Different Types of Reef

Coral reefs are among the most biologically diverse and valuable ecosystems on the planet! Globally there are 34 known animal phyla, 9 of which are found in tropical rainforests whereas a whopping 32 are found on coral reefs. Approximately 25% of all marine organisms are dependent on coral reefs at some point in their life cycle and, furthermore, 38% of coral reef organisms are so rare that they have only ever been encountered once!

Coral reefs are brilliant nursery habitats, providing refuge and complexity for smaller organisms to hide and escape from predators. They provide coastal protection, with the ability to absorb 70-90% of wave energy they are faced with, and can protect landmasses against storms and flooding. Over 1 billion people are reliant on coral reefs for survival, with the estimated global economic value of reefs being $5.5 billion/year.

Even though coral reefs occupy less than 1% of the ocean floor, approximately one third of all marine species live in them!

 

 

Anyway, enough of the fun facts. You get the picture; coral reefs are hugely important! But did you know there are different types of coral reefs? This is sometimes a point of debate, but scientists generally agree that there are four different types of coral reefs:

1. Fringing reefs
2. Atolls
3. Barrier reefs
4. Patch reefs (this is the one that is sometimes not classified, but don’t tell them)

We’re going to break them down for you, don’t worry!

 

Fringing reefs.
Fringing reefs are the most common type of coral reef. Narrow, shallow lagoons separate these reefs from the shore. These reefs grow seaward directly from the shore and form borders along the shoreline and surrounding islands. Fringing reefs are made up of the reef flat – the part which is closest to the shore – and the reef slope – the part which faces the open ocean. Some fringing reefs can be hundreds of metres wide, but most are usually less than 100 metres.

Fringing reefs are formed when free-swimming coral larvae attach to hard surfaces or submerged rocks along the edges of islands or continents and, as the coral grows, they expand seaward.

Common examples of fringing reefs include The Great Fringing Reef in the Northern Red Sea. At Operation Wallacea our Madagascar marine site on Nosy Be is surrounded primarily by awesome fringing reefs. They form a ring of coral ecosystems close to the island, which is off the northwest coast of mainland Madagascar, creating the classic structure of a fringing reef.

 

 

Atolls
Atolls are formed when a volcanic island sinks below sea level – or the sea level rises around them – and the fringing reef that was surrounding it continues to grow upwards. This process may take as long as 30 million years to occur. When the island completely subsides beneath the water, it leaves a ring of growing coral. Atolls are usually oval or circular in shape and surround a body of water in its centre, creating a protected lagoon. They can be dozens of kilometres across and the lagoon in the centre can be over 50 metres deep!

Atolls are some of the most complex structures on the planet, and the largest atoll in the world is the Kwajalein Atoll in the Marshall Islands, enclosing a lagoon of 2850 km². Unfortunately, we don’t visit any atolls just yet across our marine sites but never say never!

 

 

Barrier reefs
Next up we have barrier reefs, and these are very similar to fringing reefs because they border a shoreline but, rather than growing directly from the shore, barrier reefs are separated from land by a barrier; an expanse of water! These reefs are parallel to the shoreline but separated by a lagoon or a deep channel. Barrier reefs are larger than fringing reefs, and the lagoons or deep channels can be several kilometres wide and 30 to 70 metres deep. Barrier reefs are formed when the seabed lowers or sea level rises.

I am sure you can all name an example of a famous barrier reef, The Great Barrier Reef in Australia! The Great Barrier Reef is the world’s largest coral reef system composed of over 2900 individual reefs and stretching over an area of approximately 344,400 square kilometres! The Mesoamerican Barrier Reef is also the largest barrier reef in the Western Hemisphere stretching nearly a whopping 700 miles from the Yucatan Peninsula down to the Honduran Bay Islands, encompassing our marine sites in both Mexico and Honduras!

 

 

Patch reefs
Patch reefs are sometimes not classified in the main three coral reef types listed above because patch reefs are typically part of a larger reef system and not recorded as individual reefs. Patch reefs are small, isolated patches of coral that are in close proximity to each other. They grow from the bottom of an island platform or continental shelf. Patch reefs vary in size and shape due to the different species of coral that are present and the size of the hard substrate from which they grow. They also rarely ever reach the surface of the water.

The isolated outcroppings of coral are physically separated by sand rings, and this is the result of herbivorous fish and invertebrates that live in the patch reef. Herbivorous fish such as surgeon fish and invertebrates such as urchins eat the algae that grows on the coral and algae around the perimeter of the reef. They then excrete the waste as sand.

Patch reefs are common in the Caribbean islands, the Bahamas, Bermuda and the Pacific Islands. At Operation Wallacea our Indonesia site in the heart of the coral triangle is a brilliant place to explore some patch reefs and these awesome ecosystems!

 

 

When most people hear the word reef they do think of bright colours, warm waters and of course coral. But it is important to know that not all reefs are coral reefs. We also have deep sea reefs, cold water reefs, corals at hydrothermal vents, and rocky reefs for example! Our marine site in South Africa at Sodwana Bay is also an example of a unique reef system. It is classified as a fossilized sand dune reef, built on ancient sand dunes that solidified into rock! They are not fringing reefs in the traditional sense because they are set slightly further offshore and interspersed with patches of sand.

 

Title photo by Jack Hague