FRINGING REEF FORMATION

The geological structure of the Red Sea, with a narrow coastal shelf dropping off quickly into deep water, has constrained the development of coral reefs, since reef-building corals require shallow, warm water for photosynthesis by their symbiotic algae (zooxanthellae). Rather than the large, offshore barrier reefs found in areas such as Central America and Australia where there is a wide, shallow continental shelf, Red Sea reefs tend to be fringing reefs, forming a narrow band quite close to the shoreline. The Gulf of Aqaba, on the east side of the Sinai peninsula, is also deep and has this structure, with fringing reefs along its coast. The Gulf of Suez on the west side of Sinai is shallower, however it is influenced by Mediterranean weather, meaning that the water gets sufficiently cold to limit reef development.

REEF DIVERSITY

Despite these geological limitations to the extent of coral growth, the Red Sea has a very rich and diverse coral fauna. Conditions for coral growth are ideal. The climate is hot and arid, meaning that the water is always warm and corals get a lot of sunlight. Corals also grow best in water that is low in nutrients such as nitrogen and phosphorus, because in nutrient-rich water, algae grow rapidly and block the light. Hence low nutrient water is very clear, while nutrient-rich water is murky. The Red Sea surface waters are exceptionally clear and low in nutrients because the hot, arid climate means that population density is low and there is little nutrient input from soil, agriculture and pollution on land. It also creates a permanent surface layer of warm, nutrient-poor water which does not mix with nutrient-rich deeper water (a process called "stratification").

Coral reefs are particularly well developed in the north and central Red Sea (Egypt, Saudi Arabia, Sudan). In the south, coral growth is inhibited because nutrient-rich water from the Indian Ocean enters the Red Sea, and because stratification is reduced by very strong winds caused by the high mountains near the coast. In this area, corals are less diverse and seaweeds grow on top of the coral reefs during the summer. (Actually it is quite rare to find coral reefs with

a seasonal seaweed cover - the southern Red Sea and the south coasts of Yemen and Oman are one of the few places were ithappens regularly.) The Egyptian Red Sea supports about 200 species of reef building corals in about 50 genera (no-one can give an exact number since the taxonomic categorisation of corals is difficult and controversial, and there are likely to be species remaining undiscovered). This is about the same number of species as the Maldives, Seychelles and other Indian Ocean islands, rather less than the Great Barrier Reef and the western Pacific Islands (about 300 species) but several times more than the Caribbean.

Red Sea fringing reefs are not only important for hard corals. They support a highly diverse fauna and flora with almost every group of animals and plants represented: algae, sponges, cnidarians (corals, sea fans, jellyfish, anemones), molluscs (e.g. octopus, shellfish, nudibranchs), crustaceans (e.g. crabs, lobster, shrimp), echinoderms (e.g. sea urchins, starfish, brittlestars), worms, tunicates ("sea squirts": organisms that look a little like anemones and are our nearest invertebrate ancestor) and of course vertebrates like fish, sharks, turtles and dolphins.

BEHIND THE REEF

The fringing reefs form a barrier against the open sea, behind which (except where the reef is very close to shore) is a shallow protected lagoon. The lagoon will contain patches of coral wherever there is a patch of hard bottom for corals to attach to - they can be anything from a few metres to a few 100 metres in diameter. These patch reefs are often even more diverse and beautiful than the fringing reef itself, since they are more protected from waves. The reefs are interspersed with seagrass beds and sandy areas to form a mosaic of habitats. If you dive or snorkel in these areas you will see different animals in the different areas: e.g. juvenile fish, crabs and nurse sharks in the patch reefs; starfish, sea cucumbers, and occasionally baby reef shark in the seagrass beds; and flatfish, stingrays and textile cone shells in the sandy areas (beware the latter two: they have toxic stings).

The different species in each place might lead you to believe that the ecosystems are separate. However in actual fact they are highly interdependent, and this mosaic structure makes each type of habitat more productive and diverse than it would be if it existed in isolation. For example, many reef dwelling species forage for food in seagrass beds at night when they are safer from predation. If you are snorkelling or diving on a reef, you will be rewarded by looking carefully in all the cracks and crevices, where many of these species hide out during the day. You are likely to find crabs, lobster, juvenile grouper, moray eels and nurse sharks in reef crevices. Try not to touch, knock or stand on the coral while you are looking (it may be killed).
(Jo Gascoigne)