Coral reef zonation
Different coral species and morphological types prefer different habitats within a reef ecosystem based on varying degrees of wave energy, temperature, water chemistry and light. Such distributional patterns of corals on reefs creates the zonation found on many coral reefs. These patterns of zonation are different depending on whether the reef is a fringing, atoll or barrier reef. However, there are some general patterns found on most reefs. A typical zonation pattern includes back reef or reef flat, reef crest or algal ridge, and seaward slope or fore-reef.
The back reef (also sometimes called the reef flat) is the zone found the closest to the shore. Back reefs are shallow water areas that extend from shore to the reef crest, the highest part of the reef that separates the back reef from the fore reef. The shallow water in this part of the reef creates an area with high light intensity and temperature, an environment at the edge of coral tolerance. The geomorphology of back reefs vary depending on location and reef type; they can range from meters to kilometers in width. They also vary in depth from a few centimeters to several meters deep. Due to the shallow depth of this zone, portions of back reefs can be exposed during low tides. The reef exposure along with higher temperature and light intensity inhibits coral growth. Despite these stressors, the corals in this region are protected from the majority of the wave stress that is present on the reef crest and upper fore reef.
The reef crest is found farther away from the shore and is between the back reef and the fore-reef. The reef crest of most Indo-Pacific coral reefs (and some Caribbean reefs) consists of a seaward margin called an algal ridge, which is exposed at low tide. This is the area of the reef with the highest wave action. Calcareous red algae dominate this area, leading to the name algal ridge. There are some corals that can survive on reef crests with less severe wave action, typically short branching corals that are able to withstand moderate to high wave action. Many Caribbean reef crests are heavily dominated by elkhorn coral (Acropora palmata).
The fore-reef (also called the reef face) is the reef zone that is the farthest away from the shore, on the oceanic side of the reef crest. It slopes downward at steep angles, sometimes to great depths. The fore-reef is home to the largest corals on the reef, because it is the most hospitable environment with limited wave action. The corals are the most diverse on the reef slope between five and twenty meters of depth, and as depth increases coral species richness also decreases. The types and diversity of corals in this zone are affected by light availability and other factors. As the light decreases with depth, the number of different types of corals also decreases.
Changes in Zonation
Coral reefs are threatened by many different factors including increased nutrients, sedimentation, land use, disease, sea-level rise and thermal stress. Climate change is one of the biggest threats to coral reefs, and can lead to changes in zonation. With climate change and increasing thermals stress events, the corals in shallow areas are especially at risk because they are already living at the edge of their tolerance range. In the Seychelles, coral mortality in reef flats and areas with shallow water was near 100% during the 1998 bleaching event. Reef flats in the Seychelles were dominated by staghorn corals and Porites. When these corals died there was an increase in water depth and wave energy reaching the shore. Loss of protection against wave energy is due to a decrease in complexity. As corals die and erode away, the surface of the reef becomes rubble which is less three dimensional than the live reef, making it less effective at dispersing wave action. This can cause increases in shoreline erosion, and greater impacts from storms.
- NOAA Coral Reef Information System
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