VENRICK, McGOWAN, and MANTYLA : PHOTOSYNTHETIC CHLOROPHYLL 



tion to account for the presence of a chlorophyll 

 maximum layer in very different environmental 

 regimes. Of the numerous hypotheses which 

 have been put forward, several may be relevant. 

 The increase of chlorophyll with depth corres- 

 ponds to an increase in the number of phyto- 

 plankton (diatom) cells, but this may be 

 accentuated by an increase in the amount of 

 chlorophyll per cell. Zooplankton have been 

 shown to be concentrated above the maximum 

 layer and differential grazing pressure may help 

 to maintain the abrupt gradient at the top of the 

 maximum layer. In situ production has been 

 demonstrated within the maximum layer at very 

 low light intensities, and this will contribute to 

 the formation and maintenance of the layer. 



The strong development of a deep maximum 

 within the oligotrophic environments of the 

 Central Gyres, the effect of fluctuations of the 

 rate of downwelling on the depth of the max- 

 imum layer and the productivity in the overly- 

 ing water, and the consistent relationship be- 

 tween the depth of the maximum layer and the 

 depth of the nutricline and the nitrite maximum 

 suggest that the nutrient regime may be a crit- 

 ical factor in the development and maintenance 

 of the chlorophyll maximum layer. Our obser- 

 vations to date support the theory of Steele and 

 Yentsch (1960) that depletion of nutrients above 

 the summer thermocline leads to a reduction in 

 the buoyancy of phytoplankton, and that a sub- 

 surface maximum results from the accumulation 

 of impoverished cells at the top of the nutricline 

 where the absorption of nutrients decreases the 

 sinking velocity (Eppley, Holmes, and Strick- 

 land, 1967). The maximum layer may continue 

 to subside slowly as the nutrients at the top of 

 the nutricline are depleted, and thus it may be 

 that the depth of the maximum layer is ulti- 

 mately determined by the nutrient regime, rather 

 than ambient light intensity. As long as the 

 depth does not exceed the maximum depth of 

 the winter mixed layer the cells will be returned 

 to higher light levels during the winter. It may 

 be that the chlorophyll maximum layer repre- 

 sents a senescent stage in the annual cycle of 

 oceanic phytoplankton which is analogous to the 

 formation of resting spores by many neritic 

 species. 



It is evident that the chlorophyll maximum 

 layer, which may account for a major portion 

 of the standing crop of plant material and for a 

 substantial portion of the primary productivity, 

 is not restricted to the traditionally defined "eu- 

 photic" zone, the zone above the ISr light level. 

 There is no justification for limiting samples for 

 chlorophyll or productivity measurements to this 

 zone. These programs must be extended below 

 the chlorophyll maximum layer. We expect this 

 will result in a substantial increase in the es- 

 timates of total production within the water 

 column. 



ACKNOWLEDGMENTS 



The work was supported in part by National 

 Science Foundation Grant GB-12413 and in part 

 by the Marine Life Research Program, the 

 Scripps Institution of Oceanography's part of 

 the California Cooperative Oceanic Fisheries In- 

 vestigations, which are sponsored by the Marine 

 Research Committee of the State of California, 

 and by the National Science Foundation Grant 

 GB-2861. 



We thank Gary B. Smith for assistance in 

 many phases of this program and members of 

 the Scripps Institution of Oceanography Data 

 Collection and Processing Group for the collec- 

 tion and processing of the hydrographic and 

 chemical data. The seasonal data from station 

 100.80 was supplied by R. W. Owen and M. G. 

 Kruse of the National Marine Fisheries Service. 



LITERATURE CITED 



Allen, W. E. 



1932. Problems of flotation and deposition of ma- 

 rine plankton diatoms. Trans. Am. Microsc. Soc. 

 51:1-7. 



1939. Summary of results of twenty years' re- 

 searches on marine phytoplankton. Proc. 6th Pac. 

 Sci. Congr. 3:577-588. 

 Anderson, G. C. 



1969. Subsurface chlorophyll maximum in the 

 Northeast Pacific Ocean. Limnol. Oceanogr. 14: 

 386-391. 

 Austin, R. W., and R. W. Loudermilk. 



1968. An oceanographic illuminometer for light 

 penetration and reflection studies. S.I.O. (Univ. 

 Calif., Scripps Inst. Oceanogr.) Ref. 68-11, 10 p. 



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