Award: OCE-2023701

Coral reefs are currently imperiled from a variety of human-induced threats from climate change, coral diseases, overexploitation of important fish species, and enrichment with excessive amounts of nutrients. These threats can result in the decline in corals and fishes and the rise in seaweeds, turning coral reefs into seaweed reefs. Our research on the coral reefs of Moorea, French Polynesia examined how altering levels of herbivory, which mimics overfishing, and increasing nutrient availability, which mimics nutrient pollution alter the dynamics of coral reef ecosystems. Further, we were able to contrast the effects of two disturbances (a simulated cyclone and a natural coral bleaching event) on the reef communities and how they interacted with different levels of herbivory and nutrient pollution. A key aspect of our work was that we tracked the impact of these factors both on the macroscopic organisms, the corals and algae on the reef, as well as the microscopic organisms, the bacteria, that life on corals.

 

One of our key findings is that herbivorous fishes have strong impacts on the communities of coral reefs after a disturbance such as a cyclone that removes dead corals. After our simulated cyclone, when herbivores were present macroalgae were relatively rare but when herbivores were removed, macroalgae became very abundant, suggesting that herbivores exert strong control on benthic communities after cyclone disturbances. However, in response to the coral bleaching event that killed corals and left their dead skeletons intact, herbivores had little impact on the community and macroalgae proliferated across the experiment. It did not matter whether herbivores were present or absent, macroalgae increased in abundance and helped suppress the recruitment of new baby corals. Importantly, the dead coral skeletons after the coral bleaching event acted like hotspots of macroalgal development. Then the macroalgae from the dead corals actually grew onto the live corals and killed the corals that survived the initial bleaching event. So, the dead corals that died from coral bleaching acted like ‘zombie corals’ in that their dead bodies, and the algae that grew on them, continued to kill live corals. Thus, our results show the important contrasting impacts of herbivores after different types of disturbances. After the cyclone, which reefs have been experiencing for millennia, herbivores kept algae at bay and helped corals recover. However, after a coral bleaching event, herbivores no longer could control algae, which proliferated everywhere, and corals could not recover.

 

Another important set of results from our project allowed us to understand how marine heat waves, nutrient pollution, and overfishing affected the microbiome, the key bacteria, that live on corals. Our previous research had shown that nutrient pollution and the removal of herbivorous fishes disrupted the microbiomes of corals via competition with algae and the nutrients facilitating coral pathogens, often leading to the death of corals. However, in our current work, the marine heat wave that occurred during our project overwhelmed the effect of nutrient pollution and the removal of herbivores on coral microbiomes. The impact of the heat stress clearly disrupted the coral microbiomes for months to years with some corals showing permanent changes to the microbiome. The impact of nutrient pollution and removing herbivores only became apparent multiple years after the marine heat wave was over showing that local stressors like nutrients and overfishing can affect coral microbiomes but that warming ocean temperatures can override these local stressors.

Finally, our broader impact work focused on creating a film about the project and our work in Moorea. ‘The Moorea Microbiome’ documented the science and our interactions with the local communities of Moorea and our work on communicating our science to school groups and local stakeholders about the importance of how nutrient pollution and overfishing impact reefs. Our film can be viewed on YouTube https://www.youtube.com/watch?v=SKCj3J13yF8

 

Last Modified: 01/06/2026
Modified by: Deron E Burkepile