Morphologically driven sedimentation patterns on a coral reef

Sedimentation on coral reefs has often been analyzed with a focus on coral ecology and limited description of sedimentation dynamics, or with a focus on hydrodynamics and reef platform development, without connections to the benthic assemblages. Research on interactions between reef morphology, hydrodynamics, and sedimentation, and how they shape coral populations deserves further attention. Here we investigate the role of the spur and groove (SAG) morphology in driving water flow and sediment movement. Our study was undertaken in SAGs characterized by a well-developed coral community, and results were compared to an adjacent low coral cover area without SAGs. Mean flow and wave velocities were measured in SAG and non-SAG areas and coupled with the analysis of sediment size and sorting to calculate thresholds for sediment movement. Wave orbital velocities were an important driver of sediment suspension. The suspension threshold due to mean flow was reached 80-100% of the time on spurs. In grooves, it was reached 60% of the time during off-shore flows, but only 33% of the time during on-shore flows, suggesting that net seaward sediment transport occurs within grooves. The steeper bathymetry in grooves (8% slope) relative to the non-SAG area (4%) could have also promoted gravity-driven seaward sediment and rubble accumulation, favoring substrate stabilization at the SAG slope base. The hydrodynamics and sedimentation patterns within the SAGs likely offer a more favorable condition for coral growth. In the non-SAG area, different flow patterns are expected to promote the formation of sediment deposits, preventing a similar benthic colonization.