The Coral Reef Bleaching Environmental Sciences Essay

Coral are very old animals that have evolved over millions of years to become reef-building organisms. On Earth, coral reefs are among the most biologically diverse places to be found. They may only cover a small percent of the ocean, about .2 percent, but more than 25 percent of the marine life resides there. The two main types of coral are hard coral and soft coral. Hard coral is the major species that build reefs. Similar to tropical rainforests, coral reefs are usually located within 30 degrees latitude of the equator. Water temperatures for the reefs are between 16 and 30 degrees Celsius, though there are coral outside these regions with somewhat colder temperatures. The ideal development waters for reefs are tropical waters which are low in nitrogen, phosphorous and other nutrients, stable temperatures, as well as stable salt content. The coral reefs we see today are a result of thousands of years of production and cycling through life and death. Coral only contribute a smaller portion of life found in the reefs, though they create a habitat for various amounts of other species. There are three main types of coral reefs: barrier reefs, fringing reefs, and atolls. Respectively, these reefs are offshore, separated by water from land, continuous with land, and islands made of coral that flank lagoons. Though there are different types, the general coral makeup is the same. After years of mystery, we now know that coral is very complex in its design. Coral is a symbiotic relationship with the small animals that make up coral called polyps, and the microscopic algae called zooxanthellae that live inside the coral. [1][2]

Coral reefs lives in colonies consisting of many polyps using living tissues to connect to each other. Each polyp resembles a cupcake shape with a circle of tentacles near the opening. To capture zooplankton for food, the tentacles are equipped with stinging cells for trapping. The small polyps secrete limestone, also known as calcium carbonate. The exoskeleton of the coral is made of these secretions, just like the shell of a snail. Reefs are able to grow from the stone. [4]

The algae that live in the corals use the process of photosynthesis, taking sunlight to converts water and carbon dioxide into sustenance. The polyps receive most of their energy this way, although they are carnivorous. For providing the corals with energy, the algae are provided shelter to live, taking the coral waste for their own growth. A situation where two organisms live together gaining benefit is called mutualistic symbiosis.[15]

In every ecosystem, there are three groups of organisms: producers, consumers, and decomposers. Producers are usually organisms that take in sunlight to produce energy. The consumers are above the producers, and range from herbivore, omnivore, and carnivore. Next are decomposers that break down dead plants and animals that help provide food back to the producers. [12]

At a first glance, the initial food source may not be very apparent in coral reefs, as there are usually low levels of actual plants to the visible eye. However, the microscopic algae as stated before take in sunlight, use photosynthesis, and create energy for the other animals and the coral.

At the base of all food chains, the primary producers, also known as autotrophs are found. The producers have the ability to synthesize intricate, organic compounds like glucose using inorganic molecules and sunlight. This process is called photosynthesis. Less common producers are some bacteria. These get energy by oxidizing other inorganic compounds like hydrogen sulfide. These are better suited to live in more hostile areas such as deep sea vents.

The greater proportion of production happens in the euphotic zone. This zone is within the first 70 meters of water. The way productivity is measured, is in grams of carbon produced per meter of ocean surface per year, better written as “g C/m2/yr”. Total productivity of the ocean is estimated to be between 75 and 150 g C/m2/yr. Common producers in the coral reefs are phytoplankton, the symbiotic zooxanthellae that live with the corals, different types of algae like coralline and turf algae, along with some seaweed. One of the more important primary producers is phytoplankton. Phytoplankton is a group of organisms like diatoms, dinoflagellates, some bacteria, and smaller species known as nanoplankton which grow about 2.0-20mm long, and picoplankton. These grow to about 0.2-2.0mm.[21]

Next come consumers. Consumers rely on the producers and other consumers as their main source of energy. The first set of consumers is organisms like zooplankton, some species of corals, sea urchins, green sea turtles, herbivorous fish, and herbivorous larvae. Secondary consumers are generally carnivores, going for primary consumers. After the carnivores, come tertiary consumers, and prey on secondary consumers. These consumers tend to be carnivores as well, but can also be omnivores as well.

