Coral reefs and bleaching

A coral reef is formed by coral polyps forming their calcareous skeleton. These cnidarians usually live in colonies and, over time, their calcium deposits then form these huge structures on the seafloor.

A healthy reef in the northern Red Sea

The largest coral reef on earth is the Great Barrier Reef in Australia, off the coast of Queensland. It is characterized by a fascinating diversity of species and stretches over 2,500 km and comprises thousands of reefs and islands made up of 600 different hard and soft corals. This unique ecosystem can even be seen from space!

Satellite image showing Great Barrier reef off the coast of Australia; © Google earth Together with mangrove and rainforests, the overwhelming biodiversity of coral reefs makes them one of the most important and productive ecosystems on earth – that is also why they called 'biodiversity hotspots'.

But coral reefs are also very important for us humans, even though they cover less than one percent of the earth's surface!

Coral reefs serve as coastal protection, but also for recreation, and tourism. Also, the fishing industry benefits from intact coral reefs. It is estimated that worldwide, the livelihood of around half a billion people depends at least in part on the existence of coral reefs. In addition, it is assumed that around 30 million people, mostly residents of atolls, are completely dependent on such reefs [1].

Due to global warming, tropical corals are particularly endangered by rising ocean temperatures. This is because most of the tropical coral species live in a close symbiosis with single-celled algae. This vital symbiosis can collapse at higher water temperatures and the algae are expelled from the coral tissue, revealing the white limestone skeleton in which the cnidarians live. This process is known as coral bleaching [2].

A partially bleached Acropora coral

If this bleaching lasts only a short time, the corals have the potential to take up some algae again from the water column, and the symbiosis continues [2]. But if coral bleaching occurs over a longer period of time or more frequently, they ultimately die off [2,3], not only because they are lacking the energy that is normally provided by their algal symbionts but also because the bleaching makes them more susceptible to others threats (e.g. diseases) as well. When the corals in a reef die, also the overall biodiversity of this habitat also decreases. As a consequence, entire ecosystems or food chains can collapse; because when coral species disappear, so might also all other organisms, that depend on them, for example, some fish species that need corals as their food source or as a place to raise their offspring [4].

A bleached reef in the southern Red Sea

References:

[1] Stefan Rahmstorf, Katherine Richardson: Wie bedroht sind die Ozeane?, in: Klaus Wiegandt (Editor): Mut zur Nachhaltigkeit. 12 Wege in die Zukunft. Frankfurt am Main 2016, 113–146, S. 127.

[2] Glynn, P. W. (1993). Coral reef bleaching: ecological perspectives. Coral reefs, 12(1), 1-17

[3] Loya, Y. et al. (2001). Coral bleaching: the winners and the losers. Ecology letters, 4(2), 122-131

[4] Pratchett, M. S. et al. (2008). Effects of climate-induced coral bleaching on coral-reef fishes. Ecological and economic consequences. Oceanography and Marine Biology: Annual Review, 46, 251-296