Image © Richard Smith/OceanRealmImages.com

Overview

Welcome to the unique, stunning and hugely diverse world of coral reefs! Everywhere you look the reef harbours an incredible array of life, from cleaner wrasse precariously cleaning the mouths of big predatory grouper to nudibranches quietly chopping away at sponges, and clown fish carefully tending to their anemone home.

Under every nook and cranny is a different weird and wonderful creature, elaborately adapted to its little niche on the reef and intrinsically linked to its fellow reef species. This variety of life on the reefs is made possible largely due to the reef-building stony corals lying down their coral skeleton to form the foundations of the reef.

These ancient geological structures teeming with fish and invertebrates are the rainforests of the sea. They are the most biodiverse marine ecosystems harbouring around a third of all marine species, most of which are found nowhere else, not bad for an ecosystem that takes up only 0.2% in area of the marine environment!

Yet 19% of coral reefs have already been lost and 35% are seriously threatened by direct human actions such as over-fishing, water pollution and sedimentation. These estimates do not take into account the combined effects of climate change including ocean acidification, raising sea temperatures, sea level rise and more frequent occurrences of large storms. When climate change is factored in all coral reefs are severely threatened. Without immediate drastic reductions in atmospheric CO2 concentrations coral reefs face imminent and total collapse in the face of global climate change.

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Coral reefs are often considered the ‘rainforests of the sea’ due to their incredible biodiversity



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Coral Reef Diagram
Reef Habitat Key Features
A. Inter-tidal zone The inter-tidal zone is situated between the boundary of low and high tide, a hostile zone in which few corals can survive. Here the fluctuating tide will result in periods of submergence in water and exposure to air, as well as a constant physical barrage from moving waters.
B. The inner reef, back reef, lagoon, reef flat

C. Back reef slope
The inner reef is the section of coral reef closest to a landmass and typically consists of a shallow water area. This reef habitat is often subject to high and fluctuating water temperatures, intense UV light, high salinity and potentially desiccation as a result of low tides and exposure. Inner reefs, being adjacent to land, are also the areas of reef most affected by land based pollutants and sedimentation. In barrier reefs and atoll reefs, the inner reef will consist of a lagoon which itself may consist of further habitat types such as seagrass meadows and sand plains.
D. Reef crest The reef crest is the section of coral reef that breaches the ocean surface and divides the sheltered fore reef from back reef/reef slope habitats. Similar to other shallow water reef habitats, this section of the reef is subject to high levels of physical disturbance, high light intensity and high temperature fluctuation. Being fully exposed to the onslaught of the oceans waves, this high energy environment is often more suitable to hardy, calcareous algae.
E. Reef slope The reef slope encompasses a broad section of the reef, stretching from the reef crest to the ocean floor. These slopes are highly variable in their physical attributes; but often start as gentle slopes, potentially with a plateau, before dropping steeply into the ocean abyss. Scientists often consider reef slopes to consist of numerous distinct sub-sections such as the upper reef slope, mid-slope and deep reef slope. These sections each possess different abiotic conditions with deeper regions becoming increasingly stable but with decreasing light availability.
F. Ocean floor The ocean floor is a habitat considered distinct and separate from coral reefs but this does not mean corals will be completely absent. Some Scleractinian corals are free living meaning they are not attached to substrate. These corals can persist outside of traditional coral reef habitats and may miss out on the conservation attention afforded to reef building corals.