Coral Bleaching

A White Hot Problem

Laura Rose & Lisa Ayers Lawrence, Virginia Sea Grant, Virginia Institute of Marine Science
Grade Level
Lesson Time
1 hr.

Cnidarian, Zooxanthellae, Sedimentation, Coral bleaching, HotSpot, Degree heating week
Materials Required
San Juan water temperature graphs (pdf)
Student worksheet (pdf)
Answer sheet (pdf)

Assess coral bleaching using water temperature data from the NOAA National Data Buoy Center.

Coral Bleaching The magnificent beauty of a coral reef is a true masterpiece of Mother Nature. A reef is a sculpture of living organisms, varied in color, texture, shape, and size. The creation of these works of art takes many, many years (some reefs are thousands of years old), and they don't exist solely for show. Reefs are building blocks for rich communities, providing habitat for a myriad of organisms, and they are some of the most diverse ecosystems on the planet. In addition, they support fishing grounds, attract tourists, and protect shorelines from waves and storms.

Coral reefs exist in a geographical band 30 degrees north and south of the equator. Corals are cnidarians (related to jellyfish and anemones) and have symbiotic algae called zooxanthellae living in their tissues. These zooxanthellae are photosynthetic and produce food for the coral during the day. Corals live in waters with a salinity range of 30 to 40 ppt and can tolerate water temperatures between 16-35 °C, but the ideal temperature for growth is between 23-25 °C.

Sadly, coral reefs around the world are suffering the hardships of environmental stress. Corals are susceptible to a number of stressors, both natural and human-induced, including pollution, sedimentation, increased temperature, ocean acidification and physical damage by humans. These stresses can kill coral reefs outright, or make the corals more susceptible to disease.

Average global sea temperature has been rising gradually over several decades, 0.7 °C in the past 30 years alone, which is generally believed among the scientific community to be due to global warming. This thermal stress can cause coral bleaching, or the loss of zooxanthellae from the coral tissues. Since the zooxanthellae are what give the coral tissue their color, the loss of them make the coral appear white. With the loss of the photosynthetic algae, corals lose their primary food source and can die.

Corals which had thrived along Australia's Great Barrier Reef for hundreds of years suddenly died in 2016. It was the worst year ever recorded globally for coral bleaching up to that point, and it brought the hottest sea surface temperatures since 1998. View a NOAA animation of coral bleaching hot spots (areas where the sea surface temperature exceeds the climatological expected maximum for that region by 1° C or more, colored orange to red).

In 2005, sustained high sea surface temperatures caused coral bleaching in the Caribbean. According to some scientists, the 2005 Caribbean bleaching events were bigger than all the previous 20 years combined. The effects from this are still being felt now. Recently, there was a large die off of corals off the coast of Puerto Rico and the U.S. Virgin Islands. These corals had survived the high heat of 2005 but were weakened and succumbed to white plague disease. The following data activity examines NOAA water temperature data for San Juan, Puerto Rico for 2005 through 2008. We will compare the observed water temperature with the known stressful temperature level for coral reefs and discuss the affects.

Data Activity
Bridge - April 2006 Data Tip Corals can survive in water temperatures up to 35 °C, however the optimal growth temperature for corals is around 25 °C. Researchers have determined that, for any given area, water temperatures of 1 °C above the expected summertime maximum temperature is stressful to corals. If an area experiences this it is called a coral bleaching HotSpot. If the thermal stress lasts for a week or longer, the stress accumulates. To measure this, researchers use a degree heating week (DHW) value. If the temperature is above the expected average maximum for one week, the DHW is 1. If the temperature is 2 °C higher than the expected maximum for one week then the DHW value is 2. You can also get a DHW value of 2 if the temperature is 1 °C higher for 2 weeks.

For Puerto Rico, the expected summertime maximum temperature is 28.5 °C. This makes the DHW level 29.5 °C.

Print the following graphs of observed daily water temperature for San Juan, Puerto Rico for 2005 - 2008. These data were collected by the NOAA National Data Buoy Center. Each year has its own graph, and the last graph shows all years (2005 - 2008) compiled.

In the graphs, the vertical lines mark one-week time periods. The yellow line marks the 1 DHW level of 29.5 °C and the red line marks the 2 DHW level of 30.5 °C.

A study conducted by researchers from the University of Puerto Rico found that 54 days of 29.5 °C water temperatures or 10 days of 30.5 °C water temperatures correlated with severe coral bleaching.

Print out the student worksheet, and fill in the answers to the following questions.

Compare your answers to the Bridge Answer Sheet.

Discussion Questions

Coral reefs are considered to be the key to tropical ocean ecosystems, and marine scientists warn that their decline could be a prelude to widespread ecological damage. Want to help reverse the trend? Here are 25 things you can do to save coral reefs.