Lesson Plan

Coral Reefs: Ecosystems Dissolving

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Grade Level:
Fifth Grade-Eighth Grade
Biology: Animals, Biology: Plants, Climate, Climate Change, Conservation, Ecology, Environment, Marine Biology, Wildlife Biology
45 minutes
Group Size:
Up to 24 (4-8 breakout groups)
National/State Standards:
Standard 6: Students assess the interrelated cycles and forces that shape Earth’s surface, including human interaction with Earth. (ASDOE Elementary Science Standards: Grade 5-8, pp. 42-73)


Corals face in our modern world, a great threat due to a projected change in water chemistry in the ocean due to global warming. Just as carbon dioxide (the main greenhouse gas causing global warming) is increasing in the air, it also increases in seawater in its dissolved form. That makes seawater more acidic which, in turn, may slow the rate at which corals build their calcium carbonate skeletons.


Students will be able to:

1. Define the vocabulary terms coral reefs and ocean acidification.

2. Identify the role of pH in ocean acidification.

3. Describe coral reef vulnerabilities to human actions that can threaten its survival.



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. Scientists predict that episodes of warm water temperatures will become more frequent due to a general warming of the earth. That's bad news for coral reefs. While it's unlikely that all of the corals will die off as the environment gets warmer over the next few decades, the number and/or abundance of corals may well decline in American Samoa. That might impact American Samoa in two general ways. First, coral growth might not keep up with rising sea levels or the reef itself may begin to erode, thereby allowing more storm waves to reach our shorelines and cause damage to roads and houses. Second, a reduction in coral growth and number of species could reduce the diversity of habitats required by fish, so a downturn in reef catches could eventually occur. Both of these changes would probably occur at a slow but steady pace over the next 30 years.

What to do? Well, it's true that American Samoa has little impact on the world's changing climate, but it makes sense not to worsen the situation by further stressing the coral reefs with rubbish, sewage from piggeries, or dirt (sediment) from land-use activities that flows into streams and out onto the reefs. The brown water that can be seen entering the ocean from streams after a heavy rainfall is harmful to the corals. Additionally, we should locate and protect any areas where corals appear to be naturally resilient to bleaching events. These hardy survivors could then help re-seed other areas where the corals had died.


1. Dead coral pieces
2. Clear plastic cups
3. Vinegar
4. pH strips
5. Seawater
6. Tap water
7. Board game
8. Die
9. Checker cab

10. Playing cards
11. Energy stars
12. Power point program

Handouts & Worksheets
1. pH key


Last updated: February 28, 2015