Predicting Coral Bleaching Event Areas of the Bahamas for Aug-Sept 2006 using Remote Sensing of Sea Surface Temperature

Frank Natale

Coral bleaching refers to the loss of color of corals due to stress induced expulsion of their endosymbiotic dinoflagellates (zooxanthellae).
Below is an histological cross section of a hard coral. Notice the zooxanthellae inside the endodermic layer of the coral tissue.

                                                                                  (histological slide of Montipora capitata provided by Rutgers Univeristy Coral Lab)

    Since the 1980’s there have been 7 major bleaching events; the most severe occurring in 1998 and 2005.
    While SSTs continue to rise, bleaching events are becoming more frequent, and more severe.

     Worldwide, coral reefs cover about 110,000 square miles, less than one-tenth of 1 percent of the world's oceans.
     But they support more than 1 million species of marine life, sustain tourism industries and provide food for islanders throughout the tropics.

http://news.mongabay.com/2005/1220-reefs.html

Some major areas where bleaching events have occurred over the last 15 years…

   http://www.marinebiology.org/coralbleaching.htm

 Why the Bahamas?
-Ecological and Economic Implications

-Also, the raw SST data from this area was the easiest to get my hands on thanks to Jen Bosch and the RUCOOL Lab.  

                                                                    Factors effecting retrieval:
                                                                                -Emissivity of the ocean
                                                                                -Emission by the atmosphere
                                                                                -Reflected radiation emitted by clouds and the atmosphere

                                                                    How retrievals are made using IR:
                                                                                -Find the clouds (using visible channel ~600nm)
                                                                                -Temperature calculated using atmospheric window channels like 11µm and 3.7µm.

                                                                    Problems using IR for SST:
                                                                                -clouds
                                                                                -water vapor (especially in the tropics)



Satellites and Instruments used:

                        Operational Date:      Inclination Angle       Altitude (km)   Period (min)

NOAA 12       09/17/1991                  98.7                             804                       101.1         

NOAA 15       12/15/1998                  98.5                             807                       101.1

NOAA 17       10/15/2002                  98.7                             810                       101.2

NOAA 18       08/30/2005                  98.74                           854                       102.12

Channel          Wavelength (microns)      Primary Use

1                      0.58-0.68                            Daytime cloud/surface mapping

2                      0.725-1.10                          Surface water delineation, ice and snow melt

3A                   1.58-1.64                            Snow and ice detection

3B                   3.55-3.93                            SST, nighttime cloud mapping

4                      10.30-11.30                        SST, day and night cloud mapping

5                      11.50-12.50                        SST, day and night cloud mapping

Algorithm Used to process raw data:

Multi-channel Sea Surface Temperature (MCSST)
    -A linear algorithm using channels 3, 4 and 5 of the AVHRR

To learn more about the MCSTT algorithm, please visit:

http://fermi.jhuapl.edu/avhrr/primer/primer_html.html

If only there was a MATLAB for Dummies book…
The raw SST data files were downloaded from RU COOL server, and analyzed with MATLAB.
To reduce the affect of diurnal heating and evaporative cooling, I tried to choose images from around the same time of the day (very early morning)
Also, images with little to no cloud coverage were selectively chosen to accurately show SST throughout the region
Through some manipulation of the code, I was able to narrow the Florida Current data down to specific Lat/Long region focusing on the Bahamas only.
Next, I was able to produce images in which only sea surface temperatures of 29° C + were plotted. (*note: white areas in images are not clouds, they are SST below 29° C )

(To understand the title of each image:
 ex; RUCOOL AVHRR 060802.214.0316.n17)
                     year/month/day.#dayofyear.time(GMT).NOAA#   

 Using Satellite SST data to predict where bleaching events will occur based on high SST anomalies…

For the purpose of this project, a high SST anomaly is defined as temperatures above the average maximum monthly temperature
(For August and September in the Bahamas ~27-28°C.)

“…not only the intensity, but also the duration of elevated water temperature on a reef can be an excellent predictor of the strength of a bleaching event.” (Gleeson and Strong, 1995)

Here is an example of one of my images…

Expected bleaching sites…
  
    I would expect to see some patchy localized bleaching areas in some of the shallow lagoonal areas on the north side of Great Bahama Island and the western edge of Great Abaco with little to no bleaching around the Andros Islands (maybe some along the NW end of the island). As you saw in the animation clips, there were several weeks of warm waters in early August but they were not as warm throughout September and would probably not result in  a major bleaching event. Clearly, 2006 did not see similar massive bleaching and mortality as seen in 2005, however SSTs are continuing to rise and temperature related bleaching is beginning to become an annual occurrence.


Another Approach… NOAA’s Coral Reef Watch (CRW)
NOAA’s CRW (remember my cool image of the day?) uses a similar technique on a global scale in which it determines Degree Heating Weeks (DHWs) and HotSpots and puts out warnings of where potential bleaching events may occur. However, since there sampling area is global, they are using a 50km resolution
                 (b/c using microwave detection)
                 (a bonus to using microwave is that you don’t have to worry about clouds)
   


     -DHW = one week of sea surface temperatures one degree Celsius higher then expected maximum (A DHW > 2 often certainly implies some bleaching and >4 implies mass bleaching         and mortality)
 


  So…
    If I wanted to assess coral reef health in a specific region (ie; Bahamas), I could use this localized IR SST technique (instead of the global microwave SST technique used by NOAA’s CRW) to get an idea of where I should expect (if any) bleaching events, and after writing it into a proposal and waiting for funding, I would be diving in no time!

References:

Gleeson, M.W., and Strong, A.E. 1995. Applying MCSST to Coral Reef Bleaching. Advances in Space Research, vol. 16, No.10. 151-154.

http://coralreefwatch-satops.noaa.gov/SBA.html

http://en.wikipedia.org/wiki/Bahamas

http://marine.rutgers.edu/cool/sat_data/?product=sst&nothumbs=0

http://news.mongabay.com/2005/1220-reefs.html

http://science.nasa.gov/Realtime/JTrack/NOAA.html

http://www.agu.org/eos_elec/95274e.html

http://www.marinebiology.org/coralbleaching.htm

http://www.osdpd.noaa.gov/PSB/EPS/SST/methodology.html#dhw

http://www.oso.noaa.gov/poesstatus/spacecraftStatusSummary.asp?spacecraft=12

http://www.oso.noaa.gov/poesstatus/spacecraftStatusSummary.asp?spacecraft=15

http://www.oso.noaa.gov/poesstatus/spacecraftStatusSummary.asp?spacecraft=17

http://www.oso.noaa.gov/poesstatus/spacecraftStatusSummary.asp?spacecraft=18

http://www.sat.dundee.ac.uk/avhrr.html

Additional Coral Reef related links:

Coral Reef Alliance
NOAA's Coral Reef Watch (join the mailing list for updates on bleaching events!)

(***Information provided on this website are for educational purposes only and was created as part of a Remote Sensing class project***)