Coral Reefs Assignment - Chapter 14, Part I

ATOLLS

BARRIER REEFS

FRINGE REEFS

1. How is each reef structure formed? “Coral” is a general term for several different groups of cnidarians, only some of which help build reefs. In reef-building, or hermatypic, corals the polyps produce calcium carbonate skeletons. Billions of these tiny skeletons form a massive reef. The most important reef builders are a group known as scleractinian corals, sometimes called the stony or “true” corals. Nearly all reef-building corals contain symbiotic zooxanthellae that help the corals make their calcium carbonate skeletons. It is the zooxanthellae as much as the corals themselves that construct the reef framework, and without zooxanthellae there would be no reefs.

2. Where is each reef structure found? There are three basic types of coral reefs; fringing, barrier and atoll. Fringing reefs are located very close to shore, and because of water run off they are typically high in nutrients and the water has a high turbidity. Barrier reefs are further from shore, with a lagoon between the reef and the shore. And finally atolls are a circular reef with a central lagoon and possibly small islands formed on the reef.

3. What is the trophic structure of a reef? The trophic structure of a reef is the recycling of nutrients. Coral reefs have among the highest rate of nitrogen fixation of any natural community. Coral reefs are very productive even though the surrounding ocean water lacks nutrients because nutrients are recycled extensively, nitrogen is fixed on the reef, and the zooplankton and nutrients that occur in the water are used efficiently. The reef is able to provide some of its own nutrients.

4. How does the location and type of reef influence the trophic structure? It is theorised that each of these types of reefs corresponds to a differing age of the entire reef structure. The youngest is the fringing reef, with the corals colonising a shallow water area close to the land. If the sea levels then rise or the land subsides, then the reef structure keeps up with this changing depth by growing upward. Eventually a shallow area with no coral growth will form behind the main reef, called a lagoon, giving a barrier reef. If the sea level or land subsides so much as to cause the land to disappear below the water surface, then an atoll is formed. The overall type of the reef whether it is a turbid, high nutrient reef where the stony corals are less common and algae abounds or crystal clear, low nutrient reef where the stony corals can dominate, is dependent of several factors. These include the proximity to land (therefore water run off which will be high in nutrients), proximity to river mouths (for the same reason as land proximity), and location of deep sea currents (which typically bring nutrient rich water. Each type of reef is also divided into various zones within each reef.

5. Give examples of the types of corals found on reefs. Corals are divided into two kinds and both are stationary on the ocean bottom. Hard corals such as brain, star, staghorn, elkhorn and pillar corals have rigid exoskeletons, or corallites, that protect their soft delicate bodies. Gorgonians, or soft corals, such as sea fans, sea whips, and sea rods, sway with the currents and lack an exoskeleton.

6. Give examples of competition, predation, and grazing. Competition – Sessile coral reef organisms must compete for space. Corals and seaweeds compete for light as well. The two main ways in which corals compete for space are by overgrowing their neighbors and by directly attacking them. NOTES FOR STUDENT ONLY: Competition – The interaction that results when a resource is in short supply and one organism uses the resource at the expense of another. Predation – Predation on corals occur when a variety of animals eat corals, but instead of killing the coral and eating it entirely, most coral predators eat individual polyps or bite off pieces here and there. The coral colony as a whole survives and can grow back the portion that was eaten. NOTES FOR STUDENT ONLY: Predation – The act of an animal, or predator, eating another organism, or prey. A top predator is one that feeds at the top of the food chain. Grazing – Grazing is the process of transplantation, removal and caging. An example of the effects of grazing on reefs are damselfishes. Many damselfishes graze on seaweeds inside territories that they vigorously defend, chasing away other fishes that happen to venture inside. Many such damselfishes has actually “farm” their territories. NOTE FOR STUDENT ONLY: Grazer – An organism that feeds primarily on plants.


CITE
Marine Biology Sixth Edition, Peter Castro/Michael E. Huber - Chapter 14http://ozreef.org/library/articles/zonation.html
http://www.reefrelief.org/coral_reef_body.shtml

http://library.thinkquest.org/4305/coral.jpg
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http://z.about.com/d/animals/1/7/Z/C/GreatBarrierReef-EO.JPG.jpg
http://library.thinkquest.org/05aug/00183/Great%20Barrier%20Reef_image005.jpg
http://www.caribbeanedu.com/images/kewl/atoll.jpg
http://www.adventuresinbelize.com/packages/southerncayes/atoll-ariel.jpg