VENRICK, McGOWAN, and MANTYLA: PHOTOSYNTHETIC CHLOROPHYLL 



accumulation of pigment generally occupies a 

 layer 50-75 m thick, the core of the layer may 

 be very abrupt. From closely spaced water sam- 

 ples (Table 1) we have found that the highest 

 concentrations of chlorophyll may be contained in 

 a layer less than 5 m thick and may exceed by 

 109^ to 50% the concentrations in the adjacent 

 samples. Occasionally maximum concentrations 

 are found in more than one layer within the re- 

 gion of chlorophyll accumulation (Figure 3c, 

 Table 1, 23 August 1967). 



It is very difficult to sample such a narrow 

 core with discrete water samplers, A routine 

 cast, in which samples are usually spaced at least 

 15 m apart, is likely to miss the peak concen- 

 trations, and underestimate the chlorophyll con- 

 tent of the maximum layer. Moreover, because 

 of the rapid vertical changes in chlorophyll con- 

 centration, slight variations in the position of 

 the samples within the layer may appear as hor- 

 izontal discontinuities of the layer. Discrete 

 chlorophyll data, including those presented in 

 this paper, must be interpreted accordingly. 



SPECIES COMPOSITION 



The numbers and species of diatoms in water 

 samples collected on expeditions URSA MAJOR 



and ZETES have been enumerated (University 

 of California, 1967, 1970; Venrick, 1969). The 

 increase in chlorophyll concentration in the max- 

 imum layer is accompanied by a significant 

 increase in the number of diatom cells. Further- 

 more, the maximum layer is composed of dif- 

 ferent assemblages of species within the Sub- 

 arctic Pacific, the Transition Domain, and the 

 Central Pacific (Venrick, 1971). In August, 

 north of lat 40°N the species within the maxi- 

 mum layer were the same as those occupying 

 the overlying water mass. South of lat 38°N, 

 however, samples from the maximum layer were 

 dominated by species which were not observed 

 in shallower samples. During the winter when 

 the maximum layer had been eroded by increased 

 turbulence, the same species were present, but 

 they were distributed randomly through the 

 water column. 



More recent studies were undertaken in 1968 

 at lat 26°57'N, long 155°10'W with a series of 

 19 replicate casts over a distance of 10.5 miles. 

 Phytoplankton samples were collected from 25 m, 

 50 m, 75 m, and from the chlorophyll maximum 

 layer at 125 m. A total of 80 species of diatoms 

 were identified, of which 24, 36, and 37 were 

 observed in the samples collected from 25 m, 

 50 m, and 75 m, respectively. A total of 64 spe- 



Table 1. — Fine scale structure of the chlorophyll maximum layer. 



47 



