The diffuse attenuation coefficient of down- 

 welling irradiance was computed from the 

 corrected data with the following formula: 



lop H 



k = 



AZi 



log^ H 



AX; 



■^2 



where H is the corrected output (^a.) of the 

 submerged cell at depths Z^ and Z^ 

 (in meters); lambda (\) refers to the spectral 

 sensitivity of the immersed detector, 475 m^ 

 (except for SCOPE Expedition station 9C). 



Errors in Determination of k Values 



The errors in determination of a given k 

 value arise from two sources: depth-nneasure - 

 ment errors and the irradiance -level measure- 

 ment errors at two depths. 



Some indication of the error in measure- 

 ments of irradiance level may be seen in the 

 data obtained in two series of experiments in 

 which the depth-measurennent error was be- 

 lieved to be negligible. The first series was 

 carried put in Lake Pend Oreille, Idaho, under 

 ideal conditions, namely, a stable platform, ab- 

 sence of high waves, and vertical homogeneity 

 (Tyler, Richardson, and Holmes, 1959). The 

 depth-measurement errors were extremely 

 low--probably of the order of + 8 to 10 cm. at 

 the miaximum depth (17 m.), and much less 

 nearer the surface. The second experiment 

 was carried out at sea in the California 

 Current some 20 miles west of Oceanside, 

 Calif, Wire angles were less than 5 during 

 these latter measurements, and the sun re- 

 mained unobscured by clouds. The data were 



examined statistically by means of an analysis 

 of variance by ranks (Tate andClelland, 1957). 



The Pend Oreille data on attenuation co- 

 efficients obtained between 3 and 17 m. (see 

 table 6), according to the analysis of variance 

 by ranks, reveal that the values obtained on 

 different days and at different depth intervals 

 were not significantly different (table 6). The 

 data were also analyzed by every other depth 

 interval (4-5, 7-9, 11-13, and 15-17 m.) so 

 that the same irradiance values would not enter 

 into the k calculations more than once: the 

 results were similar to that obtained above. 

 From these analyses, I conclude the Pend 

 Oreille data were drawn from the same popu- 

 lation and that an appropriate measure of the 

 error is the standard error. The mean k value 

 for the data is 0.1805; the standard devia- 

 tion(s) equals 0.0189; and the coefficient of 

 variation, 10.5 percent. 



The data on attenuation coefficient in the 

 California Current (table 7) were analyzed in 

 the same manner as the Pend Oreille data but 

 gave somewhat different results. The probabil- 

 ity for the H value indicates that the k values 

 were not drawn from the same population with 

 respect to depth. This finding was not sur- 

 prising, since the phytoplankton standing crop 

 appeared moderately high, judging from the 

 color of the water, and any nonuniform vertical 

 distribution of the phytoplankton would be re- 

 flected in variability of k. On the other hand, 

 the k values obtained from the different low- 

 erings at the same depth showed less vari- 

 ability than would be expected (table 7). The 

 cause of this stability is not understood, and 

 the data may be suspect. Nonetheless, the 

 coefficients of variation have been calculated 

 for each of the depth intervals to gain a rough 



Table 6. --Attenuation coefficients (k) per meter observed in Lake Pend Oreille, Idaho, 15 to 20 March, 1957 



[H = 5.5; p< 0.75 > 0.50] 



""■ k was determined between 3 and 5 m. 



12 