Common types of zooplankton are copepods, amphipods and ciliates. These zooplankton, along with many more types, are the most plentiful consumers. Phytoplankton evels in the waters of coral reefs can be around 15 to 65 percent lower than neighboring open waters in the ocean because of the large amount of primary consumers that live there. Some species of coral and benthic grazers feed by filtering water to trap phytoplankton. Other creatures eat seaweed, and even the coral itself.[21]

Secondary consumers are the animals that prey on primary consumers, therefore making them carnivorous. They are split up into four main groups: plankton feeders, corallivores (feeding on coral tissues), feeders that prey on other invertibrates, and fish eaters also known as piscivores.

Barnacles, smaller damselfish, whales sharks, and some invertebrates make up the group of plankton feeders. The corallivores go a step further, being divided into four more groups: Coral tissue generalists, coral scrapers, polyp eaters, and mucus feeders. The most destructive sub group are the coral tissue generalists. Flatworms, parasitic snails and nudibranchs, and the Crown of Thorns sea star devour coral, leaving skeletons behind. The mucus feeders dine on slimy mucus that comes from the coral. Some examples animals of this group are shrimps and coral guard crabs. Parrot fish, butterfly fish, and certain species of trigger fish are some commonly known coral scrapers. The fish that are well adapted to eat coral polyps are able to stretch out or elongate parts of their bodies to cherry pick polyps singly out of a coral animal. Damsel fish, and many species of butterfly fish are some examples. Benthic invertebrates such as crustaceans and mollusks are eaten by many species of fish. Cephalopods focus on eating arthropods like lobsters and mantis shrimp, along with shelled invertebrates. Cone shells have been specialized on certain, specific invertebrates. Crown of Thorns sea stars are eaten by Tritons, or Charonia tritonis. Many species of fish, as well as some mollusks and arthropods are piscivores, feasting upon other fish for food. At the top of the food chain, where the animals prey on smaller fish, are the tertiary consumers. These animals are marine animals like dolphins, seals, and if they are around, even sea birds are considered. [21]

In all ecosystems, decomposers are one of the most important parts of the food chain. Decomposers break down deceaced biological matter and waste from other organisms. They then transfer these things into energy, meanwhile they return the important materials back to the environment. In coral reefs, the main decomposers are bacteria. Bacteria play a very important part in the nitrogen cycle in which ammonia, or NH4 is turned into nitrite, NO2 using the bacteria in the genus Nitrosomonas. Nitrite is then turned into nitrate, NO3 by bacteria in the Nitrobacter genus. Ultimately, the end result is levels in wastes that are toxic tend to be very low, and are processed into bits that the main producers can easily use. Detrivores, or animals that scavenge, have a somewhat similar job in turning out dead and waste material. Other organisms that contribute to this are sea cucumbers, bristle worms, and some species of snails.[21]Coral reef trophic structureThis is a diagram of the food chain in the coral reef. It better describes how the energy is transferred between the tiers. Solar energy is taken in by the primary producers, the primary consumers eat the primary producers, the secondary consumers eat the primary consumers, and the tertiary consumers eat the secondary and primary consumers. Each of these tiers can be decomposed and have their energy given back to the primary producers. Also between each stage of the ecosystem pyramid is the loss of heat energy. [20]

Though they may seem like a stable ecosystem, many threats are present to coral reefs. Their delicate structure can be very easy to disrupt, pollute, damage, and overall harm. Things like the acidification of the ocean, global warming, coral bleaching, levels in carbon dioxide, water pollution, sedimentation from construction, fishing practices that are destructive like blast fishing, and coral mining are good, yet terrible examples of threats.[9]

Carbon dioxide, along with other gasses that trap heat is being constantly thrown up into the atmosphere, helping the process of global warming. Acting as a sort of shield, these gasses do not allow the heat we get from our sun to leave the Earth’s atmosphere. One of the main reasons this happens is deforestation, and the burning of fossil fuels. When heat isn’t allowed to exit the Earth, water temperatures rise. This is very dangerous to coral reefs, which are super sensitive to changes in temperature, as the reefs require a certain temperature range to function. When the coral polyps become stressed from rising temperatures, they discharge the algae living within them, which give the coral most of its energy. When this happens, the colour also goes with the algae, leaving the coral looking “bleached.” As bad as this seems, there are chances for coral to recuperate if the environment returns back to normal in a fair amount of time. Most bleached coral die, however. [14]

