Abstract

Local flow dynamics play a central role in physiological processes like respiration and nutrient uptake in coral reefs. Despite the importance of corals as hosts to a quarter of all marine life, and the pervasive threats facing corals, characterizing the hydrodynamics between the branches of scleractinian corals has remained a significant challenge. Here, we investigate the effects of colony branch density and surface structure on the local flow field using three-dimensional immersed boundary, large-eddy simulations for four different colony geometries under unidirectional oncoming flow conditions. The first two colonies were from the Pocillopora genus, one with a densely branched geometry, and one with a comparatively loosely branched geometry. The second pair of geometries were derived from a scan of a single Montipora capitata colony, one with the roughness elements called verrucae covering the surface intact, and one with the verrucae removed. For the Pocillopora corals, we found that the mean velocity profiles changed substantially in the center of the dense colony, becoming significantly reduced at middle heights where flow penetration was poor, while the mean velocity profiles in the loosely branched colony remained similar in character from the front to the back of the colony. For the Montipora corals, somewhat counterintuitively, the colony without verrucae produced almost double the maximum Reynolds stress magnitude above the colony compared to the colony without verrucae. This implies that the smooth colony will have enhanced mass transport and higher bed shear stress and friction velocity values relative to the colony with verrucae.

Highlights

  • A new and important direction in understanding the coupled dynamics of corals and their hydrodynamic environments seeks to measure and model the flow inside coral reefs and individual colonies

  • The results provided an excellent visualization of the flow field inside the corals, but repeating such experiments on diverse coral structures in order to characterize how the mean velocity profile and other flow quantities differ for different geometries is expensive and unrealistic

  • Before comparing the flow profiles between the two Pocillopora structures, the results from the P. meandrina simulation were compared with the experimental results of Chang et al [31] for the high-flow morphology of Stylophora pistillata for validation

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Summary

Introduction

A new and important direction in understanding the coupled dynamics of corals and their hydrodynamic environments seeks to measure and model the flow inside coral reefs and individual colonies. The task is challenging due to the existence of a wide range of flow scales within different boundary layers around a coral reef [1, 2]. The transport of planktonic food in the reef is mainly governed by the large-scale flow motion, while diffusion [3, 4] takes place at the surface of the coral at a much smaller scale. Experimental studies performed at this smaller hydrodynamic scale [5, 6] show that the growth direction, dimensions, and sparsity of branches depend on the flow profile inside the coral [7,8,9].

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