The Coral Reef Ecosystem

Coral reefs serve as natural indicators of the health of coastal zones, have been cited as possible indicators of climate change, and are valued contributors to economies the world over through their contributions to recreation and tourism. Indeed, their biological and economic richness has led many to observe that coral reefs are the "rainforests of the ocean."
- Comments by Undersecretary Tim Wirth, at the Convention on Biological Diversity, Nassau, the Bahamas, December 7, 1994

With extremely complex physical structures, high levels of primary productivity, and the highest species diversity of any biome in their respective environments, coral reefs are in many ways the marine analog of tropical forests.
- World Resources Institute

Suggested Readings:

• Miller, D. and C. Veron. Biochemistry of a special relationship. New Scientist, 2 June 1990.
• Sebens, K.P. 1994. Biodiversity of coral reefs: What are we losing and why? Am. Zoologist 34(1): 115-133.

In this lesson, we wish to ask:

• How are coral reefs formed, and to what extent do endosymbiotic relationships play a role?
• What environments can support coral reefs, and what is the worldwide distribution of them?
• Why are coral reefs such productive and species-rich environments?
• Why are coral reefs sensitive to climate change, and in what ways can we learn about past climates from studying reefs?

Jump to: [Reef Formation] [Types of Coral Reefs] [Reef Productivity] [Species Interactions] [Threats to Coral Reefs] [Summary]

1. Introduction - Some basic facts and definitions

A coral reef is the largest and most spectacular structure made by living things. The individual building blocks are tiny, and depend upon a partnership between a coral polyp and a photosynthetic "alga" (actually a dinoflagellate - a solitary, plantlike flagellate - the order Dinoflagellata includes luminescent forms, forms important in marine food chains, and forms causing red tide) of the kingdom Protista (a group of unicellular or acellular organisms comprising bacteria, protozoans, various algae and fungi, and sometimes viruses). The relationship is an example of endosymbiosis (symbiosis - the intimate living together of two dissimilar organisms in a mutually beneficial relationship; endo - within) (see Figure 1).

Coral reefs are home to spectacular biological diversity, but the corals themselves are not exceptionally diverse. Only one type of coral (hermatypic) builds reefs. About 500 such species are found in the Indo-Pacific, and fewer are in the Atlantic, so in total there are not very many species.

The areal extent of coral reefs is small compared to the entire ocean, but large compared to shallow water areas: Cover reefs cover 600 thousand km², which is about 0.17% of the ocean surface and 15% of shallow (0-30 m depth) sea areas (see Figure 2).

2. The Making of a Coral Reef

From the film "City of Coral," shown in lecture, you should take note of the following:

1. A coral is a colony of many individual polyps.
2. Each polyp is a coelenterate (a phylum of basically radially symmetrical invertebrate animals including the corals, sea anemones, jellyfishes, and hydroids) and contains dinoflagellates capable of photosynthesis (zooxanthellae). A polyp looks like a tiny sea anemone with tentacles and stinging cells to capture animal prey.
3. By night, a polyp captures plankton with its tentacles. By day, the zooxanthellae photosynthesize. The polyp benefits from the photosynthate (product of photosynthesis), and the alga benefits from the nitrogenous wastes of the polyp.
4. Without the zooxanthellae, the polyps cannot grow fast enough to build reefs.
5. Coral can be either soft or hard (calcareous skeleton). Hard corals build reefs, creating rock out of sunlight, sea water, and minute animal prey (8 tons mile^-2 day^-1) (see Figure 3).


Figure 3: Reef growth

6. More than 65 species have been found in one reef: there are both shallow, fast-growing forms and deep, massive, slow-growing forms (living at a maximum depth of about 100 feet). The Great Barrier Reef has 350 named coral species.
7. There is intense competition for space. Corals have a rigid pecking order. When more aggressive species recognize less aggressive forms, they send out nasty filaments that wound living coral encroaching on their space (see Figure 4).

