Currents 



99 



Figure 85. Regional pattern of 

 contents of dissolved oxygen 

 and phosphate-phosphorus at 

 the surface and at a depth of 

 200 meters during the Marine 

 Life Research cruise of Febru- 

 ary 1950. Note the presence of 

 low oxygen and high phospho- 

 rus at the surface between 

 Santa Cruz and San Nicolas 

 Islands and the still lower oxy- 

 gen and higher phosphorus at 

 200 meters. 



117^21' 



20° 119° 118° 117° 



temperature and oxygen decrease with depth 

 below the sea surface and salinity and phos- 

 phorus increase with depth, it is evident that 

 some mechanism is causing subsurface 

 water to rise to the surface. When this 

 nutrient-rich water reaches the zone of sun- 

 hght, it supports a dense crop of phyto- 

 plankton (Sverdrup and Allen, 1939; Allen, 

 1945; Sargent and Walker, 1947) which 

 serves as food for many small and large ani- 

 mals. The plankton may become so abun- 

 dant that it reduces the transparency of the 

 sea water in this area (Fig. 86), but not so 

 much as sediment and plankton together re- 

 duce it near the mainland shore (Emery, 

 1954^). In addition to reducing the trans- 

 parency, high concentrations of sediment 

 and plankton change the deep blue of the 

 ocean to green far from shore and finally to 

 brown close to shore. Crews of fishing 

 boats are keenly aware of the general dis- 

 tribution of colors because they find and 

 harpoon most of their swordfish and troll 

 most of their tuna near the boundary be- 

 tween green and blue waters. 



The classical computation of currents is 

 based on dynamic topography computed in 



turn from measurement of temperature and 

 salinity at many depths in a grid of stations 

 occupied in as short a time as possible. For 

 absolute currents we must have, or be able 

 to assume, at some depth an isobaric (equal 

 pressure) level that is horizontal and, thus, a 

 surface of no motion. Currents can be com- 

 puted above a sloping surface of equal pres- 

 sure only on a basis of movement relative 

 to that of the sloping surface. Given a 

 selected isobaric level, the next step is to 

 compute the total height at each oceano- 

 graphic station of the water column above 

 that level. This computation is based on 

 the known inverse relationship of water 

 density to temperature and direct relation- 

 ship to salinity and pressure. Details of 

 computation are given by Sverdrup, John- 

 son, and Fleming (1942, pp. 289-418). In 

 addition, a complex correction for periodic 

 variations of density caused by internal 

 waves of tidal period must be made (Defant, 

 \950b). 



The maps of topography of the surface 

 relative to the 300-decibar level (1 decibar 

 equals the pressure exerted by a column of 

 sea water 1 meter high) show a variation of 



