A rough comparison may be made between 

 these results and those obtained by Jerlov 

 (1951) for Type I, II, and III by assuming that 

 45 percent of the total incident energy lies in 

 the visible region (Strickland, 1958; Withrow 

 and Withrow, 1956), Agreement for Type II 

 and III is good; it is poorer for Type I. 



CHLOROPHYLL A 



In 1957 when the present investigation was 

 initiated, the pigment analysis method of 

 Richards with Thompson (1952) was chosen. 

 This method is a selective and useful ship- 

 board technique. Although the determination of 

 chlorophyll a, b, and c and astacin and non- 

 astacin carotenoids is possible with the method, 

 only chlorophyll a concentrations have been 

 calculated and ennployed in this study. 



Chlorophyll a was selected for several rea- 

 sons: 



(1) It is well known that radiant energy 

 absorbed by chlorophyll c, certain phycobilins, 

 and xanthophylls is transferred to chlorophyll 

 a. Thus it seems likely that radiant energy 

 absorbed flows to the chemical stage of photo- 

 synthesis via chlorophyll a,. In the pigment 

 complex of the living cell, chlorophyll a_ con- 

 centration might well limit the rate of photo- 

 synthesis and also serve as a rough index of 

 plant biomass, 



(2) Of all the pigments capable of being de- 

 termined by the pigment-analysis method of 

 Richards with Thompson, chlorophyll a is 

 subject to the least error (Richards with 

 Thompson, 1952), 



(3) The likelihood of interference by phy- 

 cobilin-type pigments in the open sea is not 

 great, owing to the scarcity of plants bearing 

 these pigments. 



Although the validity of the Richards with 

 Thompson (1952) equations have been ques- 

 tioned by Humphrey (1962) and Parsons and 

 Strickland (1963), no set of revised equations 

 has appeared in the literature applicable to 

 dried chlorophyll preparations. Parsons and 

 Strickland pointed out that dried chlorophyll 

 preparations yield equation coefficients which 

 are lower than those obtained with wet prepara. 

 tions and suggested that the Richards with 

 Thompson equations may yield values with 

 dried extracts which are 10 percent too low. 

 In view of the lack of a recent rigorous 

 reappraisal of the equations for dried ex- 

 tracts, however, it seems unnecessary at this 

 time to adopt the 10-percent correction fac- 

 tor suggested by Parsons and Strickland 

 (1963). 



These equations yield a quantity called 

 "chlorophyll a,"; however, inactive chlorophyll 

 a^ derivatives (phaeophytin a and others) will 

 be determined as chlorophyll a with this 

 technique. A discussion of this problem ap- 

 pears later in the text. 



Sample CoHection 



Surface water samples used for the deter- 

 mination of chlorophyll a_ were collected either 

 with a plastic bucket or with a plastic 3-liter 

 Van Dorn (1956) sampler; subsurface samples 

 were obtained with 3 -liter Van Dorn bottles. 



In offshore tropical and subtropical waters, 

 two sampler casts were made routinely at 

 each station. Four to 5 liters of water are 

 required to obtain acetone -chlorophyll ex- 

 tracts with sufficient optical density for anal- 

 ysis, namely, a corrected extinction at 665 m^ 

 of 0.10 or more, with a 5.5- to 6.0-ml. extract 

 volume in a 10-cm. semimicro absorptioncell 

 (capacity 5.5 ml.). Thus two separate sannpler 

 casts were routinely nnade at each station; 

 0.5 to 0.7 liter of water was used in studies of 

 photosynthesis and about 0.25 liter was pre- 

 served and fixed for subsequent analysis of 

 species composition and abundance of phyto- 

 plankton. The remaining volume of water 

 (about 2.0 liters) was placed in an 8-liter 

 polyethylene bottle and combined with the 

 3 liters obtained at the same depth (based on 

 wire length and without correction for wire 

 angle) from the second cast. 



On two cruises (SCOPE Expedition and 

 TO-58-1) the depth placement of the various 

 samplers was often determined by taking into 

 consideration either diffuse downwelling ir- 

 radiance or thermal structure or both. During 

 the other three cruises (TO-58-2, TO-59- 1, and 

 TO-59-2) the sampler placement was^ stand- 

 ardized to include the following depths: sur- 

 face, 10. 25, 50, 75, 100, and 125 or 150 m., 

 and occasionally 200 m. 



Except at a very few stations, the results 

 used here were determined from samples col- 

 lected during the hour preceding local ap- 

 parent noon. 



Sample Treatment 



Before filtration, the volume of each sample 

 was estimated to the nearest 0.1 liter. About 

 20 to 25 mg. of powdered magnesium car- 

 bonate were added to each sample before fil- 

 tration during SCOPE and SCOT Expeditions; 

 on subsequent cruises magnesium carbonate 

 was not used. 



Four, and occasionally eight, samples could 

 be filtered simultaneously with the equipment 

 available; each sample required about 2 hours 

 to filter. Thus the maximum elapsed time 

 between sample collection and complete fil- 

 tration of the last sample (nornnally the 

 deepest) rarely exceeded 5 hours. In waters 

 where the standing crop of phytoplankton was 

 relatively high, the sample volun-ie was reduced 

 to 2 or 3 liters, and occasionally the sannple was 

 split and two filters were used to reduce filtra- 

 tion time. Owing to the higher density of partic- 

 ulate matter, total filtration time for these 

 samples remained similar to that given above. 



17 



