Coral Bleaching: Turning Up The Heat
OverviewCoral reefs in American Samoa have turned pure white on several occasions in recent years. They look freshly bleached, quite pretty, but that's a clear sign that they are in trouble. Two very different kinds of stress cause corals to turn white: (1)clorox bleach, and (2)warm water temperatures. Clorox bleaching happens from time to time when a foolish fisherman dumps clorox onto the reef to kill fish. This is very short-sighted because it also kills everything else in the vicinity.
Students will be able to:
1. Define the vocabulary terms global warming and adaptations.
2. Identify several coral reef species in American Samoa and explain their role.
3. Describe coral reef adaptations and how global warming threatens the health of coral reefs.
American Samoa has unusual coral reefs in some ways. First, many of the reefs are dominated by a marine plant that does not even look like it is alive: coralline algae. This is a plant that usually grows as a smooth pink coating (it looks like pink paint) that slowly spreads a thin layer across reef surfaces. It can grow over pieces of dead coral rubble and eventually cement them together and stabilize a field of loose rubble that was rolling around with the waves.
Young corals (larvae) that attach to rolling coral rubble get knocked off or smothered, so it is hard for coral to get established on loose rubble. Some coralline algae also release a chemical that attracts coral larvae to settle on it. Coral larvae are tiny ovals, about half the size of a grain of rice that swim around and then settle down and grow into corals. So the pink coralline algae help corals to get re-established after hurricane damage to the reefs. American Samoa has an unusually large amount of coralline algae for unknown reasons.
A second unusual thing about our reefs is the abundance of encrusting corals. These are corals that are fairly flat and also cover the bottom like a thick layer of paint. Corals compete for space and light (so their zooxanthellae can use the sunlight to produce food), and it seems like encrusting corals have a strategy of trying to claim as much space as possible before anyone else does. Encrusting corals can be found on coral reefs anywhere, but they are more common in American Samoa than on most reefs.
Coral communities are made up of different species in different places. There are two main types of corals in the pools, "finger coral" and “staghorns”. Finger corals have branches that look like fingers with round tips, and staghorns look like deer antlers with many branches and sharp tips.
A second coral community lives on the “reef crest” where the waves crash. Here, corals tend to be sturdy, yet even on the reef crest one common species has many small branches about the thickness of a pencil. These corals get hit the hardest by waves, yet they have some of the thinnest and most delicate branches which seem puzzling. Perhaps the branches’ being close together reduces the waves’ force.
A third set of coral communities live on the reef slopes, where the reef drops away into deeper water. Here encrusting corals and coralline algae are common, but there are some places where other communities of corals can be found. In some places, reef slopes at medium depths are dominated by a “flower coral” (Lobophyllia hemprichii) where the coral polyps are relatively large, up to 2-3 inches in diameter. Deeper on some slopes, the most common corals (Mycedium sp.) form overlapping plates almost like shingles on the slope.
Some corals grow in a shape rather like a table — they have a single stalk, often in the center of the coral, and their top is a big flat disk or table-top. Some may even have multiple layers of table-tops. These “table corals” are actually related to the staghorns (in the genus Acropora; a common species of table coral is Acropora hyacinthus).
A few corals live to become giants. Some of these are called boulder corals, often yellow or brown in color. These slow-growing corals can be found in all sizes on our reefs. One of the world’s largest is at Ta’u Island and is over 15 feet tall—it may be hundreds of years old. Boulder corals are in the genus Porites, along with the finger corals.
Most corals are firmly attached to the reef, but a few are not. One common type that isn’t attached is the “mushroom coral”. These corals, in the genus Fungia, look like a mushroom cap that has been turned over, with radiating ridges that look like the “gills” on a mushroom. Their larvae settle anywhere on the slope, but wave action can move the adults around when it is rough. On a slope, they tend to slide downwards when the waves move them, and end up at the bottom of the slope where they accumulate. Some tiny young mushroom corals have even been observed to “walk” on their tentacles.
