Fringing reef

A Fringing reef is a kind of coral reef, that is located in the tropics generally immediately near the shoreline. This type of coral reef is the most common type of reef that is found. This type of reef grows the best on some kind of hard surface, so it preferentially grows on areas with rocky bottoms. They may also grow on softer bottoms as well if there are some hard areas. Fringing reefs grow as a thin strip along the shoreCastro, Peter and Huber, Michael E. 2008. Marine Biology. 7ed. McGraw-Hill, New York] at a rate of about 2 to 7mm per year. It is Darwin’s belief that fringing reefs are the first kind of reefs to form around a landmass in a long-term reef growth process.Kennedy, D.M. and Woodroffe, C.D. 2002.Fringing reef growth and morphology: a review. "Earth-Science Reviews". 57:255-277.]

Sometimes it is hard to tell the difference between fringing reefs and another type of reef called a barrier reef. One of the ways that these two types of reefs are separated is based on the depth of the lagoon in the back reef which is the area near to shore. If the water in the lagoon is less than 10m deep it is a fringing reef. Another major difference is that barrier reefs tend to be farther away from shore than fringing reefs and have a larger lagoon.

tructure

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

Reef Flat

The reef flat is the flat, broadest area of the reef. This area is found in fairly shallow water, and is at some points in time uncovered during low tide. This area of the reef is only slightly sloped towards the open ocean.

Due to this part of the reef being found adjacent or nearly adjacent to land it maintains the most damage from freshwater runoff and sediments. Based on this there is few living corals found here, though there are some. Located on this flat area are often such things as seagrasses, seaweeds soft corals.

Reef Slope (Fore reef)

The reef slope is found at the outer edge of the fringing reef, closest to the open ocean. This area of the reef is often quite steep and may even be a straight vertical drop to the bottom of the ocean.

There is much more abundant coral growth found on this slope as well as a greater number of different species than are found on the reef flat. This is mostly because there is not much affect felt in this part of the reef from freshwater runoff and sediments, unlike the flat which is nearer to the shore. This area of the reef also benefits and has much more growth due to waves which carry nutrients to this area and take away sediments that could harm the corals.

The upper portion of this slope is called the reef crest. In this area the corals grow the best since it is closest to the surface and gets the most sunlight. The base of the slope which receives the least sunlight has the least growth out of the whole slope.

Location of Fringing Reefs

Fringing reefs are located near shores in the tropics in many areas and are the most common type of reef found. Coral reefs are found in many places within the tropics in which the water is between 18 and 30°C. [Zubi, Teresa. 2007. Ecology, Coral Reefs. http://www.starfish.ch/reef/reef.html Accessed on March 30, 2008]

Many of the reefs that are found in the Great Barrier Reef are actually fringing reefs. Of the close to 3400 reefs that make up the Great Barrier Reef 760 of them are actually fringing reefs. [Australian Museum. 2004. geoscience: the Earth, Great Barrier Reef. http://www.amonline.net.au/geoscience/earth/barrier_reef.htm Accessed on March 30, 2008. ]

Ways Fringing Reefs Grow

Keep-up: These are reefs that grow at a rate that is similar to the rate that sea level rises.

Catch-up: These are reefs that do not grow very much for a while as sea level rises, but later on when the rise in sea level slows or stops grow to reach sea level.

Give-up: These are reefs in which they are not able to grow at a certain rate and are drowned out.

Ways that Fringing Reefs Develop

When fringing reefs form the most important determinant is the available space in which the reef may grow. This is mostly determined by sea level changes. Sea level may change by either rising or falling of water levels mostly due to melting of glaciers or via plate tectonics. There are six different major ways in which fringing reefs grow and develop.

The first way in which fringing reefs develop is when the reef initially begins growing below the surface of the water. This type of reef for the most part grows vertically as far as the space below the sea surface allows. This type of fringing reef generally grows upward as sea level is also rising. It is possible that once the reef crest reaches sea level that it may begin growing horizontally in a seaward direction. This type of reef begins to grow following flooding and is built mostly from parts of the reef that have died. Since this type of reef grows vertically the oldest sediments are found lower in the reef. The reef flat will be the age of when the reef reached sea level. Catch-up reefs have younger surfaces than keep-up fringing reefs of this type.

