollars globally in ecological goods and services, but they are facing widespread degradation due to climate change. This dissertation provides insights into the dynamics of coral bleaching and recovery, responses of other key taxa such as algae, and broader ecological implications for coral reef communities in the context of thermal stress. Chapter 1 of the dissertation synthesizes results from past studies on coral bleaching: while bleaching severity was highly variable as expected, this was complicated by inconsistent response metrics and the fact that bleaching measurements are often taken at different timing with respect to the onset of thermal stress. By standardizing existing observations, this chapter allows for inter-study comparison of coral bleaching susceptibility by genus, morphology, and/or region. Chapter 2 uses a time series of underwater imagery taken yearly for the past decade from two habitats on Palmyra Atoll to quantify the cover of reef-building corals, crustose coralline algae, macroalgae, turf, and other invertebrates. One year after each of the thermal anomalies in 2009 and 2015, some sites experienced reductions in coral cover which were replaced by turf or crustose coralline algae. However, across the entire decade, benthic community structure changed minimally at the functional group level, with greater stability at the reef terrace as compared to the fore reef. Chapter 3 of the dissertation tracks the growth, discoloration (i.e., lack of pigmentation), partial or whole-colony mortality, survival, and/or regrowth of individual coral colonies on Palmyra. This chapter explores which species were more sensitive or tolerant than others when exposed to thermal stress, and whether a colony’s level of discoloration at the time of warming corresponds to its fate one year later. Finally, Chapter 4 evaluates the long-term effects of increased seawater temperatures on benthic algae. This chapter investigates the abundance of fleshy and calcareous algae on Palmyra’s reef habitats over time, and suggests that a major macroalgal genus, Halimeda spp., showed evidence of temperature sensitivity. Long-term monitoring data sets from Palmyra can be used to establish baseline information for the conservation and restoration of more-threatened reefs at risk of decline.