The fore reef zone of a coral reef is the most seaward portion of the entire ecosystem. The composition of this area is commonly almost solid coral. This zone characteristically takes the form of a downward slope towards the ocean depths.
The fore reef zone (also often called the “reef face” or “reef front”) begins at the seaward base of the reef crest and extends to the lower limits of coral growth.
Here, the sea floor begins to slope downward – somewhat gently at first, and then ever more steeply.
This zone contains the greatest mass and diversity of hard corals found in coral reef ecosystems. It also is home to the greatest number of coral reef fish species of any of the three major reef zones.
For these reasons, along with the zone’s extensive depth range and ease of access from the open sea, this is where most recreational coral reef scuba diving occurs. In many cases the fore reef plunges abruptly to great depths, creating formations popularly known to scuba divers as “walls”.
Two distinctly different segments of this zone are generally recognized, respectively known as the “upper” and “lower” fore reef.
The upper section of the fore reef begins immediately seaward of the reef crest. It is characterized by a comparatively gentle downward slope, and usually extends to depths somewhere between about 15 to 20 meters. The upper fore reef has a high diversity of corals.
Wave energy is high at the base of the reef crest, so the very shallowest parts of the fore reef are typically dominated by massive or encrusting corals that are more resistant to the destructive forces of wave impact.
As depth increases and wave energy abates, the frequency of branching corals (e.g., Acropora spp., Agaricia spp., Montastrea spp.) rapidly increases, along with boulder corals such as Diploria spp.
It is here – in the deeper portions of the upper fore reef – that we find the highest coral species diversity and greatest variety of hard coral colony forms to be found in the entire coral reef ecosystem.
Deeper yet, the “upper” fore reef gives way to the “lower” segment, characterized by different dominant corals. This sub-zone extends from the margin of the upper face into waters too poorly lit to permit coral growth.
At first, branching corals become far less frequent and massive coral forms like Montastrea spp. are predominant.
As depth continues to increase, branching forms become infrequent and massive colonies become less abundant.
Also, the relative abundance of plate-like coral colonies (e.g., Agaricia spp.) increases. These plate corals have larger surface areas relative to their total volume, and therefore are best adapted to take advantage of the minimal sunlight present at greater depths.
Eventually, diminished sunlight is insufficient to permit photosynthesis by the the corals symbiotic zooxanthellae, and hard coral growth ceases.
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