Date

Summer 2026

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)

Department

Moss Landing Marine Laboratories

Abstract

Rising sea surface temperatures have driven widespread coral bleaching and mortality, yet some corals persist through repeated marine heatwaves. In the Eastern Tropical Pacific (ETP), extreme conditions provide a natural laboratory for studying resilience, and some corals have persisted even after declines due to repeat El Niño-driven mortality events. This study investigates drivers of coral resilience by fate-tracking three dominant genera in the ETP at a restoration site on Isabela Island, Galapagos during a natural warming event. I evaluated how algal symbiont community composition, genetic lineage, temperature, and site-specific environmental conditions interacted to shape coral growth, bleaching, and survival before, during and after the 2023-2024 El Niño. Corals were monitored within a coral nursery and at two nearby depth-matched outplanting sites with varying levels of wave exposure and temperature. Pocillopora corals, a branching group, at the warmer, more protected site grew faster than corals at the cooler, less protected site, indicating strong site-level effects on performance. Symbiont communities in these corals shifted after peak El Niño conditions, with increases in proportion of heat-tolerant symbionts (Durusdinium), primarily at the warmer, more protected site. In comparison, massive corals (Pavona spp., Porites spp.) did not exhibit symbiont shuffling and maintained a higher proportion of heat-sensitive symbionts (Cladocopium) during and after the El Niño event. I also detected variation in growth and symbiont composition across Pocillopora genotypes. The Pocillopora haplotype associated with greater thermal tolerance also showed the slowest growth potential trade-off for thermal tolerance. Across linear mixed effects models, thermal history was more strongly correlated with growth than either symbiont composition or haplotype. However, because the 2023-2024 El Niño did not produce severe thermal stress or widespread bleaching in the Galapagos, the limited temperature extremes may have constrained our ability to resolve the relative influence of genetic lineage and symbiont composition versus site-level conditions on growth. These findings should therefore be interpreted as reflecting responses to a moderate, rather than extreme, disturbance scenario. Collectively, my results show that coral performance during warming is shaped by both local environmental context and individual biological variability, providing insight into the interactive roles of host-symbiont associations and environmental context in determining coral resilience. These findings suggest that climate-resilient restoration strategies should prioritize thermally tolerant coral-symbiont pairings while accounting for site-specific environmental conditions.

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