Coral reefs have existed for hundreds of millions of years and provided a habitat for some of the richest diversity on the Earth’s surface. They are the marine version of tropical rainforests. Reefs harbor a slice of the marine food chain all the way from tiny autotrophic protistans (autotrophs fix carbon through photosynthesis) to large, predatory fish. Hundreds of millions of humans live near reefs and receive important resources from them. Reefs host productive fisheries; they also provide protection to low-lying coastal areas from storms and are vital for a number of key habitats, including mangrove forests.
Examples of Reef Corals
Examples of Deep Water Corals
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The organisms that have constructed reefs, largely corals, have evolved over time, and with that change so have the locations of reefs and the dynamics of the reef community changed. Over their long history, reefs have had several intervals of crisis; in particular, they almost ceased to exist at the Permian-Triassic boundary, where over 90% of marine species became extinct, and during the Cretaceous about 100 million years ago when giant clams took over these structures for several tens of millions of years. Both of these ancient times were potentially characterized by ocean acidification. However, reefs have been remarkably resilient over geologic time and generally have been able to adapt to environmental change. For example, as we will see in Module 10, they are able to grow fast enough to keep up with very rapid rates of sea level rise.
With this background, recent human activity has placed reefs in as precarious a position as at almost any time in their history. The last fifty years have witnessed an extremely dramatic decline in the health of many of the major reefs around the world, including reefs of the Caribbean, the Bahamas, and the Florida Keys as well as those in the Indian and Pacific Oceans, including the massive Great Barrier Reef of Australia. The outlook for these rich and complex ecosystems is about as bleak as any ecosystem on Earth. As it turns out, ocean acidification is one of several environmental threats to reefs, with warming, pollution, overfishing and physical destruction all exerting major threats to reefs in the future. As we will see, acidification is perhaps the greatest of all of these threats.