Fringing reef

Afringing reefis one of the three main types ofcoral reef.It is distinguished from the other main types,barrier reefsandatolls,in that it has either an entirely shallow backreef zone (lagoon) or none at all. If a fringing reef grows directly from the shoreline, then the reef flat extends to the beach and there is no backreef. In other cases (e.g., most ofthe Bahamas), fringing reefs may grow hundreds of yards from shore and contain extensive backreef areas within which it contains food and water. Some examples of this arePhilippines,Indonesia,Timor-Leste,the western coast ofAustralia,theCaribbean,East Africa,andRed Sea.Charles Darwinbelieved that fringing reefs are the first kind of reefs to form around a landmass in a long-term reef growth process.The largest fringing coral reef in the world is theNingaloo Reef,stretching to around 260 km (160 mi) along the coastline ofWestern Australia.

A fringing reef off the coast ofEilat,Israel

Exposed reef flats duringlow tideat the Magpupungko Rock Pools inSiargao Island,Philippines

Barrier reef

There are few differences between fringing reefs and barrier reefs. Distinguishing between these two reef types involves assessing the depth of the lagoon behind the back reef. Barrier reefs feature deeper sections within the lagoon, while fringing reefs lack such depths. Additionally, a notable contrast lies in their proximity to the shore: barrier reefs are typically located much farther away from the coastline compared to fringing reefs.

Structure

Diagram of a fringing coral reef

There are two main components that make up a fringing reef: the reef flat and the reef slope.

Reef flat (back reef)

The reef flat is the shoreward, flat, broadest area of the reef. The reef flat is found in fairly shallow water and can be uncovered duringlow tide.This area of the reef is only slightly sloped towards the open ocean.Since the reef flat is adjacent or nearly adjacent to land, it sustains the most damage from runoff and sediments. Typically, few of the flat's corals are alive. Seagrasses, seaweeds, and soft corals are often found there.

Reef slope (fore reef)

The reef slope is positioned at the outer edge of the fringing reef, bordering the open ocean. Usually characterized by a steep inclination, this part of the reef either descends to a relatively shallow sandy base or extends to depths too significant to facilitate the growth of coral.The diminished presence of runoff and sediments on this slope contributes to a higher abundance of coral and a broader variety of coral species. Greaterwave actiondisperses pollutants while transporting nutrients to this specific area.A prevalent characteristic found on the fore reef involves the creation ofspur-and-groove formations,facilitating the downslope transportation of sediment within the groove.

The upper segment of this slope is known as the reef crest. This crest enjoys an optimal balance of sunlight exposure and wave action, fostering the fastest coral growth in this area. Conversely, the base of the slope receives the least amount of sunlight and consequently exhibits the slowest growth among all sections of the slope.

Distribution of Fringing reefs

Aerial view ofNingaloo ReefinWestern Australia,the largest fringing coral reef in the world and aUNESCOWorld Heritage Site

Fringing reefs are located near shore in the tropics in many areas and are the most common reef type. Coral reefs are found in the tropics in which the water is between 18 and 30 °C (64 and 86 °F).Many of theGreat Barrier Reef's components are actually fringing reefs. Of the close to 3,400 individual reefs, 760 are fringing reefs.Fringing reefs are the most common type of reef found in thePhilippines,Indonesia,Timor-Leste,the western coast ofAustralia,theCaribbean,East Africa,andRed Sea.

The largest fringing coral reef in the world is theNingaloo Reef,stretching to around 260 km (160 mi) along the coastline ofWestern Australia.

Island with fringing reef in theMaldives

Development of Fringing Reefs

See also:Formation of coral reefs

The most important determinant of reef growth is available space as determined by sea level changes.Sea levelchanges are mostly a result of glaciation orplate tectonics.Tectonic activity can have detrimental effects. An earthquake onRanonggain theSolomon Islandsmoved 80% of its fringing reef permanently above sea level. Northern reefs became elevated 1m above the high tide water height, whereas on the south side reefs moved 2 to 3m above the water height.

Keep-upreefs grow at the same rate that sea level rises.Catch-upreefs initially grow more slowly than sea level rises but eventually catch up when the rise in sea level slows or stops.Give-upreefs are not able to grow fast enough and are "drowned out". There are six different major ways in which fringing reefs grow and develop.

