Coral reefs are the constructions of organogenic limestone, located near the sea level or in small depths in the coastal zone of tropical seas or in shallow warm seas. They are presented by massive deposits of calcite (limestone), which has been formed by skeletal remnants of some colonial sea organisms – plants and animals ”“ seaweed, madreporarian corals, mollusks, foraminifers.
The formation of reefs is favourable at a depth of not more than 50 meters in clear water with normal salinity and temperature not lower than 20Â°C, rich in gases and the smallest organisms (plankton). The slight growth of reef corals can happen to a depth of sunlight penetration – the maximum of 185 meters.
The modern coral reef can be examined as a well-organized biogeocenosis. Coralline algae grow on the surface of corals and in the intervals between them forming calcareous crust, especially where the wave impact is the strongest, and water is maximally saturated with gases. They form strait extremely firm and dense surface that perfectly cements corals.
Corals and algae grow on the periphery of reef and its outer slopes better. A small column of water on the reef platform warms up, impoverishes gases, and in addition, silt accumulates there. In the full tide surface exposed reef that fatally for corals. Some species of coral and algae (especially green algae Halimeda) produce less lime under these conditions. In addition, the waves raise from the bottom of reef platform uliginous sandy sediments. Sudden changes in temperature result in dissolution and precipitation of calcium carbonate, which fills up and firms the inside part of a reef. Accumulation of fragments and coral sand often continues above the sea level, that is why coral islands are formed. The significant part of coral sand consists of foraminifer shells. In relatively shallow waters of calm lagoons the growth of some species of coral and algae also leads to the accumulation of lime. A large number of destroyed algae Halimeda skeletons accumulate at the bottom of some lagoons. In other lagoons fantastically beautiful underwater forests with branching Â«coral treesÂ» and columns, called coral heads find their place for development. However, lagoons are usually not filled with clastic material, as it is likely that thin material is removed into the open sea by inflow and flow-out streams.
There are three main types of interconnected coral reefs: bordering, barrier and atolls. Bordering, or shore reefs are usually found along the newly exposed (drained) or stable coast. This reef is a terrace-like platform at the sea level or slightly below it, stretching from the shore and often ending with highly fragmented channel edge. Corals grow faster on the outer reef slope and very slowly – on the shallow water platform.
Barrier reefs are like bordering reefs. They often surround separate islands, but they are at some distance from the coast, separated from it by the strait or calm middle-deep lagoon.
Atolls are usually ring-shaped reefs surrounding the lagoons, in the center of which there is no area of dry land. They can be ring-shaped with the inner shallow lagoon; ring-shaped with one or more ring breaks through which flood waters come into the lagoon, they can have the form of semirings, a quarter of the ring, giant rings made up of small atoll-like reefs, fringing huge lagoons.
A coral reef may emerge in any tropical or subtropical waters where the depth, temperature and salinity can promote successful growth of reef organisms. So, the genesis of bordering reefs in shallow waters surrounding islands and mainlands is quite obvious. However, specific forms of barrier reefs, atolls generated endless debates and talks about their origin. There are many doubtful hypotheses, but none of them is recognized fully as the satisfactory explanation for the contradictory geological facts.
The bad news was announced by the scientists of joint US-Australian study that coral reefs will not live long. The main threats of their existence are the diseases that in the context of today’s ecological crisis struck coral organisms much stronger and bleaching – the loss of useful algae because of global temperature increase. Researchers involved in the study of this problem, believe that human influence on coral reefs dates far back in time. The fact that coral reefs for many years silently faced people invasion, shoved the tendency of development in future.
Unusually warm water is a stressful factor for corals which ultimately leads to loss of Coelenterata’s color. In addition, algae, feeding the chemical fertilizers and sewage, threaten the corals’ life and diversity.
But at the same time scientists have concluded that as long as climate changes, micro-organisms perfectly adapt to the new conditions of habitation. There is a probability of new species appearance that will be able to live in warm water. But this is just the assumption, and researchers insist on the protection of coral reefs in the world in the form of hard controls under the pollution and increase of so-called untouchable zones.