HHMI Coral Bleaching Activity

HHMI Coral Bleaching Activity

By Ashley Weems

​INTRODUCTION

​Coral reef ecosystems are in trouble. About 20% of the world’s coral reefs have already been lost, and that number may climb to 50% in the next 20 to 40 years. The main culprits are pollution, overfishing, and climate change. In this activity, you will use satellite data to determine threats to coral reef ecosystems from warming ocean water. 

What are corals?

​Although they may look like plants or rocks at first glance, corals are animals related to jellyfish and anemones. Individual corals are called polyps and, in many species, form colonies of identical clones. Polyps secrete a hard calcium-based skeleton that creates the physical structure of coral reefs. Reef-building corals have a limited ability to acquire food and nutrients on their own, so they rely on intracellular symbiotic algae (symbiont) that supply sugars and oxygen produced via photosynthesis. 

What is coral bleaching?​

​Elevated temperatures can damage the photosynthetic system of the symbiont, causing them to create reactive oxygen molecules that can damage the coral cells. Corals respond by ejecting the symbionts, without which the polyps are colorless and the coral reef appears white (Figure 2). This is called bleaching and is a serious threat to the health of the coral reefs. Corals can survive without symbionts for short periods of time and can reacquire symbionts when heat stress subsides. However, if the bleaching is prolonged, the coral will likely die.

​This is an example of coral bleaching.

​

When does bleaching occur?​

​Heat stress makes corals vulnerable to bleaching.

Generalizing about the amount of heat stress that corals can withstand is complicated because they are adapted to local environments and are somewhat able to acclimate to changing environments.

One method to determine whether a coral is at risk of bleaching is to record when temperatures rise 1°C or more above the normal maximum for a given location. For purposes of tracking coral health, normal temperatures are determined by averaging monthly temperatures for 1985 to 1993. The warmest normal temperature is the month with the highest average temperature, called the maximum monthly mean (MMM). The temperatures are measured by satellites using an infrared radiation sensor and represent sea surface temperature (SST). Only nighttime data are used to avoid overestimating heat due to solar heating of a thin layer at the sea surface.

More on bleaching & heat stress...​

Heat stress is assessed by a measure called "degree heating weeks (DHW)." It is a cumulative measurement of the intensity and duration of heat stress that a coral reef experiences over a period of 12 weeks, equivalent to a season. Empirical observations suggest that bleaching occurs when four DHW accumulate within a 12-week window, and coral death occurs when DHW values are greater than 8.

Because heat tolerance can vary within and among different coral species, these DHW thresholds are merely guidelines; some corals may survive in areas with high heat stress, while others may perish with relatively mild stress. Further, the temperature data is averaged over relatively large areas of 5 km, but actual temperatures experienced by corals may vary greatly due to local conditions. Finally, corals can recover after the stress disappears, and the 12-week window accounts for this.​

Multiple Choice

What type of algae is found inside corals?

• Zooxanthellae

• Wakame

• Giant Kelp

• Bladder wrack

Copy & Paste the url.​

https://www.youtube.com/watch?v=MUAsFZuFQvQ​

Multiple Choice

What is the number one cause of coral bleaching?

• People swimming in the ocean

• Ocean pollution

• Ocean Warming

• Increase in invasive species

Multiple Choice

True or False: Corals become bleached before stressed

• True

• False

Multiple Choice

How much money do coral reefs yield annually in global goods and services?

• $100

• Approximately $12-15,000 annually

• Over $15 Million

• Over $30 Billion

Assignment Instructions​:

Step 1:

Your instructor will assign you a specific coral location to study by giving you a “Location Card.”

1. Download the data file for your assigned location from the Coral Temperature Data page provided in the link below.

2. ​https://www.biointeractive.org/coral-temperature-data

Assignment Instructions​:

Step 2:​

• Examine the data and graphs provided for your assigned area. (These can be found in Google Classroom)

• Next, you will estimate the number of degree heating weeks (DHW) for specific years. To calculate the number of DHWs, count the boxes under every curve above the MMM that appears during that year. For instance, 1°C above the MMM for 1 week is 1 DHW, 2°C above the MMM for 1 week is 2 DHW, and so on. Figure 3 shows several examples.

• Analyze the data by coloring in on the provided world map where your location is and if rising ocean temperatures impacted the coral for each year.

  • Use Green for 0 DHWs.

  • Use Yellow for 1-3 DHWs.

  • Use Orange for 4-6 DHWs.

  • Use Red  for 7+ DHWs.

​

hem.

 

1. ​

Assignment Instructions​:

Example:

​

Below is an example of a temperature graph for Beqa, Fiji.

​

In this example, the cumulative DHW in 2002 is greater than 8, so it is likely that corals at this location experienced bleaching severe enough to kill.

This means you would find Fiji on the world map

provided, and for the year 2002, you would color

the ocean surrounding Fiji red.

​

​

​

​

hem.

 

Assignment Instructions​:

• Using the examples in the previous slides as a guide, calculate the DHW values for your location on your location card in the years 2002, 2010, and 2014.

• Count all the DHWs within a 12-week window (calendar dates will vary from site to site).

  ​

• Tip: If the season spans January 1, then assign it to the year in which most of the heat stress occurs. For example, the 2002 hot season may actually start in December of 2001.

• Use the table on your student answer sheet to assign the risk level for your location for 2002, 2010, and 2014. As shown in the table, when DHW is greater than 8, coral mortality is likely and you can stop counting DHWs.

​

​

​

​

hem.

 

• ​

Poll

After calculating your locations DHW values, and assessing your location's risk level for a 12-week season during the prescribed years, please select the answer choice that best describes your location's past experiences of a coral bleaching event.

• There is little to no evidence to show this area experienced coral bleaching during these time periods.

• There is some evidence to suggest this area experienced at least one coral bleaching event during these time periods.

• The evidence suggests one or more coral bleaching events occurred in this location during the time period researched.

• The evidence strongly suggests the location studied warmed dramatically and experienced coral bleaching each season.