• Reefs can develop vertically as far as the space below the surface allows. The reef generally grows upward from a starting point towards the surface. Once the reef crest reaches sea level the reef may begin growing seaward. Growth begins after flooding, mostly from parts of the reef that have died. Because the reef grows upward, the oldest sediments are found lower in the reef. The reef flat's age indicates when the reef reached sea level. Catch-up reefs have younger surfaces than keep-up reefs of this type.
• Reefs can expand seaward from the shore. This requires a fairly constant sea level. If the sea level drops, the reef flat in more seaward areas slopes downward.
• Reefs can grow atop muddy sediments which can predate the reef or accrete along with the reef's growth. These reefs also grow seaward from the shore. Older sediments are closest to shore and are not buried. Coral, seagrass and algae filter sediment before it is placed on the reef crest.
• Reefs can form in a gradual, sporadic manner, with alternate vertical and horizontal growth episodes. In this type of fringing reef formation there are multiple separate reefs that are found parallel to the shore and the original fringing reef. These reefs become a single, large reef when reef sediments fill in the spaces between the different reefs.
• Reefs can develop when an offshore reef grows to sea level, forming a barrier. When the crest grows faster than the flat, a lagoon forms. The lagoon then fills with inshore sediments.
• Offshore reefs can form their barrier using storms to move coral and other debris inwards. The recurring storms continually reshape the seaward side of such reefs.

Ecology

Main article:Environmental issues with coral reefs

Uncovered reef flat during low tide onGuam

As with other types of reefs, there are many reasons of fringing reef destruction. Destructive fishing practices, such ascyanide fishing,blast fishing,bottom trawling,andmuro-ami(banging on the reef with sticks), may have detrimental effects. Bottom-trawling is one of the greatest threats to cold-water coral reefs.Overfishingaffects the ecological balance of coral reef communities, disrupting thefood chainand causing effects far beyond the directly overfished population. Tourism such as careless boating, diving, snorkeling, and fishing, with people touching reefs, stirring up sediment, collecting coral, and dropping anchors on reefs, can destroy the reefs. Some tourist resorts and infrastructure have been built directly on top of reefs, and some resorts empty their sewage or other wastes directly into water surrounding coral reefs.

Toxins frommarine pollutionare dumped directly into the ocean or carried by river systems from sources upstream. Some pollutants, such as sewage and runoff from farming, increase the level of nitrogen in seawater, causing an overgrowth of algae, which cuts off sunlight from the reefs.Erosioncaused by construction (both along coasts and inland), mining, logging, and farming is leading to increased sediment in rivers. This ends up in the ocean, where it can smother corals by depriving them of the light needed to survive. The destruction ofmangroveforests, which normally trap large amounts of sediment, is exacerbating the problem. Mining of live coral is used as bricks, road fill, or cement for new buildings. Corals are also sold as souvenirs to tourists and to exporters and harvested for the live rock trade.

Corals cannot survive if the water temperature is too high.Climate changehas already led to increased levels ofcoral bleaching,and this is predicted to increase in frequency and severity in the coming decades. Such bleaching events may cause already stressed coral reefs and reef ecosystems to completely die.

Species diversity

Reef edge and flat atTumon Bay,Guam

The backreef area has the least species diversity, which increases seaward towards the reef crest. Some of this difference is the result ofeutrophicationfrom increased nutrients, sediments and toxicity from domestic and industrial wastes.More macrophytes live on the bottom because of the increase in nutrients. This increase in nutrients has caused an increase in the number ofphytoplanktonthat are present above the coral reef. The increase in phytoplankton has led to reduced light reaching the coral species and has also led to a greater number of larger invertebrates.The sediments that are present within the environment cause increasedturbidityand may smother some organisms. The corals present on the fringing reefs use four processes to get rid of sediments which include polyp distension, tentacular movement, ciliary action and mucus production.

In the area of the reef closest to the shore there is generally a lot of fleshy algae which forms on sand and coral rubble. These types of algae includeLyngbiasp. andOscilatoriasp.Over recent years the dominant species in the reef flat have been affected by environmental changes. On fringing reefs in Barbados, species such asDiploria strigosa,Palythoa mamillosa,andDiadema antillarumare found.

The reef crest's most common species isPorites porites,a type of stony coral, although there are also significant areas covered in flesh-like algae.

See also

• List of reefs

External links

• Darwinian Literature on Fringing ReefsArchived2016-08-18 at theWayback Machine
• Algal Species Found On A Fringing Reef After A Natural Disaster
• NOAA National Ocean Service - What are the three main types of coral reefs?