FISHERY BULLETIN: \'0L. 69. NO. 4 



Rica Dome (incubator light intensity about 0.0(5 

 langley/min) and are not sig-nificantly different 

 from the mean of 8.6 ± 1.3 found by Curl and 

 Small (1965) at light saturation based on in situ 

 measurements. Anderson (1964), working off 

 the Washington and Oregon coasts, obtained ra- 

 tios of 1.6 to 9.8 (at about 0.02 langley/min) 

 with low values occurring during the summer 

 when nutrient concentrations were low and high 

 values during the spring bloom when nutrient 

 concentrations were high. In the eastern trop- 

 ical Pacific, Thomas (1970) andMalone (in press 

 a) found that assimilation ratios were signifi- 

 cantly less in nitrogen-poor than in nitrogen-rich 

 waters. These results are consistent with the ob- 

 servations of Curl and Small (196.5), supported 

 by McAllister et al. (1964), which suggest that 

 ratios below 3 are indicative of a nutrient defi- 

 ciency while those above 5 indicate nutrient- 

 rich waters. 



Both the nannoplankton and the netplankton 

 exhibited relatively constant assimilation ratios, 

 but mean nannoplankton ratios were signifi- 

 cantly higher (9.4 ± 1.5 inshore, 8.3 ± 1.2 off- 

 shore) and twice as great as those of the net- 

 plankton (4.7 ± 1.3 inshore, 4.1 ± 0.8 off- 

 shore). The constancy of these ratios over a 

 wide range of productivity values in spite of 

 large variations in ambient nitrogen concentra- 

 tions indicates that nutrient concentration was 

 not an important limiting factor and that the 

 phytoplankton were adapted to about the same 

 light intensity over the entire year. This is 

 conceivable since seasonal variations in day 

 length and light intensity tend to be dampened 

 by the seasonal pattern of cloud coverage, i.e., 

 the summer months are usually foggy while the 

 winter months are usually clear. The situation 

 is similar to that found off La Jolla (Strickland 

 et al., 1970). 



Increases in the productivity and standing 

 crop of the netplankton fraction and in the net/ 

 nanno ratio were closely coupled with the occur- 

 rence of upwelling. Each new upwelling pulse, 

 regardless of duration (CalCOFI 3) or location 

 (transect 1-B) was marked by an increase in 

 net/nanno ratios and netplankton standing croj). 

 Potentially, upwelling can affect phytoi)lankton 

 productivity in at least two ways: (1) by in- 



creasing the residence time of cells in the upper 

 reaches of the photic zone and (2) by increasing 

 the rate at which nutrients are supplied to the 

 photic zone. The settling velocities of phyto- 

 plankton cells range between and 10 m day~' 

 (for a review see Smayda, 1970), with most 

 values falling between 0.5 and 2 m day"' (Ep- 

 pley et al., 1967; Strickland et al., 1969) . Aver- 

 age upwelling velocities are of the order of 10 m 

 day ' (Hidaka, 1954), which is quite sufficient 

 to inhibit the sinking of negatively buoyant cells. 



Since the netplankton fraction was primarily 

 composed of nonmotile diatoms and the nanno- 

 plankton fraction of flagellates, it is probable 

 that vertical water movements will have a great- 

 er effect on the vertical distribution of netplank- 

 ton than on the nannoplankton. It is not sur- 

 prising, therefore, that the depth of the net- 

 plankton maximum was more closely tied to the 

 u])ward and downward trends of the isotherms, 

 both seasonally at CalCOFI 3 (Figure 6) and 

 along inshore-offshore transects of the Califor- 

 nia Current (Figure 8). The netplankton max- 

 imum at CalCOFI 3 was always found below that 

 of the nannoplankton except during strong up- 

 welling when both maxima occurred in the upper 

 10 m. During periods of subsidence the net- 

 plankton minimum was depressed to greater 

 depths than the nannoplankton maximum was. 

 This was observed during the Mixed Period even 

 though NOa-N concentrations in the surface lay- 

 ers were still high (>1.0/LiM). The reverse was 

 observed along transect 1-B in that the netplank- 

 ton maximum decreased in depth as the zone of 

 ofli"shore upwelling was approached, moving in 

 the process from a nitrate-rich layer (>5.0 yuM 

 NO.i-N) into a nitrate-poor layer (<0.5 yuM 

 NO:!-N) . The depth distribution of nannoplank- 

 ton chlorophyll (Figures 6 and 7) was more in- 

 dependent of vertical water movements and 

 maximum chlorophyll concentrations were often 

 found at the su)-face during influxes of oceanic 

 water (during both Oceanic and Mixed Periods) 

 when subsidence was most pronounced. 



Most of these trends in the depth distriljution 

 of netplankton and nannoplankton chlorojihyll 

 could be explained in terms of the vertical dis- 

 triljution of nitrate in the photic zone. How- 

 ever, during the early stages of upwelling in 



814 



