Macroalgae and Light Availability Modulate the Distribution of the Temperate Coral Astrangia poculata

Abstract

The temperate coral Astrangia poculata is rapidly becoming a convenient system for studying symbiosis, microbiome, and thermal resilience. However, the ecology of this species is poorly understood. A. poculata is a temperate coral that employs facultative symbiosis to survive in cold, nutrient-rich New England waters. The two ecotypes—symbiotic mixotrophic and aposymbiotic heterotrophic colonies—exist in the same environment, but their abundance changes across depth gradients. We conducted quadrat sampling along a depth gradient to quantify the density of A. poculata at Fort Wetherill State Park, RI, and determine the correlates of ecotype distribution. Further, we quantified light availability and macroalgal cover as possible ecological drivers of abundance. Symbiotic and aposymbiotic A. poculata exhibited a bell curve distribution along the depth gradient. In shallow environments, where both light and macroalgae are abundant, macroalgal cover was inversely related to coral density, implying a negative effect on coral abundance. In deeper environments where macroalgae were not present, coral cover was higher. However, light limitation and depth may limit A. poculata abundance, with symbiotic colonies growing no deeper than 12.5 m and only aposymbiotic colonies present from 13 to 22 m. As macroalgae abundance and turbidity increase with more nutrient loading in coastal marine ecosystems, monitoring their effect on the distribution and density of A. poculata is critical to conserve this monotypic species.