Coral Reefs

Coral Reefs

 Coral reefs are usually found in warm, clear, shallow and high salinity waters with a decent current. Coral reefs can be found near the coast or out around volcanic islands. They are composed of calcium carbonate and limestone and are deposited by living things.

Where are most coral reefs found?

Near the equator

Large Scale Coral Reefs

 Of the thousands of species of corals, only a few species produce these large reef structures.

They Can Sting

 Corals are cnidarians, relatives of the sea anemone, and only live in the polyp stage. There is one kind of reef building coral called “fire corals” are related to jellyfish and are anthrozoans.

Soft Corals

 Soft corals are not reef building corals. These include black corals, sea fans and sea whips. These lack hard skeletons and are made mostly of proteins.

Sea Whips and Sea Fans

The Coral Polyp

 The coral polyp looks a lot like a mini sea anemone, consisting of an upright cylinder with a ring of tentacles. They use their nematocyst filled tentacles to trap food which consists of zooplankton and is passed to a sac like gut. Most reefs are composed of thousands to millions of coral polyps all living together.

Colony of Polyps

Reef Building Coral

 Reef building corals begin with a planula, which is a planktonic coral that finds a hard surface to settle. Once there, the planula metamorphoses into the polyp. This polyp will divide into identical copies and form the colony. They will all share a single nervous and digestive system.

 The polyp lies in a cup like skeleton of calcium carbonate which will grow over time to form the coral reef structure. These building corals can take on many different shapes.

Reef Builders

Reef Builders

 Reef-building corals create habitats for many other organisms. The corals reefs of the Red Sea are highly diverse and unique in the world, providing shelter and sustenance for abundant fishes and other marine life.

Symbiotic Relationship

 Nearly all reef building corals are hematypic, which means they contain symbiotic zooanthellae (algae). Without the zooanthellae, reef building would be slow or non existent. The zooanthellae provide nourishment to the coral because the perform photosynthesis and pass some of the organic material onto the corals. Some corals can live without eating as long as the zooanthellae have enough light.

Feeding on Zooplankton

 The corals do prefer eating and are efficient at capturing zooplankton from the water. A current is necessary to carry a constant flow of zooplankton. They can capture the zooplankton by using their tentacles with a coating of mucus.

DOM-Dissolved Organic Matter

 Tiny cilia will then carry the zooplankton to the gut. The corals have sometimes been called a “wall of mouths”. In addition to using their mouths, corals are one of a few animals that can absorb dissolved organic matter (DOM).

Conditions for Successful Growth

 The conditions for coral growth are very specific. They can only grow in shallow water where light can penetrate to the zooanthellae. Reefs rarely develop in water deeper than 50 meters. This fact limits places where coral reefs can grow.

Temperature Range

 Corals also need clear water to succeed, because if there are small sediments suspended in the water, it can clog the mucus on the tentacles and limit the food taken in by the coral polyps. The water needs to be warm as well. Reef building corals will not grow in water below 68 or above 92 degrees Fahrenheit.

 If these conditions cold or warm temperatures are present the corals will begin to die. The coral will expel their zooanthellae and will take on a white color. This is called coral bleaching.

Color Before Bleaching

 This is because the zooanthellae are usually golden brown or green and they are what give the coral its color. Different corals will react differently to temperature change depending on what they are used to. Large changes in tide or temperature due to El Niño, could also promote the death of coral.

Some Has Begun to Bleach

Factors Other Than Temperature

 Sediments, salinity and pollution play a role in the life of a coral reef. Too much sediment is the water will inhibit the food intake of the coral, pollution can kill the zooanthellae, and if the salinity gets too low from a freshwater input, the coral will also perish.

Kaneohe Bay, Hawaii

 Another type of pollution comes in the form of sewage and fertilizers. These will cause algae and seaweed to grow. This growth will inhibit coral growth. This was witnessed first hand in Kaneohe Bay, Hawaii. This accelerated algal growth due to nutrient input is called eutrophication.

We Tried

 People in the area lobbied for a reduction in pollution and when it was stopped, the coral came back quickly. Unfortunately, in 1982, a hurricane came into the area and destroyed the new growth probably because its base structure had been damaged from the years of pollution.

Types of Reefs

 Coral reefs are broken down into three main types; fringing reefs, barrier reefs and atolls. Not all reefs can fit neatly into one of these, but most will.

Fringing Reefs

 Fringing reefs are the simplest and most common. They develop in shallow water near the surface on hard surfaces. Some can be found on soft bottom as long as they can get a foothold on something solid in the area. Once they get started, they form their own hard bottom and can grow.

Largest Fringing Reef

 Since they are close to land, they are subject to pollution, sedimentation in the water and fresh water input from streams and rivers. The largest fringing reef is located in the Red Sea and spans over 2500 miles.

