Macroalgal–Coral Interactions in New Caledonia South West Lagoon: Diversity, Abundance, and Spatial Patterns

Macroalgal-coral interactions (MCI) are an integral yet understudied component of coral reef ecology, particularly in healthy systems where they may represent stable coexistence rather than competition. This study provides the first comprehensive assessment of MCI diversity, abundance, and spatial patterns in the South West Lagoon of New Caledonia (SWLNC). Across 26 coral-dominated habitats, MCI accounted for an average of 16.4% of the benthic cover, with local values reaching 70% in high-interaction areas. A total of 43 unique macroalgal-coral genus pairings were documented, involving 16 macroalgal and 10 coral genera. <i>Lobophora</i> (47%), <i>Halimeda</i> (20%), and <i>Hypnea</i> (9%) were the dominant macroalgae, while <i>Acropora</i> (61%), <i>Montipora</i> (19%), <i>Seriatopora</i> (13%), and <i>Porites</i> (5%) were the most frequent coral interactants. The most abundant specific interactions were <i>Lobophora-Acropora</i> (29%), <i>Hypnea-Acropora</i> (15%), <i>Halimeda-Montipora</i> (10%), <i>Lobophora-Seriatopora</i> (10%), and <i>Halimeda-Acropora</i> (10%). MCI abundance varied markedly among habitat levels, differing across reef types, zonation, and benthic cover. Six recurrent typologies of physical association were identified, and the Coralgal Biotic Interaction Compass (CBIC) is introduced as a conceptual framework to distinguish the nature of macroalgal-coral associations. Overall, the study demonstrates that MCI in the SWLNC are diverse, structured, and non-random, shaped by both interactant identity and habitat filtering rather than ubiquity, providing a robust ecological baseline for future analyses of macroalgal-coral dynamics in Indo-Pacific reef systems.