The first term on the right side of equation (19) is the same as that in equa- 

 tion (14). This would be expected because the only difference in the two 

 approaches was in the statistics dealing with the fluorescence cross sections 

 and not the Xi parameters. The variance of the normalized chlorophyll a con- 

 centration contained in an algal color group is dependent upon the variance in 

 the normalized fluorescence cross section at any excitation wavelength of the 

 same algal color group. Results of calculations of 5nj/nj from equation (19) 

 for cases I and II are given in table 5. It is assumed that the normalized 

 variance for fluorescence cross sections is the same for all algal color 

 groups. 



Interpretation of the data presented in tables 4 and 5 can only be made 

 after consideration is given to the magnitude and source of the uncertainties 

 which can lead to errors in the determination of the chlorophyll a concentra- 

 tion contained in each algal color group. 



Parameters Which Can Alter Fluorescence of Chlorophyll a In Vivo 



Some of the factors which can alter the fluorescence cross section of 

 algae include long- and short-term light history, nutrient and age effects, 

 and water temperature (refs. 25 to 30). With the exception of the work of 

 Campillo, Kollman, and Shapiro, who used a laser excitation source at 530 nm, 

 all the data presented in the following sections are from experiments where a 

 broadband excitation source in the blue spectral region was used to excite 

 broadband fluorescence at 685 nm. The reported data are in terms of the ratio 

 of algal fluorescence to chlorophyll a in vivo concentration. This ratio is 

 proportional to the fluorescence cross section discussed previously. 



Long-term light history .- Changes in fluorescence levels of chlorophyll a 

 in vivo result from irradiation of the algae by different amounts of solar 

 radiation. Munday and Govindjee (ref. 25) explain this phenomenon as a 

 "membrane" reaction where different reaction centers in the cell "move" with 

 respect to each other and thus alter the fluorescence properties of the cell. 

 Kiefer's experiments in the Gulf of California and North Central Pacific 

 (ref. 26) established that algal fluorescence is inversely related to solar 

 irradiation and that this effect was limited to algae near the surface of the 

 water. Laboratory tests were performed by Blasco (ref. 27) with the marine 

 diatom Skeletonema costatum to determine the day/night effect on algal fluo- 

 rescence. Figure 13 shows chlorophyll a concentration versus fluorescence for 

 day and night conditions. It can be seen that the ratio of fluorescence to 

 chlorophyll a concentration is 80 percent larger at night than during the day. 

 Blasco concluded that prediction of chlorophyll a concentrations from fluores- 

 cence measurements is greatly influenced by the background irradiation 

 intensity. 



Short-term light history .- The transient change in fluorescence yield of 

 chlorophyll a in vivo during the initial few seconds of illumination has been 

 studied by Munday and Govindjee (refs. 25 and 28). They conducted experiments 

 into the nature of the short-term, less than 2 sec, fluorescence transient 

 associated with the algae Chlorella . A representative fluorescence curve is 

 given in figure 14. The algae were exposed to a 2-sec pulse of low-intensity 



19 



