[Paper Review] Hydrodynamics shapes annularity in coral reefs via scale-free growth processes

Eva Llabrés, Àlex Giménez-Romero+2 more|arXiv (Cornell University)|Mar 14, 2026

Coral and Marine Ecosystems Studies0 citations

TL;DR

The paper presents a numerical model coupling coral growth with hydrodynamics that generates annular reef forms and scale-free geometric patterns across scales, aligning with observed morphologies.

ABSTRACT

Atolls are traditionally explained as the result of coral reefs accreting around volcanic islands followed by gradual subsidence, yielding a hollow, ring-shaped rim that can extend for kilometres. However, satellite imagery shows that similar annular outlines also appear in much smaller patch reefs, where atoll-forming geological pathways do not apply. In some systems, small annular patches occur within the lagoons of larger atolls, producing nested ring-like patterns. The recurrence of annularity across such contrasting contexts and scales suggests that shared, self-organising processes may also contribute to shaping these reefs. Here, we test whether interactions between reef growth and marine currents can generate annular forms and explain their cross-scale geometric regularities. We develop a numerical model in which coral growth follows simple process-based rules, with local colonisation and mortality depending on resource supply and hydrodynamic stress, and water flow resolved using fluid dynamics. Simulations show that this coupling robustly produces ring-like patch reefs and atoll-like configurations across spatial scales, consistent with observed morphologies. Beyond qualitative agreement, the emergent reefs reproduce key geometric signatures reported in global datasets, including scaling laws and fractal dimensions. Together, these results identify coral-current interactions as a plausible pathway to annular reef formation and a mechanistic explanation for scale-free reef geometry.

Motivation & Objective

• Motivate understanding of cross-scale annularity in coral reefs beyond geological subsidence explanations.
• Test whether growth–flow interactions can generate ring-like reef forms in both small patch reefs and larger atoll contexts.
• Assess if emergent reef geometry exhibits known scaling laws and fractal characteristics.
• Provide a mechanistic, process-based explanation for annularity through hydrodynamic stress and resource supply.
• Evaluate the robustness of annular patterns to varying environmental and flow conditions.

Proposed method

• Develop a process-based numerical model where coral growth relies on local colonisation and mortality governed by resource supply and hydrodynamic stress.
• Resolve water flow using fluid dynamics within the reef environment.
• Couple reef growth dynamics to the resolved flow to simulate feedbacks between hydrodynamics and recruitment/mortality.
• Run simulations across spatial scales to generate ring-like patches and atoll-like configurations.
• Analyze emergent reef geometries for quantitative signatures such as scaling laws and fractal dimensions.

Experimental results

Research questions

• RQ1 Can interactions between coral growth and marine currents produce annular reef forms across scales?
• RQ2 Do cross-scale reef morphologies emerge from hydrodynamics–growth coupling that resemble observed patterns?
• RQ3 Do the emergent reef geometries exhibit scaling laws and fractal characteristics consistent with global datasets?
• RQ4 Is the annularity robust to variations in resource supply and hydrodynamic stress?

Key findings

• Simulations show ring-like patch reefs and atoll-like configurations emerge from coral–current interactions.
• Emergent reef geometries reproduce key geometric signatures reported in global datasets, including scaling laws and fractal dimensions.
• The coupling between growth rules and resolved hydrodynamics provides a mechanistic pathway for scale-free reef geometry.
• The results support annularity as a plausible byproduct of local growth–flow interactions across scales.