SECT. 4 J GEOGRAPHIC VARIATIONS IN PRODUCTIVITY 369 



7. Hydrodynamic Features Which Influence Productivity 



Throughout the world's oceans there are specific areas whose productivity is 

 influenced by local hydrodynamic features obscuring the generalized regional 

 picture presented above. The physical mechanisms involved in these situations 

 are varied and often complex, but the end result in every case is the vertical 

 transport of nutrient-rich water up into the euphotic zone by means other than 

 turbulence or diffusion. To this extent, and for the purposes of this discussion, 

 these processes may be referred to collectively as "upwelling", a term which 

 may be employed more restrictively elsewhere. Some areas of upwelling are 

 small and local ; others are extensive and may influence large parts of the ocean. 

 We have, in fact, already discussed, without so designating the process, up- 

 welling around the Antarctic Continent as the factor responsible for the high 

 fertility of that entire oceanic region. 



Graham (1941) first presented biological evidence of upwelling in the 

 equatorial Pacific. As stated earlier, subsequent investigations of the region 

 have clearly shown increased production as the equator is approached (Doty 

 et al., 1956; Steemann Nielsen and Jensen, 1957; Austin, 1957; King et al., 

 1957 ; Holmes et al., 1957 ; Jitts and Rotschi, 1958). The hydrodynamic features 

 of the upwelling have been investigated by Cromwell (1951, 1953, 1954) and 

 will not be discussed here in detail. Briefly, the effects of Coriolis's force north 

 and south of the equator on the westerly drift of the Equatorial Current results 

 in the poleward divergence of the surface layers, which are replaced with water 

 upwelling from depths within the thermocline (150-200 m). The biological 

 effects of this upwelling are detectable across the entire Pacific, as revealed by 

 expedition "Eastropic" (King et al., 1957), and are evidenced by the abundance 

 of zooplankton and pelagic fish as well as by increased rates of primary produc- 

 tion (Sette, 1955). 



The upwelling of water from sub -surface and intermediate depths on a 

 smaller scale may occur in many parts of the oceans, and it will not be attempted 

 here to record every documented case of the phenomenon. The process is most 

 conspicuous along the western coasts of continents where prevailing offshore 

 winds carry surface waters seaward, drawing colder, nutrient-rich water from 

 moderate depths to the surface. The effects appear to be heightened where 

 there are strong coastal currents. Familiar examples of coastal upwelling are 

 those along the coasts of Southwest Africa (Defant, 1936), Peru (Gunther, 1936) 

 and California (Sverdrup and Fleming, 1941). Currie (1953) has pointed out 

 that the vertical circulation associated with the upwelling in the Benguela 

 Current is somewhat more complicated than the "one-sided divergence" 

 pictured by Defant. Rather than a continuous line of divergence along the coast, 

 there appear to be formed a series of eddies of varying size and strength, 

 somewhat similar to the situation along the Peruvian coast as described by 

 Gunther. Thus high productivity associated with coastal upwelling is irregular 

 and may be highly localized. 



On the other hand, coastal upwelling may influence the fertility of large 



13— s. ii 