3. Types of coral reefs.

Reefs are constructions of calcium carbonate, made literally from sunlight and sea water, by hard corals. Some algae, called coralline algae, also secrete calcium carbonate. This is especially important in "gluing" loose sediments and reinforcing coral against wave damage (see Figure 3 above). Some encrusting invertebrates, such as sponges, complete the reef structure.

Reef-building corals require water temperatures of at least 20 degrees C, and ample light. This means clear water and shallow depths. Coral reefs are found on continental shelves, around islands, or on top of seamounts.

• Fringing reefs are the simplest and most common kind of reef. They develop near the shore throughout the tropics. Some hard substrate is necessary for the polyp to establish, but after that the corals can make their own hard bottom. Note in Figure 5 the different aspects, including the inner reef flat and outer reef slope.


Figure 5: Fringing reefs

• Barrier reefs are similar to fringe reefs, but they lie farther from shore and contain a relatively deep lagoon. Some reefs are as far as 100 km from shore, and 2,000 km in length. Figure 6 shows the structure.




Figure 6: Barrier reefs

• Atolls are rings of reef, with steep outer slopes, that surround a central lagoon (see Figure 7). They are found far from land, and mainly in the west Pacific. How they are formed was a major puzzle. Charles Darwin, famous for the theory of evolution, solved this puzzle by postulating that atolls formed around subsiding islands. If the upward growth of the coral keeps pace with the downward subsidence of the original volcanic island, an atoll is formed.

4. Reef Productivity

Coral reefs are like tropical rain forests in at least two ways. First, they are the most species-rich ecosystems of the sea. Second, they are oases of abundant life in generally nutrient-poor water. The symbiotic relationship between polyp and zooxanthellae makes for especially efficient nutrient cycling, and this contributes to the high productivity. Fish and other consumers that graze on algae and the corals themselves also excrete nutrients back into the water. In addition, blue-green algae and other specialized bacteria are capable of nitrogen-fixation, meaning they can convert nitrogen gas into NH₃, which then is then available for plant growth.

5. Species Interactions

A great deal of ecological study is concerned with how species live together and interact with one another. Because species interactions are so apparent within coral reef communities, it is useful to explore some of these ideas here.

Symbiosis simply means "living together," and refers to any close and intimate association of two species. Subdivisions of symbiosis include: mutualism, commensalism, and parasitism. In mutualisms , both partners benefit (+/+). Coral reefs probably have more mutualistic relationships than any other system. In addition to the polyp-dinoflagellate symbiosis, another famous example is the clown fish-sea anemone. Mutualisms range from "casual" to "obligate." Neither of these examples is obligate, although the coral comes close.

Commensalism is +/0, meaning one individual benefits and the other is neither helped nor harmed. A shark and a remora are good examples. Parasitism is a +/-, where one partner wins and another loses. There are many examples of small crustaceans adapted to live on the gills of fish, and obtain their nourishment from the blood that is close to the gill surface.

The video showed some very impressive time lapse photos of competition between corals. In the rich and crowded world of coral reefs, space is at a premium. All sessile organisms compete for space; in addition, corals and seaweeds compete for access to sunlight (see Figure 8). You might expect that corals might compete with one another for light in the same, mundane way that most terrestrial plants do, using rapid growth to overshadow competitors and steal the sunlight. This they do, but some also engage in aggressive interactions using modified tentacles loaded with stinging cells, as you saw in the video. Large, slow-growing corals tend to be the most aggressive. Fast-growing, upright branching forms are less aggressive, while massive, slow-growing forms must compete aggressively to hold their position for decades to centuries.

Seaweeds and filamentous algae are capable of even faster growth, and have the potential to smother coral reefs. Hungry grazers, and some degree of nutrient limitation, generally prevent this from happening.

6. Threats to Coral Reefs

Despite their great size and complexity, coral reefs are vulnerable to a large number of threats. Climate change and changing sea level pose long-term risks. Storms, rogue starfish, and a number of human activities pose more immediate dangers.

