Award: OCE-2023424

Final Outcomes

 

Background: Global change has driven the decline of corals worldwide with many reefs now lacking corals and being overrun by macroalgae. Yet coral reefs have exhibited strikingly different trajectories, with some reefs recovering from disturbances and others undergoing rapid declines. This research examined the impacts of overfishing and nutrient pollution on the health of corals and their ability to recover after large coral-killing disturbances such as cyclones and marine heat waves. Importantly, we addressed these impacts at multiple levels of biological complexity, including the whole reef community itself, the coral host, and the coral microbiome. Our overarching hypothesis was that factors such as overfishing and nutrient pollution impact coral health via changes to their microbes and physiology, and that when combined with climate change, lead to coral death and reef degradation. 

 

Experimental Design: The island of Moorea, the location of the NSF’s Long Term Ecological Research site on coral reefs, is an excellent model system for testing why some reefs are resilient and return to abundant coral while others are not and undergo persistent phase shifts to macroalgal dominance. In this 6-year experiment, we manipulated two major drivers of reef change, nutrient concentrations to mimic near shore pollution and the abundance of fish consumers to simulate overfishing. We then measured the dynamics of reef communities, coral health, and the coral’s microbiome across seasonal changes in ocean temperature, including across 2 major marine heat waves that led to bleaching events. This design allowed us to (1) link changes in coral microbiomes (e.g., a rise in pathogenic bacteria) to the trajectories of coral decline or recovery, (2) link nutrients, consumer pressure, and temperature to phase shifts in both benthic communities and coral microbiomes, and (3) track if reefs under these different scenarios are able to resist change or be resilient in the face of chronic nutrient pollution, increased fishing pressure and/or climate change.

 

Results: After 6 years of research and analysis, we found that marine heat waves cause the most increases in coral bleaching, tissue loss, and mortality. We documented significant mortality after a major bleaching event in 2019, and then continued mortality over the remaining 5 years of the experiment. During the bleaching event, potential pathogens and microbiome variability increased and were correlated with tissue loss and bleaching severity. Yet in two of the three species we tested, corals and their microbiomes dramatically recovered. Our early observations suggested that some corals have the capacity to recover and that their microbiomes might provide a mechanism of acclimatization during individual and repeated bleaching events. Another major and unexpected finding was that short-term nutrient enrichment was beneficial to our some of our experimental corals and their habitats. This finding was the reverse of what we had previously shown to be true in the reefs of South Florida, where nutrient enrichment and overfishing combined with climate change to cause increased bleaching, disease, and mortality.  In Moorea, where nutrients are much lower than in South Florida, we found that short-term enrichment improved thermal performance in at least one coral species and that growth of another coral species increased as well. We also found that nutrient enrichment altered the microbiome trajectories of corals that settled on our reefs and even altered the composition of dinoflagellates that live in symbiosis with the coral, potentially protecting their coral hosts from increased thermal stress during the marine heat waves. However, after 6 years of the experiment, we also found that after the 2019 and 2020 marine heat waves, the combination of increased nutrients and severe reductions in consumer pressure, in fact, prevented corals and their microbiomes from recovering, ultimately reducing their resilience to climate change and increasing the amount of coral mortality.

 

Outreach and Science Communication: Along with our outlined science program above, we also conducted a wide variety of science communication and education programs. We developed and ran a new high school training summer camp in microbiology for underserved Oregonians. For 3 years, we ran this microbiology summer camp where high schoolers conducted hands-on research in the topics of marine and aquatic microbiology as well as food and soil microbiology. We further published new methodologies to help democratize the work we developed and conducted during this project. Lastly, this project helped train over 5 graduate students, 4 postdoctoral researchers, and 12 undergraduate students.    

 

Conclusion: Our collaborative project with UC Santa Barbara has helped clarify how microbiomes are intimately involved in coral susceptibility or resilience to three factors known to affect reefs: nutrient pollution, overfishing, and climate change. Overall, our 6-years of collaborative and training-focused research showed that some species of corals along with their microbiomes can be resilient to thermal stress, but that preventing long-term local run off and marine species exploitation can help buffer them against increased seawater temperatures and heat waves.

 

Last Modified: 01/14/2025
Modified by: Rebecca L Vega