In partial fulfillment of the requirements for the degree of

Doctor of Philosophy in Biology
In the
School of Biological Sciences

NOAM ALTMAN-KUROSAKI

Will defend his dissertation

COMPETITION AND CONSUMPTION ON CORAL REEFS IN THE ANTHROPOCENE

MONDAY JULY 7, 2025
2 PM EST
In person location: Ford ES&T L1125
Zoom link: https://gatech.zoom.us/j/6771385277?omn=95015924449

Thesis Advisor:
Mark Hay, Ph.D.
School of Biological Sciences
Georgia Institute of Technology

Committee Members:
Julia Kubanek, Ph.D.
School of Biological Sciences
Georgia Institute of Technology

Lin Jiang, Ph.D.
School of Biological Sciences
Georgia Institute of Technology

Marc Weissburg, Ph.D.
School of Biological Sciences
Georgia Institute of Technology

Deron Burkepile, Ph.D.
Department of Ecology, Evolution, and Marine Biology
University of California, Santa Barbara

ABSTRACT: 
Coral reefs are among the most biodiverse ecosystems on the planet. However, they are increasingly threatened by stressors at both global and local scales. At global scales, stressors such as ocean warming increase the frequency and severity of bleaching events. At local scales, factors such as overfishing and nutrient pollution can bias reefs towards degraded, macroalgal dominated reef states, especially following disturbances. However, there is substantial spatial variation in coral reef resilience. Thus, understanding the processes that drive reef decline at different scales will be important for predicting and understanding changes in reef state. In this dissertation, I investigated potential processes and mechanisms that drive coral reefs towards degraded states. I first examined the mechanisms governing competition between corals and turf algae—the most common competitor with corals on reefs. I found that turfs can compete with corals via the production of allelopathic surface chemicals, but the impact of these chemicals on corals depends on the species involved in the coral-alga pairing and the season or temperature. Turf allelopathy was more potent during the austral summer, suggesting that turf impacts on corals may magnify as oceans continue to warm. I then tested the impact of nutrient enrichment on the territorial behavior of damselfish that culture and protect gardens of algal turfs. I found that increases in algal resource quality due to nutrient enrichment led to increased herbivory by both resident damselfish and roving herbivores, smaller damselfish territories, increased damselfish aggression towards heterospecifics, and reduced aggression towards conspecifics. These changes in behaviors could affect nutrient cycling processes on coral reefs or potentially expose corals that settled in damselfish territories to greater predation or algal overgrowth. When I examined the effects of consumer exclusion and nutrient enrichment on both coral growth and algal overgrowth of corals, I found that a fast-growing but thermally sensitive coral benefitted from consumer exclusion, but a slower growing thermally resistant coral experienced greater predation and algal overgrowth due to nutrient enrichment. This coral also experienced significant mortality and algal overgrowth when herbivores were excluded. Nutrient enrichment could therefore weaken resilience by enhancing predation on, and algal overgrowth of, thermally tolerant corals, and this may be exacerbated by the overfishing of herbivores. Finally, I assessed how ocean warming affects invertebrate corallivores and their impacts on corals. I found that warming enhanced the impact of a sessile predator on a thermally resistant coral by reducing the coral’s tissue reserves. Further, warming increased the metabolism of a mobile corallivore, resulting in increased predation on this coral. Anthropogenic stressors at both global and local scales can therefore threaten reef resilience by increasing the vulnerability of thermally resistant corals to predation and algal overgrowth. By extension, mitigating coastal inputs and maintaining healthy fish populations could bolster reef resilience in the Anthropocene.