Coral reefs look like they are the same day after day, but over a longer time span of decades to centuries or longer, they can change considerably. Because most corals live only in warm tropical waters, it seems odd that corals will die when the water gets slightly warmer. The reason is that corals live close to the hottest temperature that they can tolerate, so it doesn't take much to push them over the limit. To explain what is happening, recall that corals are animals with colorful plantlike cells (Zooxanthellae) living in their tissues. These cells use the sun's light to produce food which is also used by the coral animal. Many coral animals receive much of their food this way, so this relationship is quite important to the coral animal. The coral, in turn, provides the zooxanthellae with nutrients and a secure place to live. Both the coral and the zooxanthellae benefit from this arrangement.
When the coral is stressed by warmer than usual temperatures, the zooxanthellae are released from the coral, for reasons known only to them. What's left is a rather colorless coral animal overlying a bright white coral skeleton. The animal portion of the coral may eventually recover its zooxanthellae and continue living, or it may die, depending on how stressed it gets. It's easy to tell when portions of the coral die because they become covered with fuzzy green algae.
1. Dead coral pieces
2. Blank poster boards
3. Markers and/or crayons
4 Recycled magazines
6. White paper plates
7. Synthetic feathers
8. Volleyball net
9. Clothes pins
10. Power point program
Handouts & Worksheets
1. Coral Reef Illustration Key
Introduce Inquiry Questions?
What are coral reefs? What are threats to coral reefs? What is coral bleaching?
Ask: Have you ever heard the term global warming? If yes, explain where or how they heard the term? If not, what do you think it means? Write the term on the board and explain how to break the word into parts to understand its meaning. Explain that global means "earth" and warming means "heating up." Therefore, global warming means "heating up of the earth."
Ask: Have you ever heard the term adaptations? If yes, can you give an example? If not, what do you think it means? Write the term on the board and explain how to break the word into parts to understand its meaning, as you did with global warming. Explain that adaptations means "conforming to changing factors" such as lack of habitats, food, and temperature. To illustrate the concept of adaptations, show students the images of coral reefs in Ofu that are said to be climate change resistant.
Ask: What are coral reefs? Tell students the number of coral species in American Samoa and the world. Talk about habitat and Zooxanthellae. Show them slides of healthy corals. Introduce threats to corals that include global warming due to pollution. Introduce coral bleaching as a byproduct of global warming. Tell students they will watch a brief video about coral bleaching.
Before the video, ask students to pay close attention as to why corals bleach and understand what Zooxanthellae are. Have students watch the “Coral Bleaching” video. After the video, check student’s comprehension by asking the following questions:
1. What is coral bleaching?
2. What is causing coral bleaching?
3. What happened to the Zooxanthellae when the ocean temperature rose?
Make your own Coral
Have students arrange their desks and provide materials as indicated so that they may create their own corals. First, distribute a set of white paper plates to each student. Explain that the white paper plates represent their corals and they have to use materials like crayons, markers, feathers, or any other recycled materials to decorate. Tell students to be creative in that all corals come in different colors, shapes, and sizes. Hang the net and pin one white paper plate to it. Explain to students that the white paper plate symbolizes a bleached coral; writing a frowning face on this paper plate to represent how sad the coral is that it has lost its Zooxanthellae along with its color can be added.
Have students discuss their coral creations. Have student take turns sharing about their coral creations and describe the role it plays in the ocean ecosystem. To check for understanding, have students discuss reasons the corals lose their color-Zooxanthellae. After each student has shared, pin their coral creation to the net surrounding the one bleached coral in the center. Discuss with students what they see and how their coral creations differ from the bleached coral. Then revisit some of the same questions students discussed after the video. Ask: Why do coral reef ecosystems need to adapt to global warming?
Have a class discussion about how we may negatively affect coral reef ecosystems in terms of global warming and adaptations. Ask the following questions:
1. In the video, how do we harm coral reefs? (Global warming, pollution, and overfishing)
2. What can we do to help ensure the survival of coral reefs and the sea life living there?
Conclusion with Inquiry Question
What are coral reefs? What is coral bleaching?
Do your part to reuse, reduce, and recycle.