The form, function and evolution of coral reef spurs and grooves

Abstract

The fore reef spur and groove (SaG) zone is an important, but poorly understood, zone of coral reefs. Spurs (parallel ridges of carbonate material), are separated by grooves (regularly spaced channels) to form a distinct finger-like pattern around the margins of coral reefs. Few studies have collected quantitative data in SaG systems and many questions remain about their formation and evolution. This thesis aims to describe the eco-morphodynamics of SaGs by establishing the interactions and feedbacks between their geomorphology (form), hydrodynamics (function) and reef growth (evolution) across multiple spatial and temporal scales at reefs in the Indo-Pacific. This constitutes the most extensive and comprehensive study of SaG features to date. A remote sensing analysis of SaG morphometrics at 17 reefs in the southern Great Barrier Reef (GBR), Australia, revealed exposure to wave energy was the most important factor influencing SaG morphology with fore reef slope playing a secondary role. Four statistically distinct classes of SaG were also identified and described. In-situ hydrodynamic data from SaG zones at One Tree Reef, in the GBR and Moorea in French Polynesia showed that the fore reef SaG zone is a remarkably effective breakwater, dissipating up to 86% of wave energy. Currents in the zone were complex and variable. A unique suite of 38 cores from SaG zones at One Tree and Heron Reefs (in the southern GBR) and Moorea were combined with existing reef platform cores to identify three potential modes of fore reef and reef flat evolution. These modes were matched to the remote sensing defined classes, suggesting that remotely sensed data can provide insight into SaG evolution and potentially upscale the findings of in-situ hydrodynamic and reef growth studies. While further research is required, by studying multiple sites on multiple reefs this study provides globally applicable insights into SaG eco-morphodynamics across a broad spectrum of environmental conditions.