Bleaching

Corals cannot tolerate water that is too warm. The upper limit varies because corals in warmer waters tolerate higher temperatures, but above some critical temperature (often near 30 °C, but sometimes as high as 35 °C), the coral expels its zooxanthellae, and also may lose its mucus coat. Because the zooxanthellae give the coral its brilliant color, the coral is white without its symbiont. Hence this is referred to as bleaching . At too high a temperature, the coral dies (white death).

Severe bleaching occurred in many regions in 1982-83, when the El Niño brought unusually warm waters to both coasts of Central America, the Florida Keys, the Bahamas, and to wide areas of the Pacific Ocean. Coral bleaching is a natural phenomenon.

Widespread bleaching of Caribbean corals occurred in 1987-88. Bleaching may be a general response to stress. The 1987-88 event may have been due to slightly elevated temperatures, but if so, coral reefs appear to be extremely sensitive to even slight temperature elevation. If greenhouse gasses warm the earth, many coral reefs might be threatened, and provide an early warning of global warming.

Global Climate Change and Coral Reefs

Past: During each advance and retreat of the ice ages, sea level dropped and rose again. For most reef flats, a drop of a few meters means exposure and death. Over and over during the Pleistocene, vast areas of reef were alternatively left high and dry, and then flooded. We have good evidence that past glacial epochs wreaked havoc with coral reefs. What will be the effects of a greenhouse climate?

Under a "no change in emissions" global warming scenario, land surfaces will warm faster than oceans, and this will be especially pronounced in high latitudes in winter. Surface temperatures of tropical seas may not rise a lot, but even a small rise may be significant. In addition, the global sea level rise is expected to average 6 cm/decade (recently observed: 1-2 cm, Holocene maximum: 20 cm/decade).

Future ecosystem roles and responses:

1. Temperature change: the effects are likely to be serious, causing bleaching, survival only of tolerant species, or death. On the other hand, a warming trend would extend the range of habitable seas. Local effects may be serious to local human populations that depend on the reefs.
2. Sea level rise: the vertical growth rate of reefs is 1-10 mm/yr. Given the present best estimate of sea level rise of 6 mm/yr, it should be within reef-building capabilities to keep pace (water clarity is important also). In fact, reefs today are constrained vertically by sea level, which has held steady within 1-2 m over the past several thousand years. In all likelihood, a sea level rise would be positive for coral reef growth. There also would be changes in current and wave action, which can damage reefs. The effects are hard to predict, but they could be locally important.
3. Carbon flux: The CaCO₃ of coral reefs is a major global carbon reservoir. Reefs often are thought of as carbon sinks, because CO₂ from the atmosphere is converted into calcium carbonate rock. Actually, coral reefs are a minor source of atmospheric carbon. Although reef-building stores CO₂ in coral rock, the formation (precipitation) of CaCO₃ lowers the pH of the water, and this alters the balance of CO₂, HCO₃^-, and CO₃^-2, known as the bicarbonate buffer system. More bicarbonate is converted to CO₂, which outgases to the atmosphere (estimated to be 1% of annual anthropogenic releases).
4. Anthropogenic damage is still the most serious problem facing reefs. Serious and immediate human impacts include mining for cement, dynamiting to kill fish, and anchoring of pleasure boats.

Overall, coral reefs are not as seriously threatened by climate change as by human impacts.

Corals as indicators of past climates:

Coral skeletons have annual and sometimes sub-annual growth bands like tree rings. Annual rings appear as alternating dark and light bands when corals are sectioned and x-rayed, due to a seasonal cycle of high verses low density of coral secretion. The causes of this cycle are uncertain but probably nutritional. Some coral colonies on the Great Barrier Reef are 600 to 1,000 years old. Supra-annual peaks in skeletal density correspond to El Niño years, which is useful for examining past climates on a 10 to 1,000 year time frame.

7. Summary

Allan: AOSS 171 Introduction to Global Change