Coral Forests: How Mangroves Provide Refuge to Corals from Light Stress on the Shallow Reef

Abstract

Given increasing reports of corals living among roots under mangrove canopies, we used field surveys and reciprocal transplants to test the hypothesis that mangroves are a refuge for corals from the high solar irradiance and temperatures of adjacent shallow reefs. We found that live coral cover and species richness in mangroves rivaled or exceeded that of reefs. Our experimental manipulation of light in mangroves and on adjacent reefs indicated that light intensity is a key environmental parameter mediating coral bleaching and survival. We suggest the role of mangroves acting as coral refugia will become increasingly important with global climate change. Photo 1: Large Siderastrea siderea coral colony observable just below the water’s surface under the mangrove canopy. These large corals can make up 15–51% of the coral cover in the mangrove forest of Bocas del Toro, Panama. Surveying the corals within the mangrove forest requires both snorkeling and climbing the trees to avoid harming the corals growing in the shallow water. Photo credit: Heather A. Stewart, McGill University/Smithsonian Tropical Research Institute. Photo 2: To characterize the coral community in the mangrove forest and adjacent shallow reef, coral surveys were conducted with 1-m2 quadrats quantifying cover, richness, diversity, abundance, and identity of live coral species. Photo credit: Heather A. Stewart, McGill University/Smithsonian Tropical Research Institute. Photo 3: Treatments included natural conditions in each of these habitats, plus experimental addition of shading on the reef to mimic the mangrove canopy, and experimental removal of the mangrove canopy to mimic light conditions on the reef. The corals represented two species observed more frequently in mangroves (Montastraea cavernosa and Pseudodiploria clivosa), two species observed more frequently on the reef (Diploria labyrinthiformis and Porites astreoides), and two that were observed in similar frequency in both habitats (Colpophyllia natans and Orbicella faveolata). We found that the proportion of coral bleaching and mortality increased with light intensity across all coral species except Diploria labyrinthiformis. Photo credit: Heather A. Stewart, McGill University/Smithsonian Tropical Research Institute. Photo 4: Close-up of Montastraea cavernosa colony with polyps extended within the mangrove canopy. This coral species typically grows in a plate morphology and with coloration such as blue or green hues that differ from domed morphology and red, yellow, or brown hues on the adjacent reef, indicating some differences in traits associated with light conditions under the canopy. Photo credit: Heather A. Stewart, McGill University/Smithsonian Tropical Research Institute. These photographs illustrate the article “Caribbean mangrove forests act as coral refugia by reducing light stress and increasing coral richness” by Heather A. Stewart, David I. Kline, Lauren J. Chapman, Andrew H. Altieri published in Ecosphere. https://doi.org/10.1002/ecs2.3413.