What is a coral reef?
Coral reefs are one of the most attractive and most complex biological communities on the planet. The word “coral” is used to describe a group of tiny organisms, from the phylum Cnidaria, that secrete hard calcareous skeletons and live in colonies in the form of polyps.
In the Caribbean, there are more than 70 stony corals, whose skeletons are complex structures we call coral reefs. Associated with these are soft or horny corals, some Zoantharia, and millepora or “stinging corals”.
Biology and ecology of coral reefs
The biological combination of colonial organisms and associated flora and fauna make up the coral reef, one of the biological systems in which nature expresses its greatest splendor and complexity. Slow but steady coral growth takes thousands of years; the accumulation of sediment and fragments that are generated in the reef allows corals to survive, despite increases in sea level. The vertical growth, at the rate of 0.5 to 1.5 cm per year, allows the reef to adjust to these changes. Many of the modern reefs were established less than 15 thousand years ago when sea level was 85 meters (279 feet) below the current level, and island platforms began to flood as a result of the rapid rise in sea level.
Coral polyps have tentacles with which they capture zooplankton that swims freely in the water. The most unique characteristic of polyps is that they have unicellular algae, known as zooxanthellae, inside.
Numerous hiding places between coral blocks provide shelter to a wide variety of animals including sponges, worms, mollusks, crustaceans, sea urchins, sea stars, holothurians, and fish that are characterized by vivid, contrasting colors. This complex community of organisms, closely integrated as a result of a long evolution, gives rise to the ecosystem of coral reefs. The deployment of colors found in the reef is not an accident but rather the result of the complexity of that biological community.
From an environmental point of view, the development of coral reefs is limited to relatively stable places with very specific ecological conditions.
• Warm temperatures, never lower than 21-22°C (70°F)
• Good lighting
• High salinity
• Low tolerance of suspended sediments
• Waves and currents
• Low tolerance for prolonged emersion
Distribution and types of reefs
The above conditions only occur in shallow areas of the tropical seas. In the Caribbean, there are favorable conditions for the growth of corals in the West Indies, especially along drier coasts where there are no rivers or sediment supply. In Puerto Rico, there are important coral areas on the east, south, and southwest coasts. The north coast has no large coral development probably because plentiful rivers flow into it and bring large amounts of sediment. The continental shelf north of the island is also very steep and falls deep near the coast. The space available for the establishment of this type of community is limited by those factors.
In Puerto Rico, there are three types or forms of reefs:
• Fringing reef – This type of reef surrounds a non-coral coastline. It is often separated from the coast by a lagoon or shallow, narrow body of water whose floor is covered in calcareous sands and seagrass. This type of reef is one of the most common in Puerto Rico; but because of its proximity to the coast itself, it is also the one most degraded by human activity.
• Barrier reef – This type of reef occurs further away from the coast. In Puerto Rico, this type is represented by a reef which lies at the edge of the island shelf at depths of 20 meters (65 feet).
• Bank reefs – Bank reefs are located on the platform between the two previous types. This type of structure is known as coral platform or bank-barrier. These reefs often acquire crescent shapes because the coral grows and consolidates preferably towards the waves.
Coral reefs respond to disturbance
Coral reef systems are highly complex and, therefore, damage caused by natural disturbances or human beings can take many decades to be repaired. However, the species that constitute the reef include relatively fast-growing species; this allows the “healing” of damage caused by disturbances that leave no residual actions. Unfortunately, the actions of human beings on these ecosystems often do not allow these mechanisms of natural regeneration to operate, causing chronic disturbances that lead to the deterioration and eventual collapse of the system.
• Storms – In our geographical region, hurricanes and storms are one of the most violent natural disturbances that act on coral reefs. These storms generate waves of great strength that break corals and detach blocks of coral rock, which in turn cause extensive damage when they slide or roll over the ocean floor. Paradoxically, these changes tend to renew the vital processes of the system and, in the long run, may be beneficial as they contribute to further growth and development of new habitats.
• Changes in sea level – The scientific community has expressed great concern with regard to the changes in sea level that we are experiencing because of the so-called “greenhouse effect” caused by emissions of certain gases into the atmosphere. In the case of corals, there is concern that these systems are experiencing severe degradation that could make it impossible for them to persist given the speed of sea level changes that have been predicted.
• Coral bleaching – Coral bleaching is a phenomenon that is happening in many areas of the Caribbean, but its causes are unknown. Bleaching, which is caused by the expulsion of zooxanthellae, has been attributed to small increases in the average temperature of surface waters in the area of the greater Caribbean.
Disturbances caused by human activity
• Sedimentation and enrichment of the waters – In Puerto Rico, some rivers transport up to 100 metric tons (200,000 lbs.) per hectare of suspended sediment per year. These sediments reach the sea where they scatter and degrade the quality of coastal waters. Reef systems subject to sedimentation are quickly destroyed or degraded. Nutrients that enrich the water cause phytoplankton and the organisms (zooplankton) that feed on them to appear. This appearance reduces the transparency of the water and causes sedimentation of organic material. It also favors the colonization of reef substrates by fleshy filamentous algae that grow on live corals and destroy them.
• Overfishing – Although the reef is a system of very high productivity, these systems are highly vulnerable to over-exploitation. Selective fishing of certain species can destabilize the system when those species have vital functions in its maintenance.
• Removing corals or “live rocks” – The removal of corals leads to destruction of habitat and impoverishment of the areas. It also reduces the aesthetic value of these areas. In Puerto Rico, the removal of corals is forbidden by the Reglamento para Controlar la Extracción, Posesión, Transortación y Venta de Recursos Coralinos (Regulations for controlling, gathering, possessing, exporting, and selling of corals).
• Ship grounding – On February 15, 1985 a ship named “A. Regina” was grounded east of Mona island causing coral destruction on a system of spur and grooves that characterizes the external slope of the reef edge located in that area. The direct destruction due to the movement of the hull of the ship, which weighed over 3,600 tons (7,200,000 lbs.) affected approximately 5,875 square meters (19,270 feet) of reef floors. The pulverizing of the floor and the dispersion of fine sediments affected an area of over 15 hectares -an area more than 25 times larger than that destroyed by the hull of the ship.
Importance of coral reefs
The importance of coral reefs is:
• They protect us from heavy tides and waves during storms and hurricanes;
• They significantly modify the speed and direction of ocean currents, allowing the establishment of communities associated with this system such as seagrass prairies and mangrove swamps;
• They export nutrients to terrestrial systems where sea currents go from sea to land, and receive nutrients from these terrestrial systems when currents go from land to sea;
• They serve as habitat for many marine species of human consumption and as a bridge of commercial and recreational fishing;
• They have many aquatic animals that are food sources for higher organisms;
• They are valuable landscapes because of their variety and diversity of colors and shapes;
• They are sources of recreation for many people who practice diving; and
• They are sources of natural products of biomedical potential and in manufacturing industries.
The productive potential of the coral reef ecosystem and its ability to generate goods and services (e.g as tourist attractions) depends on maintaining its structural integrity and protecting the quality of the water that surrounds these systems. We also have an inescapable responsibility of protecting them so that they can be used by future generations of Puerto Ricans.
These extremely fragile systems require special handling to ensure their persistence. A significant element in handling these resources is the awareness of citizens about the importance of these systems, their vulnerability, and the care they need in order to be protected. The protection of these resources is a task that must be shared by all.
Author: Grupo Editorial EPRL
Published: August 27, 2014.
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