Table l.--Prel imi nary calculations of light energy absorbed by a coral reef 

 community. Surface PAR light levels averaged 1.47 x 10 17 quanta/cm 2 /s during 

 the experiment. Measurements were carried out on Dancing Lady Reef, Discovery 

 Bay, Jamaica, W.I. 



Calculated quanta Percent of Incident 

 Depth Substrate Absorbed (q/cm 2 /s) Downwelling PAR 

 (400-700 nm) (400-700 nm) 



10 m sand 2.679 x 10 16 87.0 



coral 2.429 x 10 16 92.7 



30 m sand 7.42 x 10 15 68.0 



coral 1.08 x 10 16 95.7 



FIELD STUDIES 



In 1978 we developed a small submersible spectroradiometer to study the 

 photobiology of zooxanthel lae (Booth and Dustan, 1979). I used the instrument 

 to characterize the light field on Dancing Lady Reef, Discovery Bay, Jamaica, 

 with special emphasis on the photobiology of Montastrea annularis , one of the 

 principal reef-building corals on both the deep and shallow reefs of the region 

 (Dustan, 1982). As part of this study, I made systematic measurements of 

 upwelling and downwelling irradiance and reasoned that it might be possible to 

 measure the amount of light energy that the reef community "absorbs." In this 

 experiment measurements were taken over sand, which was reasoned to be an 

 almost perfect reflective surface, and over areas covered with living coral 

 (see table 1). 



The calculations suggest that corals resemble black bodies in their 

 absorption properties in both shallow and deep water environments. Reef sand 

 is not a very good reflective surface, as I had first thought, but does absorb 

 a smaller fraction of the incident light than corals. The difference in 

 absorption between coral and sand is greater in deep than shallow water, possibly 

 suggesting photoadapti ve or geometric change in the members of the coral 

 community. The percentage of incident light absorbed by both substrates seems 

 high, but since a reference of known standard reflectance was not used to 

 "calibrate" the methodology, the degree to which light scattering and other 

 possible nonlinear effects may distort the data is unknown. In addition, the 

 sandy environment of coral reefs contains a variety of photosynthetic organisms, 

 each with its own particular light absorption characteristics. 



A second set of spectroradiometer measurements was made in August 1982 on 

 the Molasses Reef in the Key Largo National Marine Sanctuary, Florida. Spectral 

 measurements were taken with a 15 channel spectroradiometer (Biospherical 

 Instruments Mer-1015) during a research expedition in support of remote sensing 

 of coral reefs (P. Dustan, C. R. Booth, and A. R. Hibbs, unpub. data). Data 

 from a sandy zone on Molasses Reef gave a net absorbance of 1 x lO 1 ^ quanta/cm^/s, 

 which was 51% of the downwelling irradiance at the substrate surface (fig. 2). 



192 



