VENRICK, McGOWAN, and MANTYLA: PHOTOSYNTHETIC CHLOROPHYLL 



internal waves. However, on a wider scale, 

 there appears to be no relationship between the 

 depth of the maximum layer and any one isoline 

 of temperature, salinity, or density. 



Plant nutrients are present in very low con- 

 centrations in the upper 100 m. Phosphate val- 

 ues were less than 1.5 /xg at./liter in the North 

 Central Pacific and less than 0.2 /xg at./liter in 

 the South Central Pacific. Nitrate was less than 

 0.6 fxg at./liter in the North and less than 

 0.8 fxg at./liter in the South, while corresponding 

 values of silicate ranged between 1 and 7 fig 

 at./liter in the North and between and 3 /xg 

 at./liter in the South. The concentrations of 

 these three nutrients increase systematically and 

 significantly at, or just below, the level of the 

 chlorophyll maximum. Concentrations of am- 

 monia are low and irregular throughout the up- 

 per 200 m, showing no pattern with depth. In 

 contrast, high values of nitrite in the upper 

 200 m (occasionally as high as 4.5 /xg at./liter) 

 were observed only within or just below the 

 chlorophyll maximum, and may indicate recent 

 phytoplankton assimilation of nitrate-nitrogen 

 (Vaccaro and Ryther, 1959). In all of our stud- 

 ies, the relationship between the vertical distri- 

 butions of chlorophyll and nutrients was far 

 more predictable than the relationship between 

 chlorophyll and any of the physical properties. 



We have found no evidence of any accumula- 

 tion of zooplankton within the chlorophyll max- 

 imum layer. Total zooplankton biomass (ani- 

 mals greater than 35 /x) was greatest above the 

 maximum layer during both day and night. This 

 appears to be true for all size categories. 



THE SEASONAL CYCLE 



Seasonal samples from the western edge of 

 the California Current (station 100.80 at lat 

 30°00'N, long 120°07'W) during 1969 demon- 

 strate a seasonal change in the vertical distribu- 

 tion of chlorophyll a (Figure 5). We have evi- 

 dence that this maximum layer is continuous 

 with that observed within the Central Pacific 

 Gyre and we expect their seasonal cycles to be 

 comparable. For a short period in February, 

 chlorophyll is essentially homogeneous through 

 the upper 50 m. This corresponds in time to 



the maximum development of the mixed layer. 

 When the water column begins to stratify in 

 March, chlorophyll concentrations at the surface 

 decrease abruptly and a subsurface maximum 

 layer develops. As the summer progresses, the 

 maximum decreases in concentration and the 

 layer subsidies, reaching its maximum depth just 

 prior to the breakdown of the density stratifi-|i 

 cation in December. Figure 5b illustrates the' 

 lack of temporal relationship between the depth 

 of the chlorophyll maximum and any one iso- 

 pleth of density. This would seem to preclude 

 the formation of the maximum layer from the 

 accumulation of cells regulated solely by their 

 physical density. 



The vertical distribution of chlorophyll dur- 

 ing August along long 155°W is presented 

 in Figure 2. This may be compared with the 



MLR STA. 100.80 30°00'N I20*'07'W 

 CHLOROPHYLL - a 



100 - 



150 



200 



SONDJFMAMJJASONDJFMA 

 1969 



25.50 



26.00 



SONDJFMAMJJASONDJF MA 

 1969 



Figure 5. — Annual development of the subsurface chlo- 

 rophyll maximum layer at lat 30°00'N, long 120°07'W. 

 Chlorophyll a concentration is contoured with respect to 

 depth (A) and density (B). 



49 



