FISHERY BULLETIN: VOL. 71, NO. 1 



(Allen, 1939; Lorenzen, 1965, 1967). Initial 

 results of the EASTROPi^ C survey (Love, 1970, 

 1971) indicate a chlorophyll maximum varying 

 in depth between 50 and 100 m over large areas 

 of the eastern tropical Pacific. Anderson (1969) 

 has studied the chlorophyll maximum layer off 

 the Oregon coast which is present between 50 

 and 75 m during the summer. The layer is con- 

 tinuous over a broad region in the Eastern Sub- 

 arctic Pacific and maybe transpacific. Chloro- 

 phyll maxima have also been reported from the 

 other major water masses of the Pacific (El- 

 Sayed, 1970; Sorokin, 1970). 



Different workers have attributed the exis- 

 tence of the maximum layer to different processes 

 including the concentration of detrital chloro- 

 phyll in the pycnocline (Lorenzen, 1965), differ- 

 ential zooplankton grazing (Lorenzen, 1967), an 

 increase in the chlorophyll/carbon ratio in plant 

 / cells, without an accumulation of cells (Steele, 

 1964) , horizontal advection and layering of dif- 

 ferent water masses and plant populations 

 (Sano, 1966), the sinking of active or senescent 

 cells from shallower depth (Allen, 1932; Steele 

 and Yentsch, 1960) , and in situ production (An- 

 derson, 1969). In short, the tendency has been 

 to consider deep chlorophyll maximum layers as 

 discrete and sporadic phenomena and to inter- 

 pret them strictly according to local conditions. 



The accuracy with which surface productivity 

 reflects the productivity throughout the water 

 column has been investigated by Koblenz-Mishke 

 et al. (1970) by means of log-log scatter dia- 

 grams. There is a linear trend in their trans- 

 formed data, but the spread of values around the 

 regression line is broad. Lorenzen (1970) 

 showed a significant linear regression, after 

 transformation to logarithms, of total produc- 

 tion on the concentration of surface chlorophyll. 

 The regression, however, removes only half of 

 the variability of the dependent variable, and 

 the author advises that precise values of total 

 production must depend upon direct measure- 

 ments. He also cautions that extrapolations from 

 surface values are based upon averages and will 

 easily miss unexpected events. 



There have been very few attempts to mea- 

 sure productivity in the deeper maximum layers. 



Anderson (1969) made one series of in situ mea- 

 surements within the chlorophyll maximum layer 

 off the Oregon coast. There was a peak in pro- 

 duction within the layer and positive photosyn- 

 thesis as deep as 90 m, the 0.1% light level. Im- 

 plicit in most studies to date is the assumption 

 that pigment concentrations below the level of 

 1% light are nonphotosynthetic and represent 

 a loss of plant material from the "euphotic zone." 

 The two extensive surveys from the Sargasso 

 Sea and the eastern tropical Pacific both adjusted 

 the depth of the lowest sample to the depth of 

 1% light, and rarely sampled below 100 m, even 

 though the maximum pigment concentrations 

 were frequently obtained from the deepest sam- 

 ple. 



In the present paper, the authors have sum- 

 marized a large amount of data accumulated 

 over the past 8 years, all of which indicate that 

 a deep chlorophyll maximum layer is a regular 

 and continuous feature of much of the oceanic 

 Pacific. It is frequently observed below the tra- 

 ditionally defined euphotic zone, yet it is dom- 

 inated by photosynthetically active chlorophyll a 

 which is present in concentrations as great as 

 10 times those at the surface. The development 

 of this maximum layer appears not to be a lo- 

 calized process, but a widespread and regularly 

 occurring phenomenon. Because of its limited 

 vertical extent and great depth, the existence, 

 extent and significance of this maximum layer 

 has been overlooked by most previous surveys 

 of chlorophyll and productivity. Evidence sug- 

 gests that a better understanding of this layer 

 will necessitate revision of existing estimates of 

 total primary production in the ocean. 



METHODS 



Since 1964 we have been mapping and study- 

 ing the subsurface chlorophyll maximum in the 

 Pacific on a series of expeditions (Figure 1). 

 In 1964 (URSA MAJOR Expedition: Univer- 

 sity of California, 1967) and 1966 (ZETES Ex- 

 pedition: University of California, 1970), chlo- 

 rophyll pigments were determined with a D U 

 Spectrophotometer; on other expeditions chlo- 

 rophyll a and phaeophytin were assayed with a 



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