Phd Defense by Nicole Johnston

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In partial fulfillment of the requirements for the degree of


Doctor of Philosophy in Biology

In the

School of Biological Sciences


Nicole Johnston


Will defend her dissertation


Climate Change and the Behavior, Physiology, and Ecology of Coral Reef Organisms

Wednesday, March 4th, 2020

10:00 AM

Krone Engineered Biosystems Building

Children’s Healthcare of Atlanta Seminar Room (1005)


Thesis Advisor:

Dr. Mark Hay

School of Biological Sciences

Georgia Institute of Technology


Committee Members:

Dr. Julia Kubanek

School of Biological Sciences

Georgia Institute of Technology


Dr. Frank Stewart

School of Biological Sciences

Georgia Institute of Technology


Dr. Lin Jiang

School of Biological Sciences

Georgia Institute of Technology


Dr. Valerie Paul

Smithsonian Marine Station

Smithsonian Institution



The magnitude of ocean acidification (OA) and warming predicted to occur within the next century could have significant negative effects for organisms that inhabit coral reefs. Our understanding of how these stressors will impact coral reef organisms is complicated by the diverse behavioral and ecological interactions that exist on these reefs. In a series of experiments, I explored interactions between coral reef organisms, evaluated how some of these interactions may be affected by OA and warming, and then studied how environment may shape an organism’s response to a changing climate. First, through a sensory manipulated tank and a two-chamber choice flume, I demonstrated that anemonefish respond to both chemical and visual conspecific cues, but they require a combination of these two cues to correctly identify conspecifics. Given that previous research indicates that fish behavioral responses to chemical cues are altered under conditions of future OA, this inability to compensate for the loss of one cue through a second cue could affect their ability to acclimate as climate changes. Second, I found that the common Caribbean mounding coral Porites astreoides, is unaffected by competition with Montastraea cavernosa and Orbicella faveolata under ambient environmental conditions, but exhibits significant reductions in photosynthetic efficiency in areas of direct contact with M. cavernosa and O. faveolata under conditions of elevated CO2 and temperature that are anticipated to occur by the year 2100. These results demonstrated that climate change can interact with competition to alter the rate and severity of coral-coral interactions on reefs of the future. Next, I compared the effects of  OA and warming on the physiology of two congeneric coral species (Oculina arbuscula and Oculina diffusa) representing temperate (O. arbuscula) and tropical (O. diffusa) environments and found that, although both corals were negatively impacted by ocean acidification and warming, the temperate coral was slightly more resistant to these stressors. This suggests that temperate species may not be as disadvantaged by climate change as one might expect and may not be easily displaced by more tropical species moving poleward as global oceans warm. Finally, I evaluated the effect of elevated temperature on the well-being of the temperate coral, O. arbuscula when collected from deeper more physically stable environments versus shallower more physically variable environments. I found that corals from both deep and shallow sites were negatively impacted by elevated temperature, but that corals from deeper sites were more strongly impacted. These findings suggest that the physiologies,  biotic interactions, and behaviors of reef organisms may all be affected by climate change and that outcomes of these interactions may not be simple to predict as global oceans warm and acidify and as tropical organisms shift poleward and intermix with temperate species to form novel communities.




  • Workflow Status: Published
  • Created By: Tatianna Richardson
  • Created: 02/18/2020
  • Modified By: Tatianna Richardson
  • Modified: 02/18/2020


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