How does sedimentation affect a coral reef?

Coral reefs - underwater paradises - Formation, ecology and threats -

Isabel Koch & Franz Brümmer, Stuttgart

Part 1: Coral reefs - then and now - Today's distribution - Reef types

Part 2: Master builder of the reefs: Cnidaria - Nettle cells - Zoological keyword - Coral reefs: a symbiosis of algae and corals

Part 3: Ecology - Disturbances in the ecosystem

Part 4: Outlook - Literature - Summary - Keywords - Addresses

Note: Coral reefs are complex habitats, diversely designed and endowed with an immense number of species. It is therefore impossible to cover all the reef-related aspects in this article; the authors ask for your forbearance and understanding.

Coral reefs - then and now
Today coral reefs are considered to be the most species-rich habitat on earth, alongside the tropical rainforest. Scientists suspect that there are over 400,000 species, most of which are small organisms that drill into the limestone of the reefs or live in narrow crevices that are quite safe from discovery. So far, only around 60,000 species are known. Whether the gap between the two numbers can ever be closed depends on how quickly and completely humans destroy this habitat. This almost unmanageable abundance of species is only a copy of what has already existed on reefs in the history of the earth! Today, in principle, only scleractine corals are able to form reefs. The first reefs arose 2 billion years ago in the Precambrian: Back then, cyanobacteria were the builders of the so-called stromatolite reefs. This was followed by reefs from hydrozoans as well as tabular and rugose corals in the Silurian and Devonian over 450 million years ago. Red algae, pebble sponges, brachiopods, bryozoa, mussels, tube worms ... they were all reef builders. The scleractin corals that predominate today did not appear as master builders until the Jurassic 190 million years ago. Despite the greater variety of reef-building organisms, the general conditions for the formation of reefs were always the same: shallow, clear and warm water!

Fig. 1: Divers explore one of the most biodiverse habitats weightlessly: the coral reef.

Today's distribution
But before we deal with the distribution and formation of coral reefs, one more remark about the term "reef". Seafarers who were probably the first to become acquainted with reefs refer to every shoal in the sea as a reef. Biologists and geologists have a somewhat narrower definition: "A reef is a mostly bank-shaped structure that is mainly composed of living organisms and extends from the sea floor to the surface of the water. Due to its size, it has a lasting influence on the ecological properties of its surroundings and is of a sufficiently solid structure to withstand the forces of water and thus be able to create a living space of their own over several years ". Coral reefs are built from hard corals, more precisely from the reef-building (hermatypical; Greek: herma = support, cliff, reef) hard corals. Accordingly, coral reefs should also occur where hard corals occur. But it's not that easy. Hard corals are found in all seas, but reefs are not. With the exception of the reefs built by the hard corals Lophelia and Amphihelia off Norway, the "typical coral reefs" only occur in the warm, shallow seas of the tropics. The demands of the reef-building corals on their environment are the same as they were millions of years ago. Since the position and depth of the oceans has changed significantly since then, coral reefs now only exist in tropical seas between the 20 ° C isochryms, i.e. roughly between the tropics on both sides of the equator. Only here find (or can one say: found?) The sensitive reef builders find the ideal conditions:

Temperature: There are clear differences within the distribution limits. The coral reefs are practically always missing on the west coasts of the continents. One explanation for this are the warm ocean currents. Where they are absent there are no coral reefs. The optimum temperature for most species is 26-27 ° C. The reefs in the Persian Gulf can cope with the enormous temperature fluctuations for corals from 13 ° C in cold winters to 38 ° C in summer.

Light requirement: All reef-forming corals live in symbiosis with unicellular algae (zooxanthellae), which are eminently important for calcification. Like any other plant, these algae can only carry out their photosynthesis when there is sufficient light. The light factor prevents reef formation in the middle of the sea at great depths and in plankton-rich seas.

Sedimentation: Corals as fixed organisms cannot run away when they are sanded in. Too high a sedimentation practically suffocates the small polyps. An example of this is the different colonization with corals in the Gulf of Aquaba and in the Gulf of Suez. In the shallow Gulf of Suez, on average only about 20 meters deep, the sandy bottom is constantly being stirred up and thus prevents a larger coral settlement. In contrast, the neighboring golf course (steep banks, up to 1800 meters deep) with its great coral reefs.

Water: The salinity should be 28-40. There are no reefs in the area of ​​river mouths, where the salinity fluctuates greatly due to changing freshwater input and where a lot of sediment is transported.

So water temperature and sedimentation are the factors that determine the horizontal distribution of coral reefs. Light, on the other hand, mainly affects vertical occurrence.
Worldwide, the reefs only take up about 0.2% of the total sea area, but in the tropical shallow water areas it is up to 15% of the sea floor.

Reef types
Depending on the shape and location, there are four main types:
The most common type is the fringing reef. It arises directly on the coast and grows seaward from the low water limit. The surface remains evenly close to the water surface. The extent to the open sea is limited by the slope of the seabed and the strength of the coral growth. This creates reefs that are many kilometers long, but hardly more than a hundred meters wide. In some old fringing reefs, a lagoon is formed on the side facing the coast due to erosion. Examples of fringing reefs are the coral reefs in the Red Sea.
Barrier reefs lie far off the coast, but did not grow there from the shore, they were created there. A lowering of the subsoil and a rise in sea level were necessary to allow the reef barrier and lagoon to grow to such dimensions. Because of these geological processes that are necessary for their formation over a long period of time, barrier reefs are rarer than fringing reefs. The best known example is the Great Barrier Reef off the east coast of Australia.
The development of a platform reef is not tied to land masses. In contrast to fringing and barrier reefs, it grows in all directions and is surrounded on all sides by deep water. An eroded platform reef is called a pseudo atoll because it cannot be distinguished from a real atoll without closer examination. Examples of platform reefs can be found on the Mascarene Bank in the Indian Ocean.
The atoll is probably the most famous type of reef. Here a ring-shaped coral reef surrounds the lagoon, whereby this lagoon is always connected to the open sea by at least one passage. The formation and morphology of an atoll are diverse and complex. The most famous atolls are in the South Seas and in the Maldives.