SECT. 4] GEOGRAPHIC VARIATIONS IN PRODUCTIVITY 371 



Expedition, the maximum production (2.4 g carbon/m 2 /day) occurring in 

 Walvis Bay, Angola. 



The vertical circulation of the equatorial Pacific, discussed above, becomes 

 obscured at the eastern end of the ocean by coastal topographic and bathy- 

 metric features, the influence of the Peru Current, and other complicating 

 factors. Upwelling is somewhat less common, but over large areas the thermo- 

 cline occurs at relatively shallow depths of 25-50 m. Since this lies within the 

 euphotic layer, plant production may draw upon nutrients in the relatively 

 rich thermocline waters and hence the productivity of the area is relatively 

 high. One such location, off Costa Rica (9°N, 90°W), is of particular interest 

 since the thermocline depth is extremely shallow and may frequently "out- 

 crop" during the winter months. The productivity of this "dome" area in 

 November was 0.41-0.80 g carbon/m 2 /day (Holmes et at., 1957). The physical 

 basis of this and similar areas of "ridging" was investigated by Cromwell (in 

 press). While the hydrodynamic features were not completely elucidated, it 

 was apparent that the thermocline depth of the Costa Rican dome is related 

 to the strength of the trade winds and cyclonic shear over the dome. 



Another physical process which increases the fertility of the sea surface is 

 the vertical turbulence which occurs at current fronts and, particularly, 

 between two opposing current systems. Sette (1955) has presented evidence of 

 high fish production in the North Pacific (34°N) in the region which he refers 

 to as the shear zone between the North Equatorial Current and the North 

 Pacific Drift. A temperature profile across this zone of mixing revealed numerous 

 isolated spots where isotherms rise from thermocline depths to the surface, sug- 

 gesting eddy formation in the shear zone. As with most vertical transport 

 mechanisms in the sea, the physical explanation of this situation is not entirely 

 clear. 



Finally, rich waters from moderate depths may be carried into the euphotic 

 zone by turbulence or mass uplift where currents or drifting water-masses cross 

 submarine ridges or banks, or when the horizontal flow is interrupted by out- 

 cropping islands. Again Sette (1955) has related the presence of a small but 

 productive fishery near Hawaii to large scale, semi-permanent eddies, both 

 convergent and divergent, downstream from the Hawaiian Islands. 



In the boreal North Atlantic, which has already been discussed in some 

 detail, the presence of land-masses, submarine ridges and numerous current 

 systems and water-masses cause upwellings by all the afore-mentioned proc- 

 esses. According to Dr. Unnsteinn Stefansson (in litt.), the productivity of 

 the waters around Iceland is influenced by (1) vertical turbulence to the south- 

 east at the boundary between the Atlantic and Arctic waters, (2) vertical 

 turbulence to the northeast at the polar front and (3) upwelling off the south- 

 west coast of Iceland and at the shelf along its west coast. Steemann Nielsen 

 (1958) describes productive areas around Greenland as (1) at the front between 

 the North Atlantic Current and the Irminger Sea, (2) at the edge of the East 

 Greenland Current, (3) at the front between the Labrador Current and the 

 ocean water in the center of the Davis Strait, and (4) in an area of upwelling 