The second type of reef begins to grow at the shore when sea level is fairly constant and grows horizontally in a seaward direction. If the sea level drops the reef flat in newly forming more seaward areas will become lower. In this type of reef the older sediments are found closest to shore and are not buried. This type of reef also grows mostly through the accumulation of reef sediments.

The third type of reef forms in a way similar to that of the second type, in which the reef grows horizontally towards the sea beginning at the shore. The difference is that the fringing reef grows on top of muddy sediments which may be older or the same age as the reef. Coral, seagrass and algae filter sediment before it is placed on the reef crest, and is found where there is a lot of sediment.

The fourth type of fringing reef forms in a gradual, sporadic manner. 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.

The fifth type of fringing reef, which is fairly common, develops when a reef located offshore at depth begins to grow vertically to sea level by either keep-up or catch-up methods. The crest grows faster than farther inshore and a lagoon forms when it reaches sea level. The lagoon then fills with sediments from the reef mostly.

The sixth type of reef forms in a way that is similar to the fifth type, but the offshore reef develops in a different way. The seaward barrier portion of the reef forms when storms move coral debris inwards and create a pile. It then becomes larger through more coral being placed on it by storms and just from part of it from its surface. A shallow lagoon is thus formed and becomes filled in with reef sediments prevalently brought in by storms. On the side closest to the open sea a lot of erosion occurs due to waves and hurricanes.

Effect of Tectonic Activity

In some cases tectonic activity can have a very detrimental effect on a reef. An earthquake on Ranongga Island in the Solomon Islands moved 80% of the fringing reef surrounding it to a point that was permanently above sea level. The earthquake had slightly different effects on different parts of the island. In the north reefs became elevated 1m above the high tide water height, whereas on the south side reefs moved 2 to 3m above the water height at high tide. [Albert, U., Udy, J., Baines, G. and McDougall, D. 2007. Dramatic tectonic uplift of fringing reefs on Ranongga Is., Solomon Islands. "Coral Reefs" 26:983.]

Differences in Species Diversity in Different Areas of the Reef

The area with the lowest number of different organisms is the backreef area of the fringing reef. Coral species get more diverse as you move farther seaward from the backreef to the reef crest. Some of this difference is due to eutrophication. The eutrophication is mostly due to increased nutrients, sediments and toxicity due to domestic and industrial wastes.Tomascik, T. and Sander, F. 1987. Effects of eutrophication on reef-building corals: II. Structure of scleractinian coral communities on fringing reefs, Barbados, West Indies. "Marine Biology". 94:53-75]

They feel that more macrophytes are found on the bottom due to the increases in nutrients. They also feel that this increase in nutrients has caused an increase in the number of phytoplankton that 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 to be found.

The sediments that are present within the environment cause increased turbidity and 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. The corals that are present then are thus likely those that can get rid of the sediments the best.

They have also found that the corals that are present in the greatest numbers are those that reproduce via brooding their larva, which in turn leads to higher rates of survival.

They find that in the area of the reef closest to the shore that there is generally a lot of fleshy algae which forms on sand and coral rubble. These types of algae include 'Lyngbia' sp. and 'Oscilatoria' sp.

They have found on the reef flat that over the years the dominant species in this area have been affected by environmental changes. On fringing reefs in Barbados, species such as 'Diploria strigosa', 'Palythoa mamillosa', and 'Diadema antillarum' are found.

The reef crest's most common species is 'Porites porites'. They also found though that there was a lot of this area covered in fleshy algae too.

ee also

*Coral reef
*Coral

References

External links

* [http://www.darwin-literature.com/Coral_Reefs/5.html Darwinian Literature on Fringing Reefs]
* [http://www.springerlink.com/content/v2177302x82|0821/fulltext.pdf Algal Species Found On A Fringing Reef After A Natural Disaster]