DEEP MAXIMA OF PHOTOSYNTHETIC CHLOROPHYLL IN 



THE PACIFIC OCEAN 



E. L. Venrick, J. A. McGowAN, and A. W. Mantyla^ 



ABSTRACT 



Data collected on several expeditions through the temperate and tropical Pacific Ocean 

 show that during most of the year the maximum concentrations of chlorophyll occur 

 below the surface, typically in a narrow layer near or below the depth of penetration 

 of 1% of the surface light. The layer appears to be continuous across most of the Pacific 

 although the depth and chlorophyll concentration vary regionally. The depth of the 

 layer is more closely related to the depth of the nitrite maximum and to the position 

 of the nutricline than to either light or density regimes. Productivity within the layer 

 is low but positive, and contributes substantially to the total production of the water 

 column. The maximum layer may be a seasonal phenomenon developing in the summer 

 after the stabilization of the water column and mixing to the surface during the winter. 

 Year to year fluctuations of depth and concentration of chlorophyll within the maximum 

 layer may be related to large-scale meteorological fluctuations. 



Doty and Capurro (1961) have tabulated the 

 position, date, depth, and values of chlorophyll 

 and productivity in the world's oceans. There 

 are several thousands of these measurements in 

 the Pacific. Most are in the Northern Hemi- 

 sphere, and most are near land masses or is- 

 lands (e.g., Hawaii, Luzon, Hokkaido, New Cal- 

 edonia, New Zealand), along the equator, or 

 north of lat 40°N. Of the values from the oceanic 

 Pacific, between lat 50 °N and 50 °S, less than 

 10% of the chlorophyll values represent depths 

 greater than 25 m; in the same region, over half 

 of the productivity measurements were obtained 

 at the sea surface. Koblenz-Mishke, Volkovinsky, 

 and Kabanova (1970) have used these data and 

 additional data available to them to estimate 

 the plankton primary production of the Pacific, 

 to construct tables and charts of its geographical 

 variability, and to compare production in the 

 Pacific with their estimates from other oceans. 

 Their estimates of primary production, ex- 

 pressed in milligrams carbon per square meter 

 of sea surface per day, represent production in- 



^ Scripps Institution of Oceanography, University of 

 California at San Diego, La Jolla, CA 92037. 



Manuscript accepted August 1972. 



FISHERY BULLETIN: VOL. 71, NO. I, 1973. 



tegrated through the water column. However, 

 many of their production values are extrapolated 

 from surface measurements, and, in large areas 

 of the temperate gyres of the North and South 

 Pacific, production is estimated from the avail- 

 able chlorophyll data, or from "oxygen or hy- 

 drogen saturation" values. 



All values of total production in the water col- 

 umn are strongly dependent upon the assumed 

 (usually) depth of zero productivity. This is 

 traditionally taken to be the depth at which the 

 light intensity has been reduced to 1% of the 

 incident radiation, and this criterion has been 

 used to divide the water column into a euphotic 

 zone and an aphotic zone. 



Evidence is accumulating that major concen- 

 trations of plant material in the ocean usually 

 occur below the surface, typically within the 

 thermocline and near the bottom of the euphotic 

 zone. Maxima of chlorophyll or phytoplankton 

 as deep as 100 m have been reported from the 

 Indian Ocean ( Yentsch, 1965) , the Sargasso Sea 

 (Menzel and Ryther, 1960), the Gulf of Mexico 

 (Steele, 1964), and the Kuroshio and adjacent 

 regions (Motoda and Marumo, 1963; Saijo, 

 lizuka, and Asaoka, 1969). Shallower maxima 

 are characteristic of the California Current 



41 



