FISHERY BULLETIN: VOL. 71, NO. 1 



distribution along the same transect observed 

 during January (ZETES Expedition) when a 

 similar program of chlorophyll measurement 

 was carried out. In January, north of lat 32°N, 

 the mixed layer extends below 100 m. Concen- 

 trations of chlorophyll are uniform throughout 

 this layer, decreasing abruptly below the mixed 

 layer. Between lat 26°N and 32°N a weak 

 chlorophyll maximum is still present near 120 m 

 below the mixed layer which does not reach its 

 greatest depth of 200 m until February (Rob- 

 inson, 1951). 



The evidence accumulated to date suggests 

 that a subsurface concentration of plant material 

 can persist only in the presence of a density 

 gradient which isolates the layer from the ef- 

 fects of wind-driven turbulence. Thus, any sea- 

 sonal fluctuations in the strength or depth of the 

 pycnocline may be expected to affect the presence 

 of the deep maximum layer. We can postulate 

 with some assurance that in any environment 

 in which the winter mixed layer regularly ex- 

 ceeds the depth of the chlorophyll maximum 

 layer, the maximum layer must be a seasonal 

 phenomenon. At any locality, the duration of 

 the maximum layer will be determined by the 

 duration of seasonal stratification of the water 

 column and thus will be progressively shorter 

 at higher latitudes. 



This observation has important implications. 

 Over most, if not all, of the ocean, the phyto- 

 plankton within the maximum layer do not rep- 

 resent a permanent loss to the epipelagic com- 

 munity. Neither need there be a balanced energy 

 budget within the maximum layer. Sufficient 

 energy may be produced and stored in a brief 

 period prior to stratification of the water column 

 or the depletion of nutrients from the surface 

 waters to maintain the population within the 

 maximum layer for considerable periods of time, 

 even though photosynthesis may be depressed 

 or absent. 



scale. In September 1969, the standing crop 

 and productivity were higher and more variable 

 throughout the water column than in the same 

 month of the previous year. As a result, in 1969, 

 the chlorophyll maximum layer was less sharply 

 defined and was occasionally obscured by high 

 chlorophyll concentrations in the overlying 

 water. These fluctuations are of considerable 

 interest. Namias (1971) has investigated the 

 meteorological and oceanographic conditions ac- 

 companying a vast pool of abnormally warm 

 water in the southern portions of the North Pa- 

 cific during the summer and fall of 1968. He 

 concludes: 



The abrupt and extensive anomalous warming of the 

 southeastern quarter of the Pacific Ocean north of 

 20°N from May-June, 1968, appears to have been 

 due largely to increased isolation and horizontal con- 

 vergence of the surface layers of the sea and associated 

 downwelling, .... These warming factors in the heat 

 budget were associated with the development and 

 maintenance of a strong and deep Pacific anticyclone 

 in June which appears to have been persistently re- 

 generated by an unusually strong mean jet around 

 40°N. 



This period of stronger subsidence was ac- 

 companied by a clear sharpening of the maxi- 

 mum layer and by reduced standing crop of phy- 

 toplankton and productivity above the layer. The 

 observations of Namias suggest that the gener- 

 alized downwelling in the Central North Pacific 

 anticyclone, which is an important factor inhib- 

 iting the vertical diffusion of nutrients into the 

 euphotic zone, is also closely related to the depth 

 and concentration of photosynthetic material be- 

 low the mixed layer. Extrapolation leads us to 

 expect to find similar chlorophyll maxima well 

 developed in other large, persistent temperate 

 gyres, such as the South Atlantic and the south- 

 ern Indian Ocean. 



DISCUSSION 



LARGE-SCALE TEMPORAL 

 FLUCTUATIONS 



In the Central Gyre of the North Pacific we 

 have recorded temporal fluctuations on a larger 



It is evident that a deep chlorophyll maximum 

 layer is a well-developed and consistent feature 

 of the major gyres of both the North and South 

 Pacific. In view of its geographic continuity, 

 we must reevaluate the mechanisms postulated 

 for its development, and seek a single explana- 



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