Coral Reef Bleaching

This diagram shows how the process of bleaching occurs. Healthy corals have zooxanthellae all around the body. As conditions worsen, the zooxanthellae leave the coral. The coral starves and dies. After a while, another kind of algae covers the coral, making it harder for the corals to gain back their zooxanthellae. [9]

Carbon dioxide content in the air has increased by about one third over the past few decades. The carbon dioxide also mixes into the water, which also is thought to be dissolving the coral, letting other threats like wave damage, fishers using destructive means, and possibly even tourists who do not care too much about being careful become much more of a threat than usual[9]

Pairing with global warming, the depletion of the ozone layer is caused by chemicals in the atmosphere, one of which is commonly known as CFCs, or chlorofluorocarbons. These chemicals further the depletion of the ozone layer, pumping up the amount of the sun’s radiation. In more shallow areas, it is believed that the damage done to corals increase.[9]

One of the leading causes for the destruction of coral reefs scientists have found is pollution. Pollution of all sorts damage the reefs, be it oil, gas, or pesticides. These reefs and the organisms that live within the system are poisoned whenever animal and/or human wastes, fertilizers, and chemicals are dumped into the ocean, or get carried by rivers into the ocean into the waters that reefs exist. When these bad things are introduced, levels of nitrogen in the coral reefs grow, resulting in an abundance of algae. The extra algae then start cutting off sunlight from the coral, starving it of important nutrients. Floating trash and other similar wastes can cover and block the reefs from getting sunlight as well. Mistaking plastic bags for sea jellies, turtles will often go after them and eat the bags. If they do, the plastic inside will starve them to death because of blockage in the digestive tract. “Ghost nets” or nets that have been lost, discarded, or forgot about may catch hold of reefs, strangling many fish, turtles, and other marine mammals. [9]

Careless tourists and things affiliated with tourism can pollute damage and kill corals reefs as well. Resorts near sites with reefs might have septic tanks that leak, seeping sewage into the waters nearby. These establishments might also simply dump waste they want to get rid of into the waters. The people themselves can damage the reefs as well by either accidentally or purposefully step on, kick or grab corals or the sediment around them, stirring up sand and sediments that can block sunlight as well as damage the corals directly. Setting down boat anchors nearby can destroy corals too. This usually happens when people attempt to mine the reefs of coral colonies. Broken pieces of coral can be used for bricks, road filling and when ground up into limestone, will be mixed into cement for construction. This is not the only reason corals are taken. They can be sold as tourist souvenirs, jewelry, and decorative pieces.[9]

Certain ways of fishing are pretty destructive to the environment as well. Practices like over-fishing, using blasts or charges to kill fish, and even fishing with cyanide are all threats to reefs, and any other ecosystem. Most fish caught using cyanide are sold in primarily in Asian countries, and given to customers in restaurants. Catching fish with cyanide involves fishers actually diving down into reefs and squirting cyanide into the crevices of corals and onto fast moving fish, stunning them for catching. Other fish nearby along with coral polyps are poisoned by this.[9]

Over fishing is another big cause of coral damage. A lot of times, a population near a reef will catch too many fish from a single reef to feed themselves. Things like banging sticks against the reef destroy formations of coral that provide homes for fish. Blast fishing, or fishing with explosives, is very bad for coral. When a charge is set off, the surrounding area is blown to bits, leaving little intact.[9]

Numbers of people that live along coasts have risen, leading to higher uses of the resources from the coastlines. This is causing many problems for waters near land developments, especially for coral reefs. Some cities like Hong Kong, Singapore and Honolulu had beautiful coral reefs. Due to the rising populations, however, things went sour for the reefs. Pollution, construction, docks and piers to name a few have been made. Some establishments have even been made directly on top of the reefs, both destroying them and leaving the little coral that survived barely any sunlight. Reefs are still experiencing this fate as more human communities grow along the shores.

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This is a diagram showing some -but not all- threats to coral reefs. By following the arrows, it is evident that just one these threats can lead to a multitude of consequences. This diagram does not even list all the potential threats. It is safe to assume that if all the threats were listed and shown this way, the result would be extremely overwhelming. The more word spreads about how much can harm a coral reef, the better.