Physiological consequences of nitrogen enrichment for corals in the Caribbean

Nutrient pollution has been a major contributor to coral decline throughout the Caribbean. Coral physiological responses to excess nutrients vary with nutrient forms (e.g. nitrate or ammonia), concentrations and nitrogen-to-phosphate (N : P) ratios. However, how these responses differ across nutrient contexts remains understudied. We show that Orbicella annularis photosymbiont densities respond differently to excess nitrogen in phosphorus-limited versus nitrogen-limited environments. Along Curaçao's leeward reef, excess nitrogen significantly decreased (p < 0.05) photosymbiont density under phosphorus-limited conditions (N : P > 16) with low phosphorus (mean = 0.07 µM ± 0.06). In contrast, data from Barbados indicate a significant increase (p < 0.01) in photosymbiont density under nitrogen-limited conditions (N : P < 16). These findings highlight how nutrient contexts shape coral responses to nitrogen inputs, emphasizing the need to consider nutrient dynamics in coral conservation strategies.