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Investigating the effects of local stressors on the life cycle of a brooding coral, Pocillopora acuta

Sunlight enters the lab as I sit down to peer through the microscope. I always make sure to slowly increase the light, adjust the ocular lenses, set the magnification to 4x, and look carefully. After some fine adjustments, microscopic circular objects come into view. These tiny objects are algal endosymbionts (Symbiodiniaceae), and there are thousands of these cells living in the tissues of their coral hosts. Reef-forming corals are the foundation of the most elaborate and diverse marine ecosystems in the world, and share a complex symbiosis with their algal endosymbionts that dictates aspects of their physiological response to multiple environmental parameters. I will never forget my first time seeing these microscopic powerhouses in a coral tissue sample. Conducting this research enticed me to explore and understand coral ecophysiology for my master’s research project.

Coral reef ecosystems are hubs of biodiversity supporting a multitude of complex ecological interactions within our oceans. They captivate the imagination of millions of people and provide essential services for societies worldwide. Yet, human impacts are rapidly degrading them. Since these impacts are becoming ever more prominent, I want to dedicate myself to research that closes the gap between fascination with coral reefs and action. Over the past seven years, I have dedicated myself to countless hours of academic, volunteer, laboratory, and field work in that pursuit. These experiences fueled my passion for research-based science and mentoring, preparing me to share my scientific knowledge through collaboration and outreach. For my graduate research, my focus was to study how local and global anthropogenic stressors interact to affect coral physiology and thermal tolerance. More specifically, we investigated the influence of nutrient and sediment loading on coral thermal tolerance in Mo'orea, French Polynesia at the Richard B. Gump South Pacific Research Station.

Figure 1. Our six sites along the western, central, and eastern regions of the north shore fringing reefs in Mo'orea, representing differing nutrient and sedimentation regimes.

Local scale stressors, such as nutrient and sediment loading, can make corals more vulnerable to global climate change by suppressing carbonate productivity (growth) and reducing their photosynthetic capacity. Interactions between global and local scale stressors have become more frequent and are projected to increase with anthropogenic climate change. Therefore, a better understanding of the ecological ramifications of thermal anomalies on coral reef ecosystems is necessary. While many studies have used respirometry (techniques for obtaining estimates of rates of metabolism) to determine the physiological response of numerous invertebrate species (clams, snails, insects, etc.) to temperature changes, the role of nutrient and sediment loading in determining thermal tolerance is unknown. To address this research gap, we collected coral fragments of Pocillopora acuta along a nutrient and sedimentation gradient in Mo'orea, French Polynesia (Image 1) to test their performance (photosynthesis, respiration, and calcification) across a range of temperatures (20-36°C) (Image 2).

Figure 2. Danielle Becker monitoring P. acuta coral fragments during a light net photosynthesis and calcification trail to measure coral metabolic rates over temperature exposures.
Photo By: Danielle Barnas, Silbiger Lab Technician

My project as a California State University, Northridge (CSUN) Master’s student in Dr. Nyssa Silbiger’s Quantitative Marine Ecology Lab, supported by the 2018 AAUS Master’s Research Scholarship, was to determine the influence of nutrient and sediment loading on coral thermal tolerance in Mo'orea, French Polynesia. We collected adult P. acuta coral colonies from our six sites along the northshore fringing reefs in Mo'orea by using SCUBA. At each site, we categorized coral abundances through the use of benthic surveys, deployed HOBO loggers to measure environmental parameters, installed sediment traps, and set up cages for coral recovery after fragmentation (Image 3). I felt comfortable in completing my research due to scientific diving techniques strongly developed through my AAUS dive training at the Bermuda Institute of Ocean Sciences (BIOS) and CSUN. I was so thankful to be able to dive in such a dynamic tropical coral reef ecosystem (Image 4).

Figure 3. A temporary coral recovery structure at one of our sites along the eastern region of the north shore fringing reef with corals that were used in respirometry trials.
Photo By: Dr. Nyssa J. Silbiger

The 2018 AAUS Masters Research Scholarship provided me with the means to travel to Mo'orea this past Summer 2019 to complete research pertaining to the first chapter of my master’s degree. Our study found that along an increasing nutrient and sedimentation gradient, metabolic responses of corals such as photosynthesis, respiration, and calcification significantly decreased. Mechanisms linked to thermal sensitivity, performance and overall rate processes were involved, illustrating the role of local scale anthropogenic inputs, and how they may exacerbate the detrimental impacts of global anthropogenic stressors on coral reef ecosystems. Once we understand these effects, management of land use, dredging, pollution and nutrient influx can be adjusted to reduce our impacts on coastal coral reef ecosystems experiencing the most damage from local scale stressors.

Figure 4. Danielle Becker surveying possible candidate coral colonies to be collected and used in respirometry trials.
Photo By: Danielle Barnas, Silbiger Lab Technician

Not only has the 2018 AAUS Masters Research Scholarship provided me with the opportunity to complete my research in a tropical marine ecosystem, it helped me emphasize the importance of diversity in STEM fields by mentoring undergraduate students at CSUN and providing the opportunity to take an undergraduate student to Mo'orea to engage in hands-on laboratory and field research experiences related to my project. I was also able to collaborate with the Mo'orea Coral Reef Long-term Ecological Research (MCR LTER) site and the local community through the Coral Reefs of Mo'orea Education Program (Image 5). Through this collaboration, we educated various public schools about the ocean, coral reefs, and the research of the MCR LTER program. Since island nations will be among the first communities to experience the detrimental effects of anthropogenic climate change, involving local communities in coral reef conservation initiatives can help protect the ecosystems they depend on for shoreline protection and food security. By understanding these mechanisms, my graduate research can help advise management strategies that protect coral reefs most vulnerable to these stressors. Ever since my first time observing the Symbiodiniaceae cells at work in coral tissue, I wanted to answer the question of how to preserve reefs. I believe my graduate research findings will help provide valuable insight to the scientific community that may bring us closer to reducing our impacts on coastal coral reef ecosystems.

Thank you AAUS for awarding me the 2018 AAUS Masters Research Scholarship. I appreciate the AAUS Foundation’s continued commitment to supporting graduate student research.

Danielle Becker
2018 AAUS Foundation Scholar

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