34
buttress zone , which consists of alternating coral-capped ridges,
channels and furrrows. This irregular topography tends to disrupt swell
,because an uneven wall face has much more surface area than does a
flat wall and and , therefore,receives less wave energy per unit area
under the same wave conditions. The reef face extends downward from
the buttress zone . Here living corals are absent , because they rarely
grow in water depths below 40 meters (132 feet) because there is so
little light at those levels.
The reef terrace is a broad reef flat just landward of the algal ridge , its
floor lies at mean low water ,so that much of its surface is exposed at
low tide.
Recently, scientists have been alarmed by a disturbing trend .Perhaps as
much as 30% of the world’ s reefs have been seriously damaged or
killed outright. Locally , reef decline is an even more serious problem ,
as for example in Jamaica and Costa Rica where 90% of the coral reefs
are now dead .
Bleaching events :
Sea surface temperatures of tropical water have increased during the last
few decades , possibly due to global warming . Coral are intolerant of
elevated temperatures and respond by ejecting the phytoplankton , the
zooxanthellae , which live in their tissue and are responsible for the
polyps variegated colors. This, a bleaching events causes the coral to go
white.
Overfishing :
A variety of nonsustainable techniques are used to kill or capture fish
that live in reefs. These include trawling the reefs ,blasting fish with
dynamite , and poisining them with sodium cyanide.
Most of these methods kill organisms indiscriminately and often
seriously damage the reef edifice or disturb ecological relationships that
affect the reef ’s health.
Agricultural practices :
In the tropics , deforestation of the land and the use of fertilizers ,
herbicides , and pesticides allow soil
and chemicals to be swept