Global change is driving the decline of corals on reefs worldwide, yet some coral communities have remained resilient in the face of escalating stressors. On the island of Mo’orea, French Polynesia, coral communities have exhibited strikingly different trajectories, with some reefs recovering from disturbances and others undergoing protracted coral decline. This diversity in coral community dynamics makes Mo’orea 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 algal dominance. We will conduct a 6-year manipulative experiment on Mo’orea to examine what levels of herbivory are needed to provide resistance and resilience to benthic communities and coral microbiomes when reefs are exposed to elevated nutrients and ocean temperatures. We will measure the dynamics of benthic communities, coral demography, and the coral microbiome across seasonality in ocean temperature, allowing us to (1) link changes in coral microbiomes (e.g., a rise in pathogenic bacteria) to the trajectories of coral decline or recovery and (2) link nutrients, herbivory, and temperature to phase shifts in both benthic communities and coral microbiomes. Importantly, we will then test the resistance of phase shifts of benthic communities and coral microbiomes by measuring their changes after removing the nutrient enrichment treatment at the end of year 3 and tracking recovery of the system for 3 more years. Thus, our project will begin to answer whether reef and microbial community phase shifts can be easily reversed once they occur. Many studies have focused on the factors that disassemble coral reef communities, but ours will be the first to examine how reef communities can be reassembled from the microbiome upwards.
Our experiment represents novel field examinations of the impacts of multiple stressors on corals and their microbial associates. This research will allow us to uncover mechanisms of resistance and resilience of coral communities to disturbance and anthropogenic stressors. Because many of these mechanisms likely operate in the microbial domain, our combination of experimental field ecology and microbiology will lead to novel insights that integrate across biological scales. The bulk of previous research addressing both the macrobial and microbial ecology of corals has been either small-scale laboratory experiments of limited duration (days to weeks) that may not be ecologically realistic or large-scale correlative studies that suggest patterns but cannot test processes. Thus, our research is not only timely due to the global and local threats to reefs but also unique in its scope and scale. Importantly using our six-year time-series datasets of reef communities, we can identify critical tipping points in the macroscopic and microscopic organisms that influence and potentially reinforce phase shifts on coral reefs. Understanding how human impacts interact to influence coral, algal, and microbial communities is critical for predicting the future of coral reefs and for identifying conservation priorities and management interventions that will help protect and conserve coral reefs as the climate crisis unfolds.
Our field experiment provides a realistic platform to test questions about how local management of fisheries can alter reef health and provides data about the recoverability of reefs should new water quality management be put into place. This interdisciplinary work will train a new generation of both marine ecologists and microbiologists. The project will train: 1 postdoctoral researcher, 2 graduate students, as well as numerous undergraduates. Our main international outreach effort will be to direct a co-sponsored science project to map the microbiome of the island of Mo’orea. Mo’orea is ~130km2 in area and has 5 major watersheds that transport sediment and nutrients to the nearshore coral reef ecosystems. Thus poor stewardship of these watersheds likely contributes to the local phase shifts we are currently seeing in several areas of the lagoon. Therefore we have engaged the local community to help collect microbiome samples from 50 terrestrial, 50 stream, 25 coastal sites, and 25 offshore sites around the island. We will generate an island-wide map of the microbial communities associated with the soils, streams, and coastal waters that can be linked to adjacent coral reef health—The Moorea Microbiome! As part of this outreach effort, we will also collaborate with filmmakers to make a trilingual (English, French, and Tahitian) film about the project to serve as local engagement and teaching tool to help educate school groups and different stakeholders about both the seen and unseen connections between land and sea on their island.