Moating

Abstract

Moating is the process of constraining the ebbing tide over the reef to produce moats in which low tide water levels remain perched above open water levels (see Moats). Moating truncates the lower part of each tidal cycle at an elevation determined by the height (in the tidal frame) and permeability of the moating structure (Figure 1a). Moating can occur at any intertidal elevation and may only be active during the lowest spring tides. Where moating occurs behind stable and impervious features, like algal rims (see Algal rims) the water level is consistent on every (lower) low tide. In contrast, at mesotidal locations like the central Great Barrier Reef (GBR) open water spring and neap low tides can differ by > 2 m. Moating allows some reef biota to survive above their open water limits, and must be considered when interpreting intertidal sea-level indicators, especially coral microatolls (see Microatoll) (Figure 1a). Moating also has important implications for sedimentation on reefs. It controls the tidal windows when tidal currents are active, and modifies reef flat depths and thus where reef top waves may entrain and transport sediments.