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red sea : formation
and sturcture :
reefs

Red Sea
Coast

Reefs

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)

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