Wilson and Kiefer, 1979). Present models that describe the light-limited growth 

 rate of cultured phytoplankton have been formulated using the estimated quantum 

 efficiency of photosynthesis and the cellular cross sectional area for light 

 absorption (Laws and Bannister, 1980; Kiefer and Mitchell, 1983). The formulation 

 of similar models for coral reefs might provide estimates of primary production 

 that would be applicable to different reef systems, habitats, or zones on a 

 scale that is commensurate with the scale of these ecosystems. Formulation of 

 such models requires the ability to make reliable estimates of light reflectance and 

 absorption for the reef community. 



Morel (1978) defined a measure of light absorption termed Photosynthetical ly 

 Useable Radiation (PUR). PUR defines the fraction of Photosynthetical ly 

 Available Radiation (PAR) that a photosynthetic organism can absorb and have 

 available for chemical work. PUR is the integrated product of normalized cellular 

 spectral absorption and the spectral irradiance available to the organism 

 (Morel, 1978). Measures of PUR are relative measures, though, and cannot be 

 related to an absolute measure of the quantity of light being absorbed. However, 

 much can be learned by comparing the relative light absorption of photosynthetic 

 organisms from different light environments. 



Calculations of PUR for isolated zooxanthel lae from the reef-building coral 

 Montastrea annularis (Ellis and Solander) have shown that the fraction of light 

 absorbed vanes at different depths, suggesting that the capture of light 

 energy by these zooxanthel lae varies as a function of the available light field 

 (fig. 1; and Dustan, 1982). 



METHODS 



Optical oceanographers have developed precise terminology that describes 

 the behavior and measurement of light in the underwater environment. Spectral 

 irradiance is defined as the spectral components of the light field that impinge 

 on a flat-plate, or cosine collector. Across each horizontal plane in the 

 hydrosol are upward and downward flowing flux defined as upwelling and downwelling 

 (Tyler and Preisendorfer, 1962; Gordon, et al . , 1980). The ratio of upwelling 

 to downwelling is termed spectral reflectance and is the basis for the measurement 

 of remotely sensed optical signals (Gordon, et a_L_, 1982; Smith and Baker, 

 1978b_). The difference between upwelling and downwelling at the surface of a 

 substrate is the amount of light that is absorbed by the substrate. It is 

 presently impossible to directly measure the upwelling irradiance close to the 

 substrate because the shadow of the instrument alters the light field. However, 

 the upwelling and downwelling irradiance at the reef substrate can be estimated 

 by taking sets of upwelling and downwelling measurements above the substrate. 

 From these data, the diffuse attenuation coefficient for spectral irradiance is 

 calculated, and this quantity is used to calculate the light intensities at the 

 surface of the substrate (Smith and Baker, 1978b). These derived data will 

 vary with water depth, species absorption properties, and the ambient light 

 field. Thus, measurements must be taken throughout the daytime period and at 

 systematically varying depths to use these measurements as an index of light 

 energy absorption and a community-specific spectral absorption signature. At 

 this time, we do not fully understand the relationship between this derived measure 

 and the actual amount of light absorbed by photosynthetic reef organisms, but, 

 as a first approximation, the measure represents a maximum value which includes 



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