surface in temperate and tropical areas, the 

 coefficient of variation does not exceed 27 

 percent and most frequently is much less. 



The coefficient of variation for samples 

 collected at depth is greater, except for40-m. 

 samples in the D series, than that among the 

 surface samples. As a similar trend is ap- 

 parent in the data of Humphrey ( 1960), it seenns 

 likely that sampling at depthpresents problems 

 somewhat different from surface sampling. 



The cause of this apparently greater vari- 

 ability in chlorophyll a with depth is not 

 understood completely. Evidence that this in- 

 crease arises from the failure of the sampler 

 to sample in exactly the same stratunn on 

 repeated lowerings is indirect but merits 

 consideration. 



The hypothesis that alpha (a), the beam 

 transmission coefficient, is related to chloro- 

 phyll a in offshore tropical waters, was tested 

 on cruise TO-61-1. In all, some 49 measure- 

 ments were made at 10 stations at local 

 apparent noon with the beam transmission 

 meter detector covered with a Wratten 45 

 filter {see table 4). A similar number of 

 observations were made at midnight, with a 

 Wratten No. 61 filter (peak transmission at 

 520 m/i). A nonparametric statistical analysis 

 (Spearman rank correlation coefficient-- 

 Siegel, 1956) showed that chlorophyll a and a 

 were positively correlated at better than the 

 0.01 level in the upper 20 m. in both sets of 

 data. Below 20 m. and down to 46 m., the 

 maximum depth reached by the a meter, the 

 observations were not correlated. The 

 seemingly logical assumption, that a causal 

 relation exists between these two variables, 

 suggests that a profile of a at closely spaced 

 intervals of depth should give a good indication 

 of the vertical distribution of chlorophyll a, at 

 least in the upper 20 m. 



Two such detailed a profiles were obtained 

 at stations 24 and 31 (see fig. 4). At both of 

 these stations a and chlorophyll a were posi- 

 tively correlated (significance levels: <0,01 

 and <0.05, respectively). The similarity in 

 trend between chlorophyll a and a is apparent 

 in both of these sets of data. The major feature 

 to consider in these figures is the manner in 

 which a can vary between the chlorophyll 

 sampling depths below 10 to 12 m. These 

 variations indicate the existence of con- 

 siderable gradients in light absorbing and 

 scattering materials which are probably 

 chlorophyll a-bearing. It is quite evident that 

 failure to repeat a sampling depth within 1 or 

 2 nn. may lead to differences in concentration 

 of 50 percent or more between "duplicate" 

 samples. Such sampling faults could easily 

 contribute to an increase in the standard 

 errors of replicate samples taken at depth. 

 The standard error could be further increased 

 by internal waves, patchiness, and diurnal 

 rhythms during any extended sampling series. 



Interpretation of Results 



Although an estimate of the total error in 

 field nneasurements of chlorophyll a is given 

 in the previous section, questions remainasto 

 the exact nature of the entity called chlorophyll 

 a derived from the measurement, and the 

 success of the extraction itself. 



Photosynthetically inactive chlorophyll a 

 derivitives in sea water could, if present in 

 appreciable quantities, affect the interpreta- 

 tion of the chlorophyll a determination. Chloro- 

 phyllide a will always interfere because it has 

 the same absorption spectrum and specific 

 extinction coefficients as chlorophyll a (Patter- 

 son and Parsons, 1963). Fortunately it does 

 not appear to be common (see below). The two 

 most frequently mentioned compounds of this 

 nature are phaeophytin a and phaeophorbide a. 

 The possible interference of these two pig- 

 ments with the chlorophyll a measurement can, 

 however, be postulated on the basis of their 

 similar absorption spectra, even though 

 relevant data are for different solvents 

 or different solvent concentrations. 

 Vernon (1960) examined chlorophyll a 

 and phaeophytin a absorption spectra in 80- 

 percent acetone and the red chlorphyll a peak 

 in 90-percent acetone. In 90-percent acetone 

 the peak is 664 m/i. In 80-percent acetone it is 



665 mfi, whereas the peak for phaeophytin a is 



666 to 667 m/j. If we assume a similar shift in 

 90-percent acetone, then the two peaks would 

 be separated by only 1 or 2 m/j. Such a dif- 

 ference cannot be resolved with a DU Spectro- 

 photometer used under field conditions. A 

 similar but greater shift (about 5 m;;) toward 

 the red has also been reported by Zscheile and 

 Comar (in Smith and Benitez, 1955, p. 148) in 

 ethyl ether. Thus we must conclude that 

 phaeophytin a can interfere with the deter- 

 mination of chlorophyll a if present in sea 

 water extracts. 



Patterson and Parsons (1963) investigated 

 the occurrence of chlorophyllide a, phaeophytin 

 a, and phaeophorbide a in Departure Bay, 

 British Columbia, water samples, in four 

 cultures of different marine species, in littoral 

 mud, and in a net tow in which zooplankton 

 predominated. These authors concluded that 

 most of the water samples and cultures tested, 

 except for Skeletonema costatum , contained 

 insufficient amounts of the phaeo-pigments to 

 cause errors in the Richards with Thompson 

 (1952) chlorophyll a method. 



Yentsch and Menzel (1963) used a differ- 

 ential fluorescence method to estimate the 

 phaeo-pigments by measuring the change in 

 chlorophyll a fluorescence before and after 

 acidification of the acetone extract. Their 

 results (table 4) on natural populations are 

 limited but show that phaeo-pigments are 

 present in the upper 75 m. in the tropical 

 Sargasso Sea in appreciable amounts. Below a 



20 



