Effects of heat stress and local human disturbance on the structure of coral reef ecosystems at multiple scales of biological organization




Magel, Jennifer

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The world’s coral reefs are being impacted by myriad disturbances, from localized overfishing and nutrient pollution to global climate change-induced temperature increases and ocean acidification. Conservation of coral reefs in the face of increasing variability and uncertainty requires an understanding of the interacting effects of multiple stressors on the diverse components of these vital ecosystems. In this thesis, I use data from reefs around Kiritimati atoll (Republic of Kiribati) in the central equatorial Pacific Ocean to examine the effects of a severe pulse heat stress event and local human disturbance on two important components of the coral reef ecosystem – three-dimensional (3D) structural complexity and reef fish assemblages. Using 3D reef models constructed through structure-from-motion photogrammetry, I examined changes in reef structural complexity in the year following the 2015-2016 El Niño and mass coral bleaching event. I found that exposure to prolonged thermal stress and subsequent coral mortality resulted in declines in reef structural complexity, particularly reef surface rugosity and terrain ruggedness. Baseline levels of structural complexity were also negatively influenced by local human disturbance, while complexity was positively related to the densities of branching and massive coral growth forms. These findings have important implications for the maintenance of healthy reef ecosystems, as high levels of structural complexity are important for supporting diverse reef-associated fish assemblages. Next, using underwater visual censuses of reef fish assemblages, I quantified fish abundance, biomass, species richness, and assemblage structure before, during, and after the 2015-2016 El Niño. Total reef fish abundance, biomass, and species richness declined during the El Niño, suggesting that pulse heat stress events may have short-term, negative consequences for reef fish. Although these metrics did not vary substantially across the local human disturbance gradient, recovery of assemblages following the heat stress event was impeded by higher levels of local human disturbance. Reef fish assemblage structure was influenced by a more diverse array of factors, showing significant shifts in response to heat stress, human disturbance, and net primary productivity. Given the many important roles that fish play on coral reefs, declines such as those observed here may impair the ecological functioning of these ecosystems. Together, my results highlight the negative impacts of heat stress and local human disturbance on coral reefs, demonstrating ways in which these stressors may interact to limit reef resilience in the face of increasing anthropogenic pressures.



coral reefs, heat stress, reef fish, structural complexity, coral bleaching, local disturbance