The reef crest zone is the highest (shallowest) part of the reef, and lies between the shoreward, protected back reef zone (lagoon) and the outer fore reef zone. It is the narrowest of the three main coral reef zones, standing like a massive wall that absorbs and dissipates the energy of incoming waves, resulting in the calm waters of back reef zones (lagoons).
The reef crest zone is readily observed from above the sea surface as an irregular, darkish band paralleling the shore, rimmed by a bright line of breaking waves along its outer margin. The composition and structure of the reef crest vary considerably with the prevailing wind direction, severity of wave action, type of coral reef, and geographic location.
The reef crest zone is best developed where regular exposure to powerful waves creates a high energy environment. Under these conditions, the continuity of the reef crest is broken at irregular intervals by channels cut by waves and currents, providing paths of tidal water exchange between the open sea and the back reef zone (lagoon).
On high energy windward reefs of the Indo-Pacific region (and to a far lesser extent on some reefs of the Greater Caribbean region), a massive algal ridge forms the seaward margin of the reef crest. This is composed primarily of encrusting calcareous red algae that forms a cement-like barrier to the initial impact of incoming ocean waves. The uppermost part of the algal ridge is generally emergent (exposed to the air) at low tide.
Where wave action is generally more moderate, heavy branching corals such as Acropora spp. and fire corals (Millipora spp.) typically dominate windward crests on both Pacific and Caribbean reefs. On the leeward sides of islands and atolls, where wave action is minimal, the reef crest is usually poorly developed or absent in places. Many poorly developed fringing reefs have minimal reef crest zones, if present at all.
The shoreward part of the crest, called the “reef flat”, also varies in structure and composition depending upon prevailing wave energy. On high energy reef crests, the force of the incoming waves breaks off parts of large coral colonies and tumbles smaller colonies about, creating a shallow “rubble zone” composed of accumulated dead coral fragments. Where wave action is moderate, the reef flat tends to be smoother and made up mainly of carbonate sands (finely ground coral) deposited periodically by large storm waves.
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