Parts of the Fringing Reef

 A fringing reef consists of an inner flat reef and an outer reef slope. Sometimes the inner flat is exposed during low tide. The bottom of this area is usually covered with sand and mud.

 The steeper reef slope is protected from the environment and that is where most of the corals will grow. Here is where the waves break so there is usually a constant flow of nutrients to these corals.

 The reef crest is the shallow edge of the reef slope constantly hit by wave action. Here is where you find the most luxurious corals.

Since the reef slope is so steep, sediment and rubble can fall down the face, causing the water to become shallower and promote more growth.

Barrier Reefs

 Barrier reefs are similar to fringing reefs except they are usually found farther from the shore, usually about 60 miles or so.

 The reefs will be separated from the shore by a lagoon. This area is protected from wave action and usually contains a lot of sand. A barrier reef will have the reef flat and reef crest but will also have a back reef slope as well as a fore reef slope.

 Coral growth will be slower on the back reef slope because of less water current and more sedimentation. Like in fringing reefs, the greatest coral growth will be seen on the reef crest.

Largest Barrier Reef

 The largest barrier reef is located in Australia and stretches over 1200 miles and covers 80,000 square miles. Technically this is not one reef but a combination of over 2500 smaller reefs. The largest single barrier reef is located off the coast of Belize, Central America.

Atolls

 Atolls are a ring of reef surrounding a central lagoon. These are typically found in the Indo-Pacific. They are rare in the Caribbean and the Atlantic. These atolls are found near land that rose out of the ocean.

No Land in Sight

 With no land around to pollute the water, have freshwater runoff or sedimentation, the corals grow with breathtaking colors and shapes. Most atolls range from 1-30 miles in diameter with many similarities to fringing and barrier reefs.

 The wind plays a major part in the look of the atoll. The windward side (wind comes from) will have an algal ridge and the leeward side will not.

Charles and the Atoll

 At first it was a mystery how coral was growing in the middle of the ocean. It was Charles Darwin the proposed that these coral structures grew around volcanic islands that were once there. The islands subsided and the coral kept growing on top of the coral that was there. He said there should be a layer of volcanic rock under the calcium carbonate reef.

 It was later shown that there was volcanic rock under the reef. In some places the reef was over 4000 feet thick.

Changes in Sea Level is Bad

 Changes in sea level affect the atolls in many ways. If sea level drops, the coral dries out and dies. If sea level rises to fast, the water gets too deep. Either way, the underlying structure is still there so colonization is always possible if water conditions improve.

Rich Ecosystem

 Coral reef ecology is amazing. They are the richest and most complex of all marine ecosystems.

Symbiotic Relationship

 Most corals are found in nutrient poor salty water. They can live so efficiently because of a relationship with photosynthetic algae. The zooanthellae provide organic material for the coral and the coral provide a place to live as well as nitrogen and phosphorus.

 Any waste product produced by the coral will be recycled by the zooanthellae. This is a major reason the corals can exist in such nutrient poor water.

 Recycling also occurs when fish graze on the corals. The pieces of algae created by the munching of the fish will be taken up by the algae. Since the feeding fish are necessary, the coral provide a shelter and the fish stay. Some of the nutrients are carried away by the current so this system is not perfect.

Competition

 There is a lot of competition on the reef. Space is a premium on the reef. They need space to attach and to reach the light for photosynthesis. Faster growing corals will grow tall and they spread out. This will allow for maximum photosynthesis while shading the others. Other corals will come in and begin to digest away the filaments of other corals causing them to weaken and die. Others still will use sweeping tentacles to sting other corals in the area.

Is Too Much Algae a Problem?

 Algae are always a problem. Good thing is since coral grow in nutrient poor waters, the algae will grow slow. Also there are many grazing fish that help control the algae growth.

How Could Soft Corals Have an Advantage?

 Soft corals have an advantage because they can grow fast. They lack hard calcium carbonate shells so they use spicules for protection. They also use chemicals to deter predation. These chemicals can also be released to kill hard corals around it.

Hard or Soft?

 With all of these mechanisms how can hard corals survive?

 Hard corals can live for a much longer time and soft corals are easily swept away by waves.

Predation

 Predation on corals is extreme. Many organisms do not eat the coral but eat the individual polyps. This is good because the corals can grow back. The crown-of-thorns sea star will move over the corals and pushing their stomach out and digesting the corals they touch. Believe it or not these sea stars are preferential to some corals and not others.

Grazing is Good

 Grazing is the least problematic for the coral. Many grazing fish will eat the algae that grow on the reef. In actuality, this is beneficial because sometimes the algae will grow over the corals and kill them if not properly contained.