By Carl Schmidt, Teresa Steely, John Hardy, Suzanne Strom, Maria Bynagle, Adrienne Miller, and Emily Peterson
Shannon Point Marine Center, Western Washington University, Bellingham, WA 98225-9181

Elevated temperature and ultraviolet-B (UVB) radiation can induce coral bleaching, i.e. the loss of symbiotic zooxanthellae and their pigments.  It may be possible to detect bleaching by remote sensing through measured changes in the reflectance spectra. We examined the relationship between coral pigments and in vivo optical spectra. We collected 2.1 cm diameter cores of Montastraea faveolata at 15± m depth (29.5 °C) from Lee Stocking Island, Bahamas in July 1998. Replicate samples from three colonies were exposed in an outdoor incubator for 96 hours at 31 °C at three levels of solar radiation.  Radiation treatments included in situ doses of photosynthetically active radiation (PAR) and UVB, in situ PAR and enhanced UVB, and enhanced PAR and UVB.  Coral pigments were measured by HPLC analysis and in vivo reflectance of coral was measured using a portable fiber optic spectrofluorometer.  After 96 hours of enhanced UVB treatment, a linear relationship of chl-a surface densities: reflectance emerged between treatments.  A dose rate of 43 kJ m^-2 day^-1 induced bleaching exponentially with depth, when the attenuation coefficient was applied. These UVB doses were 1 to 2 orders of magnitude greater than the in situ dose. Results suggest that small changes in pigmentation can be detected in vivo as changes in optical reflectance.

INTRODUCTION
Coral reef bleaching occurs in response to a variety of anthropogenic and natural stressors.  Ultraviolet-B radiation (UVBR; 290 to 320 nm) and high levels of photosynthetically active radiation (PAR; 400 to 750nm) induce coral reef bleaching (Brown et al. 1994, Gleason and Wellington 1993, Shick et al. 1996). Temperatures higher or lower than long-term means also result in bleaching events (Bunkley-Williams and Williams 1990, Goreau and Hays 1994, Glynn 1993, Strong et al. 1997).

During the bleaching process, both the zooxanthellae symbionts and their photosynthetic pigments (chlorophylls a and c,diadinoxanthin and peridinin) disassociate from the coral host (Glynn 1993, Kleppel et al.1989).  This loss of pigment exposes the white calcareous skeleton of the coral animal, decreasing the amount of  irradiance the animals absorb and increasing their reflectance.

Remote sensing might be useful in measuring large-scale bleaching events. To quantify coral bleaching remotely, data are needed on the relationship between pigmentation and passive spectral measurement, or reflectance spectra, of coral.  The water column attenuates reflected light; therefore a water column attenuation coefficient is necessary for determining reflectance remotely (Smith and Baker 1978).

Our objectives were to:

1. Determine changes in coral pigmentation in response to elevated temperature and elevated UVBR.
2. Characterize the reflectance spectra of an ecologically important coral species and relate it to stress-induced changes in pigmentation
3. Determine wavelength-specific water column attenuation-coefficients to apply to spectral data for remote sensing.
4. Estimate depths at which bleaching will occur with a given dose of irradiance over time.

MATERIALS AND METHODS

Experimental Set-Up

• 2.1 cm diameter samples of Montastrea faveolata were collected from three different colonies at 15 +/- 1m depth and 29.5°C near Lee Stocking Island, Bahamas (Figure 1) in July 1998.
• Four samples from each colony were exposed to three light treatments at 31 °C for 96 h in an outdoor temperature controlled incubator (Figure 2). Light treatments are as follows:

1 window screen (Enhanced treatment)
3 window screens (Reef + UVB)
3 window screens, and 1 layer of Mylar (Reef)

• Coral zooxanthellae were removed from coral samples every 24h by the water pik method (Muller-Parker et. al., 1994).
• Pigments were separated and quantified by High Performance Liquid Chromotography (HPLC) analysis (Mantoura and Llewellyn, 1983).

We measured wavelength-specific irradiance (300-800 nm) underwater at a depth of 16 m. Irradiance levels below instrument range (290-300 nm) were extrapolated using an exponential regression of the 300-305 nm irradiance trend.The wavelength-specific attenuation coefficient (k) was calculated using the equation:

Km^-1 =   -1  * (ln kd₁)
            d₂-d₁     kd₂

where d = depth and kd = irradiance at depth.

2.  Benthic Spectrofluorometer (BSF, Figure 3)

The BSF (Mazel, 1997) was calibrated by scanning a 60% reflectance gray standard.The optical probe was held 1.0 in from the surface of coral plugs in full solar light and wavelength-specific coral reflectance measurements were taken every 24 h for 96 h.

• Compared to surface waters (0.5 m), irradiance at 16 m was greatly reduced at all wavelengths and virtually absent < 350 nm and > 600 nm (Figure 4).

• Attenuation coefficients (k (m^-1)) were calculated from irradiance measurements at 0.5 m (surface) and 6 m.  Values were lowest in the 400-550 nm range (Figure 5).

• Daily dose was calculated by summing an hourly PAR and UVB surface irradiance measurement taken from dawn to dusk and multiplying by percent irradiance screened by each experimental treatment.  The experimental reef + UVB treatment best approximated the in situ PAR measurement at North Perry Reef and the experimental reef treatment resembled the in situ UVB measurement (Figure 6).

• The experimental enhanced treatment resulted in the most damaging UVB irradiance when weighted using the EXP-300 biological action spectrum (Behrenfeld et al. 1993).  Totals represent the sum of irradiance at all UVB wavelengths (Figure 7).

• Reflectance of corals maintained in the enhanced treatment for 96 h was greater than initial (time zero) reflectance of coral at all wavelengths (Figure 8).

• Plotting percent change from initial conditions for reflectance values and chlorophyll a surface densities resulted in a linear relationship between treatments after 96 hours (Figure 9).

• A dose rate of 43 kJ m^-2 day^-1 induced bleaching over a period of 4 days with a cumulative dose of 172 kJ m^-2.  When the attenuation coefficient is applied to this dose, our results indicate that at 31 °C the time for severe bleaching to occur increases exponentially with depth, assuming reciprocity (Figure 10).

Conclusions:

1. The three irradiance treatments produced a linear relationship, wherein reflectance increased as surface pigmentation decreased.
2. Attenuation data from this experiment will be useful in future monitoring of coral reef reflectance spectra at different depths within the water column via remote sensing.
3. A dose-response model suggests that during summer months the time required for coral (M. faveolata) bleaching will increase exponentially with depth.

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ACKNOWLEDGEMENTS
This reseach is part of the interdisciplinary project Science Education and Research for Undergraduates (SEARUN) funded by the National Science Foundation grant C-RUI DBI 97-11075.  We would also like to thank principle investigators Gisèle Muller-Parker, Suzanne Strom, and Jack Hardy, Erin Macri, Western Washington University, the staff at Shannon Point Marine Center and Caribbean Marine Research Station on Lee Stocking Island, Charles Mazel for the BSF, Kelley Bright, and Scientific Technical Services of WWU.