Coral Reefs: Ecosystems Dissolving
OverviewCorals 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
4. pH strips
6. Tap water
7. Board game
9. Checker cab
10. Playing cards
11. Energy stars
12. Power point program
Handouts & Worksheets
1. pH key
Introduce Inquiry Questions?
What are coral reefs? What is ocean acidification?
Ask: Have you ever heard the term coral reefs? 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 understand its meaning. Explain that corals are sea life whose habitat is the ocean. Various sea creatures make their homes in coral reefs including Zooxanthellae. Tell students the importance of Zooxanthellae to corals including their symbiotic relationship. Show power point slides of healthy corals found here in American Samoa.
Ask: Have you ever heard the term ocean acidification? If so, what about pH? Write the term on the board and explain so that students understand its meaning. Explain the process of how the accumulation of carbon dioxide is being absorbed by the ocean causing it to become acidic. The pH refers to the levels of acidity and alkalinity; a change in pH by only a few degrees can be devastating to ocean life. To illustrate, show students timeline of ocean acidification in the power point and explain further about pH and what it is.
Divide students into groups and then distribute clear plastic cups containing small quantities of vinegar, seawater, and tap water. Provide students with pH strips to measure the pH of each liquid and compare. Take one strip and dip it into liquid for approximately five seconds. Remove after five seconds and leave to dry. Cross reference the color that appears with the pH and the pH chart so as to identify whether the liquids are acidic or alkaline.
Next, give out small coral pieces to the group to place in the vinegar to visualize what it’s like for creatures with calcium carbonate shells when the ocean starts to get acidic. Note: For safety concerns, make sure that students do not inhale or consume the vinegar or seawater. Students can leave this to experiment to fizzle while they pay attention to the video.
Watch Video (21 mins, 35 secs)
Before the video, ask students to pay close attention to the negative impacts on the reef and the relationship between what we are doing on land. Have students watch the video “Ocean Acidification.” After the video, check student’s comprehension by asking the following questions and then refer back to the results of placing the coral pieces in vinegar:
1. What is ocean acidification doing to smaller organisms?
2. What is causing ocean acidification?
3. What kind of sea life did you see in the video?
4. Share what we are doing today to help reduce the effects of ocean acidification.
5. What happened to the dead coral pieces that were placed in the vinegar?
“Save the Environment” Board Game
Divide students into groups. Each group must choose a captain. The captain will be the group speaker. The captain will roll the die first. Whatever number appears on the die, move the checker cab that many boxes. If you land on a green box, your captain will look for that number on the playing cards and read it out loud to the group. The card will indicate whether you collect energy stars or move back. The group should talk about what the captain just read out. If it's something we can do to help save the environment, then collect the number of energy stars as indicated by the playing cards from the teacher. The teacher will be in charge of giving out energy stars.
After you land on that box, the next group member will roll the die. If you move back and land on the green box, you do not read the playing cards but the next member will roll the die. Whichever team gets the most energy stars, wins the game. Once groups have reached the “Finish” box, the game is over. The purpose of this game is for everyone to understand and learn what was discovered from the playing cards.
Have a class discussion about how we are negatively affecting coral reef ecosystems because of ocean acidification. Ask the following questions:
1. In the game, what were three ways that we can harm coral reefs? (i.e. Leaving the light on; pollution from driving cars; and not recycling.)
2. What can humans do to ensure the survival of coral reefs and the sea life living there? (Responses may include an attempt to reduce how much carbon we emit into the atmosphere; invest in green energy technology; public awareness campaign.)
Conclusion with Inquiry Question
What are coral reefs? What is ocean acidification?
Do your part to reduce your carbon footprint. Reduce, reuse, and recycle.