Lab 3

CORAL BLEACHING LAB
INSTRUCTIONS
In this lab, you will be assessing coral bleaching using water temperature data from the NOAA National Data Buoy Center. Please write your answers in the boxes that are provided.
OBJECTIVES
Describe the relationship between corals and zooxanthellae.
Identify stresses to corals.
Explain coral bleaching and the processes that cause coral bleaching.
Examine water temperature data and compare to levels known to induce coral bleaching.
Predict the effects of prolonged, increased temperatures on coral reefs.
INTRODUCTION
4889545339000514353032125Figure SEQ Figure * ARABIC 1. An example of a coral reef.
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Figure SEQ Figure * ARABIC 1. An example of a coral reef.
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 (parts per trillion) 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 stresses, both natural and human-induced, including pollution, sedimentation, increased temperature, and physical damage by humans. These stresses can kill coral reefs outright, or make the corals more susceptible to disease.
03425825Figure SEQ Figure * ARABIC 2. The process of coral bleaching. Zoom in on your iPad to view the details of this infographic.
Figure SEQ Figure * ARABIC 2. The process of coral bleaching. Zoom in on your iPad to view the details of this infographic.
036131500Average 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 for hundreds of years suddenly died in 1998. It was the worst year ever recorded globally for coral bleaching up to that point, and it brought the hottest sea surface temperatures since 1982. View a NOAA animation of 1998’s 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) and coral bleaching events (marked with asterisks).
In 2005, sustained high sea surface temperatures caused even worse 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 effects.
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.
DATA ANALYSIS
Examine 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. Zoom in on your iPad to see the graphs in more detail.
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.

Fill in the following table.
Question 2005 2006 2007 2008
For each year, at what date did the water temperature first hit 1C above expected summertime maximum? April 23rd April 20th May 21st March 26th
For each year, at what date did the water temperature first hit 2C above expected summertime maximum? June 15th September 10th June 24th May 28th
What date did the water temperature drop and remain below the 1C above expected maximum for the remainder of the year? November 27th November 27th October 28th November 19th
What date did the water temperature drop and remain below the 2C above expected maximum for the remainder of the year? October 20th October 30th October 15th October 23rd
Approximately how many weeks over the year were temperatures 1C above expected maximum? 27 weeks 31 weeks 22 weeks 31 weeks
Approximately how many weeks over the year were temperatures 2C above expected maximum? 18 weeks 7 weeks 16 weeks 21 weeks
How many Degree Heating Weeks (DHW) were there for the year?
1 week @ 1C above expected maximum = 1 DHW
1 week @ 2C above expected maximum = 2 DHW 45 DHW 38 DHW 38 DHW 52 DHW
As you know, 2005 was a year of high water temperatures. How do the years 2006-2008 compare to 2005?
While 2005 recorded 45 degree heat weeks, meaning a prolonged period of high water temperatures, 2006 and 2007 recorded fewer degree heat weeks of 36DHWs. Concerningly, 2008 recorded the highest number of degree heat weeks, approximately 52, surpassing 2005.
What trend, if any, do you see in the number of DHWs over the 2005-2008 time period? Is this a long enough period of time to determine a trend? If not, how long a time period would you suggest?
The trend is not conclusive enough to enable one make inferences. However, a period of 6-10 years would be enough to determine a trend.
Based on the 2005-2008 data, what would you expect to see for water temperatures in San Juan, PR for 2017? Access the San Juan, PR NOAA buoy data to see the current water temperature. How does your prediction compare to the actual temperature?
My prediction is 29.5ºC summer temperatures. However, from the NOAA data, the 2018 water surface temperatures are 28.89ºC. My prediction is way over the actual ones however I reached at it due to the increased rates of global warming that have been experienced over the last decade.
Over the next few decades, if water temperatures continue to remain high for long periods of time at a stretch (resulting in significant DHWs per year), how do you think this will affect coral reefs? What impact would this have on the ecosystem? On the local economy?
If the temperatures continue to remain high, coral reefs ecosystems are bound to become extinct. This will result from the continued bleaching and death of the zooxanthellae algae that provides the corals with food. As a result, the ocean ecosystem will depreciate due the unprotected shorelines from waves and storms. Also, the local economy will suffer from reduced revenue from tourism as well as inadequate fishing grounds for fishermen.
Adapted from:
Rose, L. & Ayers Lawrence, L. (n.d.) Coral bleaching: A white-hot problem. The Bridge. Retrieved from http://www2.vims.edu/bridge/
Reproduced with the following credit:
Virginia Sea Grant, the Virginia Institute of Marine Science, and the National Marine Educators Association