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

## TL;DR

This study explores how seaweeds and corals interact in a healthy reef system in New Caledonia, finding that these interactions are diverse, common, and influenced by the environment.

## Contribution

The study introduces a new framework, the Coralgal Biotic Interaction Compass, and provides baseline data on macroalgal–coral interactions in a healthy reef system.

## Key findings

- Macroalgal–coral interactions (MCI) covered 16.4% of the reef surface on average, with some areas reaching 70%.
- Lobophora–Acropora interactions were the most common, making up nearly a third of all observed interactions.
- Six distinct types of physical associations between macroalgae and corals were identified.

## Abstract

Coral reefs host both corals and macroalgae (seaweeds) that often live in close contact. While these interactions are well studied on degraded reefs, much less is known about how they occur in healthy reef systems. We surveyed macroalgal–coral interactions (MCI) across 26 habitats in the South West Lagoon of New Caledonia and found that these interactions are common and varied. On average, they covered 16% of the reef surface, involving 43 combinations of coral and macroalgal genera. The most frequent interactions involved Lobophora, Hypnea and Halimeda macroalgae with Acropora, Montipora, Seriatopora and Porites corals. Some interactions were far more common than others—for example, Lobophora–Acropora represented almost a third of all interactions. Their distribution also depended on habitat type, showing that these interactions are not random but shaped by the local environment and the identity of the taxon involved. We also describe six main forms of association between macroalgae and corals and introduce a new framework, the Coralgal Biotic Interaction Compass, to guide future studies. These findings show that MCI are a normal and structured feature of undisturbed reefs, providing essential baseline data for understanding coral reef ecology and resilience.

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. Lobophora (47%), Halimeda (20%), and Hypnea (9%) were the dominant macroalgae, while Acropora (61%), Montipora (19%), Seriatopora (13%), and Porites (5%) were the most frequent coral interactants. The most abundant specific interactions were Lobophora–Acropora (29%), Hypnea–Acropora (15%), Halimeda–Montipora (10%), Lobophora–Seriatopora (10%), and Halimeda–Acropora (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.