- Part 5

How Does a Coral Reef Form?

We even have a picture with labels today, don’t get used to it. This is courtesy of Fiona please see the credits below.

A coral reef is an accumulation of the limestone skeletons of dead reef organisms and the algae that glues them together.

The coral reef builds upwards, growing towards the light much like trees in a rainforest, competing for space and light. Once the coral reef reaches sea level it cannot survive the harsh surface conditions so begins to grow outwards. Creating spectacular formations of coral that spawn further growth and spreading of the reef ecosystem.

Through time, animals grow and the sand, rubble and debris of life is broken down by waves and eroding animals, such as worms and sponges. A complex reef ecosystem is built over time. Today’s underwater gullies and caves were formed because of that erosion. These are the same formations scuba divers enjoy exploring today because they are shelters for an abundance of marine life.

Coral reefs tend to grow where there is a lot of water movement, bringing nutrients, oxygen and new species. Most reef-building corals cannot grow in waters shallow enough to expose them at high tide or deeper than 50 metres, making them highly sensitive to changing sea levels.

The Reef is continually evolving and changing as climate and sea levels change. Healthy, diverse reef ecosystems are more resilient, that is, they are able to adapt to change. The Great Barrier Reef Marine Park Authority is concentrating on maximising reef resilience through research and management.

Thanks to Fiona Merida for this great information. Fiona currently runs the Eye on the Reef Program for the Great Barrier Marine Park Authority. A very successful program that brings together and unites the Great Barrier Reef Marine Park Authority, the Great Barrier Reef tourism industry and the reef research community for the greater good of protecting the reef and educating the community.

A Brief History of the Great Barrier Reef

A lot of our readers have asked for a brief history and background on coral reef formations. I’ll try not to bore everyone. Believe me it makes a better discussion when we are sailing or diving on the reef. The Great Barrier Reef is that like every other ecosystem in world. It’s life-cycle is far greater then man’s living history and we are only just starting to understand this complex underwater world. Most research only dates back 30 years and many data points are close to shore and don’t cover the full extent, depths and variety of coral reefs.

As far as the history of the world goes, reefs appear to be a fairly new phenomenon, only one million years old, a bit older than all of us. The earliest signs of the Great Barrier Reef appeared just over 500,000 years old. Just some background here that a lot of people are unaware of, regardless of what the press etc have been reporting, seas rise and fall naturally and have been doing so since time began. During the rising phase, without human interaction, seas rise about 10m every thousand years. Over the last 500,000 years we have undergone the rise and fall of oceans just under 20 times. The current Great Barrier Reef started growing between 6 and 9,000 years ago, that is when sea levels rose over old underlying reefs and corals started growing. Sleepy yet?

So when the sea level rises the coral reefs take full advantage and often the grow faster then the sea level rises. The coral keeps growing towards the surface, like a plant towards the sun, until the reef flat is exposed at low tides (sea level). When the coral reefs reach the surface, the growth stops or slows and the battle between mother nature and the reef continues. Strong winds, storms, predators, the summer heat, as well as numerous other factors are continually impacting the reef.

So how does the reef survive this continual attack? Well, corals have evolved over thousands of years to survive this delicate ecosystem. So a couple of things:
-When coral spawns it can only successfully seed and grow if the spot it comes to rest on is clean. That means no algae or live coral. So all the damage above actually makes way for the next generation of corals.
– These cycles are natural and the Great Barrier Reef has survived these impacts for hundreds of thousands of years.
– We know from core samples taken around the world that reefs are very resilient to natural events. Only a few species have become extinct over the last 100,000 years.
– Different types of corals grow at different rates so reefs have the ability to reclaim themselves very quickly and then diversify over time as long as there is “seed” coral remaining or another reef nearby.

Coral Reefs build up slowly towards the surface, building upon the growth of previous generations as well as dead coral caused by the impacts above. This is how snorkellers and divers get to enjoy the reefs everyday around the world. By minimizing our impact on the reef we can make sure that this cycle continues.
-Follow reef safe diving and snorkeling practices.
-Reducing major pollutants that have impact on coral growth to ensure that we do not wipe out entire reef systems.

Comparing samples from older reefs to the newer reefs we see that much more damage is happening with much more frequency and to date most reefs have proven resilient. Reefs closer to major human population centres have suffered the most impact. As long as we minimize our impacts, we can be pretty sure that these amazing ecosystems will continue to inspire generations to come.

Coral Reef Growing from Seabed to the Surface

The Barracuda – Fish Species of the Reef

Very popular with scuba divers on our trips to the outer Great Barrier Reef, barracuda are an impressive sight on the reef. The fearsome looking barracuda has a reputation for being and aggressive fish but it is mostly undeserved. They really use their incredible speed to shock and attack its victims of smaller fish. They do not see humans as a meal or as a threat unless actively provoked. They have been known to get aggressive where potential food is involved, another good reason not to feed the fish or support operators that feed fish.

They are often seen in large groups in the early morning or late afternoon hanging around at the back of the reefs. What are groups of barracuda called? Batteries. The can grow to about 1.6 metres or just over 5ft, the guys in the photo above are just over a metre each. They have a large swim bladder (gas filled bladder) they use to control their buoyancy in the water. This allows them to hover perfectly for long periods of time with very little use of energy.

Moorish Idols – Species of the Reef

The Moorish Idol is a common fish around the reefs and prefer to swim around the top of the corals feeding on sponges, and small invertebrates. Our snorkellers and divers often encounter the idol as it grazes amongst the coral gardens of the Great Barrier Reef.

It is easily recognized as it has a thin body with a round shape. This is highlighted but very distinctive yellow and black bands with white contrasts that run vertically up the body. During the night these colours actually become more dull and sink to the base of the reef for protection from predators. They also have several long dorsal spines that extend from the main part of the fin to flow over the back of the idol.

These fish are very popular with aquarium owners because of their bright colours and trailing dorsal fin. However they are notoriously difficult to keep alive, they can be very picky eaters and require large tanks as well as good water quality.

Moorish Idols mate for life. The release their eggs to drift where they drift until their larvae reaches about 7cm or 3in, when they as juveniles start to swim on their own and develop into mature adults.