(Ref. 31). As the mixed layer increases in depth, the maximum concentration of phyto- 

 plankton (microalgae) gradually deepens, leaving a zone of nitrate-deficient water between it 

 and the upper pycnochne.* North of 45°N, nitrate remains at relatively high concentrations 

 throughout the year. South of 45°N, nitrate is present at the surface during winter and late 

 fall but becomes depleted in spring and summer. The maintenance of relatively high con- 

 centrations of nitrate north of 45°N is attributed to relatively intensive entrainment of deep 

 water into the upper zone coupled with a slow rate of removal of nutrients by primary 

 producers (Ref. 31). 



For areas south of 40°N in the eastern Pacific Ocean there have been two major 

 surveys of nutrient and other oceanographic parameters. The California Cooperative Ocean- 

 ic Fisheries Investigation Program (CALCOFI) has been studying oceanic properties on the 

 California Current region from approximately 40°N to 20°N since 1950 and the Eastern 

 Tropical Pacific (EASTROPAC) cruises were fielded during 1967 and 1968 to survey the 

 tropical and subtropical eastern Pacific from about 25°N to 20°S and from the coast west to 

 130°W. 



Near-surface nutrient concentrations from 40°N to about 35°N tend to be low 

 during spring and summer but with slightly higher concentrations in the winter. Concentra- 

 tions of 30 [JLg-at N03-N/liter are generally located between 150 and 300 meters of depth. 



Figure A. 2 shows nitrate levels increasing rapidly from near jLig-at/liter at about 50 meters 

 of depth to 20-25 Mg-at N03/hter at 200 meters in area 11. 



South of 35°N, near-surface NO3 concentrations tend to be very low, 0.1 /xg-at/liter 



or less. Figures 14-17 (from CALOFI Atlas, Ref. 32) show the nitrate concentrations at 10 

 meters for the four seasons. Near-shore concentrations can be very high in certain areas 

 along the Baja California and California coasts during the spring and summer upwelling 

 periods. Concentrations as high as 16 jug-at/Uter NO3-N are observed at Pt. Eugenia, 27° 



30'N and Pt. Antonia, 30°N, both in Baja California; and at California's Pt. Conception, 

 34°30'N (Fig. 15, Ref. 32). Relatively high concentrations are observed off San Francisco 

 in the fall (Fig. 17). South of 35°N, nitrate concentrations at distances greater than 8 

 kilometers off the coast are consistently low. Figures 18 through 2 1 show the seasonal 

 concentration of NO3 at 100 meters of depth. 



Seasonal variations are not as apparent as areal trends. Nitrate concentrations at 

 100 meters of depth decrease from approximately 25 )ng-at/hter at 35°N near the coast to 

 about 5 /xg-at/liter at 35°N but 240 to 300 kilometers farther west. To the south there is 

 also a trend of generally decreasing nitrate concentrations but with higher values at greater 

 depths. Figure A. 1 shows three profiles for spring in area 10 (Fig. 1 ) taken in the vicinity of 

 27 N, 1 17 W. The nitrate concentration is at undetectable concentrations down to about 

 50 meters of depth, but then the concentration rapidly increases to between 7 and 22 Mg-at/ 

 liter at 1 00 meters of depth and 30 ^tg-at/liter between 250 and 350 meters (Table 2). 



"Vertical zone of maximum density gradient: usually corresponds with llie thermocline. 



