354 RYTHEB [OHAP. 17 



however, no mechanism exists for such regular and persistent vertical trans- 

 port ; the only process contributing to surface enrichment is vertical turbulence. 

 This varies seasonally and geographically and its variability determines, by 

 and large, the variability of the ocean's fertility. To some extent, then, pro- 

 ductivity may be correlated directly with the degree of vertical mixing. Here 

 again, however, light and nutrients must be considered together, for it is the 

 effect of turbulence upon these two factors which controls plant production. 

 For example, vertical mixing not only brings nutrients to the surface, but also 

 carries plant cells downward. When the mixed layer exceeds the depth of the 

 euphotic zone by several-fold, the organisms may spend a majority of their 

 time in darkness, unable to grow. The concept of a "critical depth" of the wind- 

 mixed layer, the depth below which respiration exceeds photosynthesis for the 

 whole population within the mixed layer, has been employed by Gran and 

 Braarud (1935), Riley (1942) and Sverdrup (1953), and a quantitative treatment 

 of the subject by Riley (1957) will be discussed later in some detail. 



Thus in providing the phytoplankton with one essential requirement, that 

 is nutrients, turbulence deprives the organisms of their other basic need, 

 namely illumination. For this reason neither vertical stability nor strong 

 turbulence, in themselves, contribute to high productivity. Favorable condi- 

 tions for growth stem rather from an alternation of these conditions, turbulence 

 enriching the surface layers followed by stability allowing the phytoplankton 

 to remain in the euphotic zone long enough to utilize the nutrients thus pro- 

 vided. Over a large part of the ocean the alternation from vertical instability 

 to stability is a seasonal phenomenon, and it is this periodicity which causes 

 the well known seasonal cycles of phytoplankton maxima and minima. In the 

 following discussion we shall see how variations in the degree of turbulence 

 and/or stability, as well as incident radiation, water transparency and depth, 

 determine geographic variations in the general level, annual rate and seasonal 

 cycles of primary production. 



5. Latitudinal Variations in the Stability of Surface Water 



Before discussing the productivity of specific parts of the ocean, we may 

 examine briefly, and in the light of the preceding discussion, the variations with 

 latitude of the thermal structure of the surface layers. We have already des- 

 cribed the latitudinal variability of incident radiation (Fig. 1) but pointed out 

 that this factor per se could not be assessed without simultaneous consideration 

 of nutrients. Since vertical turbulence affects, in opposite ways, the availability 

 of both light and nutrients, it is clear that the large difference in thermal 

 structure between tropical and polar surface waters must play an important 

 role in determining the relative fertility of the two regions. This effect is 

 heightened by the accompanying changes in incident radiation. 



Fig. 4 shows the seasonal cycle of temperature in the upper 300 m at latitudes 

 of 0°, 20°, 40° and 60° in the North Atlantic Ocean. The figure represents a 

 rather subjective interpretation of bathythermograph records on file at the 



