Reef Crest

The reef crest is the highest (most shallow) part of the reef, and lies between the shoreward, protected back reef and the outer reef face.

Reef crest. © Fotolia.com

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 the back reef zone (lagoon) of the coral reef biome. The crest is readily observed from the sea surface as an irregular, darkish band paralleling the shore.

When large waves are present plumes of spray splash skyward as the waves break over the leading edge of the crest. When this occurs, the crest is easily located from higher altitudes by a bright line of breaking surf along its outer margin.

Composition and Structure Of The Reef Crest

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 crest is best developed where regular exposure to powerful waves creates a high energy environment.

Under these conditions, the continuity of the 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.

On the leeward sides of islands and atolls, where wave action is minimal, the crest is usually poorly developed or absent in places. Many poorly developed fringing reefs have minimal crest zones, or none at all.

There are generally two distinctive sub-habitats contained within well-developed reef crests of both the Indo-Pacific and Caribbean regions: the algal ridge, and reef flat.

The Algal Ridge

On high energy windward reefs, particularly those of the Indo-Pacific region, a massive algal ridge often forms the seaward margin of the crest.

Pacific algal ridge partially exposed at low tide. © Istockphoto.com

Algal ridges are composed primarily of encrusting calcareous red algae that form 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 (see photo, left). Sponges and green algae adapted to such areas are highly tolerant of brief exposures to sunlight and air.

Where wave action is generally more moderate, heavy branching corals such as Acropora spp. and fire corals (Millipora spp.) typically dominate windward crests of Pacific reefs.

Well-developed algal ridges are comparatively rare on Caribbean reefs, where windward crests are typically dominated by a single massive branching coral (Acropora palmata).

The Reef Flat

The most shoreward part of the crest is called the reef flat.

NOTE ON TERMINOLOGY; The term “reef flat” is also occasionally used to denote the entire back reef zone (as defined herein). Such inconsistancies in coral reef terminology are a source of confusio for all.

This sub-zone varies widely in structure and composition, depending upon prevailing wave energy and other factors.

On high energy reefs, the force of the incoming waves breaks off parts of large coral colonies and tumbles smaller colonies about, creating a shallow reef flat composed mainly of accumulated dead coral fragments or “rubble”.

Where wave action is more moderate, the reef flat tends to be smoother and composed of compacted carbonate sands deposited periodically by large storm waves.

What Are Coral Reefs? | Where Are Coral Reefs Located? | How Are Coral Reefs Formed? | Types of Coral Reefs | Coral Reef Zones

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