Low-intensity long-exposure excitation of algae, which is used in fluores- 

 cence spectrophotometer studies, must be correlated with high-intensity short- 

 pulse laser excitation of algae before spectrophotometer measurements of algal 

 fluorescence cross sections can be used in laser fluorosensor system investiga- 

 tions. Also, additional research needs to be conducted on the variation of 

 fluorescence yield which occurs in algae because of varying ambient light 

 levels and nutrient concentrations. The uncertainties associated with these 

 factors directly affect the accuracy of the remote chlorophyll a in vivo con- 

 centration measurements. 



The use of optimized excitation wavelengths in a four-wavelength laser 

 fluorosensor system can reduce uncertainties in the chlorophyll a concentration 

 measurement by more than a factor of 3 from the present ALOPE (Airborne Lidar 

 Oceanographic Probing Experiment) system. Assuming optimistic estimates of 

 power measurement accuracies and of uncertainties in attenuation coefficients 

 and algal fluorescence cross sections, a laser fluorosensor system with opti- 

 mized excitation wavelengths would produce chlorophyll a concentration uncer- 

 tainties in all four color groups of less than 53 percent. 



The use of a "ground-truth" point to calibrate the laser fluorosensor sys- 

 tem can only be used in large bodies of water where the water conditions change 

 slowly over long distances. However, under those circumstances the variation 

 in algal types would not be expected to change significantly, and a single- 

 wavelength system could be used for the chlorophyll a measurement. 



In water where marine conditions vary widely over short distances, supple- 

 mental remote measurements of marine attenuation coefficients and environmental 

 factors which affect the fluorescence cross section of algae (e.g., nutrient 

 concentration and ambient light intensity) are necessary at each sample point 

 of the laser fluorosensor system. Without this additional measurement capa- 

 bility, both the single- and multiple-wavelength laser fluorosensor systems can 

 only provide qualitative information about the chlorophyll a contained in the 

 algae, and the single-wavelength system yields results which are comparable to 

 a multiple-wavelength system with optimum excitation wavelengths. 



Langley Research Center 



National Aeronautics and Space Administration 



Hampton, VA 23665 



April 7, 1977 



24 



