Context-dependent regulation of coral–macroalgae interactions across heterogeneous reef environments

Coral reefs worldwide are declining under multiple anthropogenic stressors, and competitive dynamics between corals and macroalgae are key determinants of reef condition and trajectory. Yet how environmental variation and herbivory jointly shape these interactions remains incompletely resolved. We conducted an 11-month observational field study across four fringing-reef sites on Culebra Island, Puerto Rico, spanning natural gradients in sedimentation, nutrient stoichiometry, seawater temperature, salinity, and Diadema antillarum density. Integrated benthic surveys, coral–macroalgae interaction assessments, and exploratory multivariate analyses revealed strong site-level differences in benthic organization. The two sites with higher sedimentation were characterized by lower coral cover and interaction patterns less favorable to coral persistence, whereas the lower-sedimentation sites retained higher coral cover and more mixed benthic assemblages. Macroalgal functional identity significantly influenced competitive outcomes with coral: Filamentous turf algae (FTA) won significantly more contact interactions than either fleshy macroalgae (FA) or coralline algae (CA), while FA was disproportionately represented at coral tissue margins at the two higher-sedimentation sites. Exploratory ordination was broadly consistent with these patterns, identifying sedimentation, D. antillarum density, and salinity as the variables most strongly associated with among-site differences in community structure, although the limited four-site observational design precludes strong causal inference. Overall, the results support a context-dependent view of coral–macroalgae interactions in which herbivory appears most likely to contribute to favorable benthic states where local environmental conditions are comparatively less stressful.