Editing Coral reef (section)

== Zones ==
[[File:Coral reef diagram.jpg|thumb|upright=1.15|left|The three major zones of a coral reef: the fore reef, reef crest, and the back reef]]

Coral reef ecosystems contain distinct zones that host different kinds of habitats. Usually, three major zones are recognized: the fore reef, reef crest, and the back reef (frequently referred to as the reef lagoon).

The three zones are physically and ecologically interconnected. Reef life and oceanic processes create opportunities for the exchange of [[seawater]], [[sediment]]s, nutrients and marine life.

Most coral reefs exist in waters less than 50 m deep.<ref>{{cite web |url=https://www.marinebio.org/creatures/coral-reefs/ |title=Coral Reefs |website=marinebio.org |date=17 June 2018 |access-date=28 October 2022}}</ref> Some inhabit tropical continental shelves where cool, nutrient-rich [[upwelling]] does not occur, such as the [[Great Barrier Reef]]. Others are found in the deep ocean surrounding islands or as atolls, such as in the [[Maldives]]. The reefs surrounding islands form when islands subside into the ocean, and atolls form when an island subsides below the surface of the sea.

Alternatively, Moyle and Cech distinguish six zones, though most reefs possess only some of the zones.<ref name=MoyleCech556>{{cite book |last1=Moyle |first1=Peter B. |url={{google books |plainurl=y |id=sZYWAQAAIAAJ}} |title=Fishes : an introduction to ichthyology |year=2004 |publisher=[[Pearson Education|Pearson]]/[[Prentice Hall]] |location=Upper Saddle River, N.J. |isbn=978-0-13-100847-2 |page=556 |edition=Fifth |first2=Joseph J. |last2=Cech}}</ref>

[[File:Propagation du tsunami en profondeur variable.gif|thumb|right|Water in the reef surface zone is often agitated. This diagram represents a reef on a [[continental shelf]]. The water waves at the left travel over the ''off-reef floor'' until they encounter the ''reef slope'' or ''fore reef''. Then the waves pass over the shallow ''reef crest''. When a wave enters shallow water it [[Wave shoaling|shoals]], that is, it slows down and the wave height increases.]]

'''The reef surface''' is the shallowest part of the reef. It is subject to [[tidal surge|surge]] and [[tide]]s. When waves pass over shallow areas, they [[Wave shoaling|shoal]], as shown in the adjacent diagram. This means the water is often agitated. These are the precise condition under which corals flourish. The light is sufficient for [[photosynthesis]] by the symbiotic zooxanthellae, and agitated water brings plankton to feed the coral.

'''The off-reef floor''' is the shallow sea floor surrounding a reef. This zone occurs next to reefs on continental shelves. Reefs around tropical islands and atolls drop abruptly to great depths and do not have such a floor. Usually sandy, the floor often supports [[seagrass meadow]]s which are important foraging areas for reef fish.

'''The reef drop-off''' is, for its first 50 m, habitat for reef fish who find shelter on the cliff face and [[plankton]] in the water nearby. The drop-off zone applies mainly to the reefs surrounding oceanic islands and atolls.

'''The reef face''' is the zone above the reef floor or the reef drop-off. This zone is often the reef's most diverse area. Coral and [[calcareous]] algae provide complex habitats and areas that offer protection, such as cracks and crevices. Invertebrates and [[epiphytic]] algae provide much of the food for other organisms.<ref name=MoyleCech556 /> A common feature on this forereef zone is [[spur and groove formation]]s that serve to transport sediment downslope.

'''The reef flat''' is the sandy-bottomed flat, which can be behind the main reef, containing chunks of coral. This zone may border a lagoon and serve as a protective area, or it may lie between the reef and the shore, and in this case is a flat, rocky area. Fish tend to prefer it when it is present.<ref name=MoyleCech556 />

'''The reef lagoon''' is an entirely enclosed region, which creates an area less affected by wave action and often contains small reef patches.<ref name=MoyleCech556 />

However, the topography of coral reefs is constantly changing. Each reef is made up of irregular patches of algae, [[sessility (zoology)|sessile]] invertebrates, and bare rock and sand. The size, shape and relative abundance of these patches change from year to year in response to the various factors that favor one type of patch over another. Growing coral, for example, produces constant change in the fine structure of reefs. On a larger scale, tropical storms may knock out large sections of reef and cause boulders on sandy areas to move.<ref>{{cite journal |journal=[[Science (journal)|Science]] |date=March 24, 1978|volume=199 |issue=4335 |pages=1302–1310 |doi=10.1126/science.199.4335.1302 |title=Diversity in Tropical Rain Forests and Coral Reefs |first=Joseph H. |last=Connell |pmid=17840770 |bibcode=1978Sci...199.1302C}}</ref>