FISHERY BULLETIN: VOL. 74, NO. 1 



however, the growth rates were determined at in- 

 cubator light levels which were not representa- 

 tive of in situ conditions. The regression of light 

 level on the ratio of the growth rates (/u net:/u, 

 nanno) is significant (P < 0.01) during the up- 

 welling months (Figure 6). Light levels approxi- 

 mately equivalent to full incubator light are 

 found at depths of 8 to 15 m during the upwelling 

 period, and the upper one-fourth to one-third of 

 the euphotic zone receives light which is in excess 

 of incubator light levels. 



o 



c 

 c 

 o 



c 



c 



CO 



0) 



o 



.06 0.6 



lO'^langley min' 



Figure 6. — Regression of incubator light levels on the 

 net'.nanno growth rates. 



Distribution in the Water Column 



Since nannoplankton concentrations were rela- 

 tively homogeneous in the water column, max- 

 ima were often not well defined. Net plankton 

 maxima, however, were usually apparent and cor- 

 responded to the depth of the seasonal pycnocline. 

 There was no regularly observed depth relation- 

 ship between nannoplankton and net plankton 

 maxima, and they often were at the same depth. 

 Phaeophytin peaks appeared at the surface and in 

 conjunction with, or just below, the chlorophyll 

 maxima. High NH3 concentrations in the deeper 

 phaeophytin maxima may be indicative of grazing 

 on the phytoplankton stocks in the chlorophyll 

 maxima (see Figures 7-10). 



During the Davidson Current period there is 

 little vertical stability in the water column, and 

 the net plankton stocks are poorly developed 

 (Figure 7). With the onset of upwelling net 

 plankton stocks develop above the strong, shal- 

 low pycnocline (Figures 8, 9) and the nanno- 



NOj ;ug atomt liter'' 



NH, IO"'>jg atoms liter"' 

 10 20 



a. 



0) 



Q 



100 



25.00 



26.0O 



«^ 



Figure 7. — Vertical distribution of phytoplankton standing 

 stocks, phaeophytin, and hydrographic parameters during 

 the Davidson Current period. 



NOj^jg atoms liter'' 

 NH3 IO''*jg atoms liter'' 

 10 20 



Ot 



x: 50 



o. 

 Q 



100 



25.00 



26.00 



O-f 



Figure 8. — Vertical distribution of phytoplankton standing 

 stocks, phaeophytin, and hydrographic parameters during 

 upwelling period. Station was sampled during a flowing tide. 



plankton stocks decline. With strong or persistent 

 upwelling, the pycnocline may intersect the sur- 

 face and the phytoplankton stocks are concen- 

 trated in a relatively shallow layer (Figure 9). 



After a slacking of upwelling the denser waters 

 subside and the pycnocline depths become pro- 

 gressively deeper. The surface layer can be 

 strongly stratified by the onshore movement of 

 warmer, low salinity oceanic water, and nutrient 

 concentrations in the near surface waters are low 

 during the oceanic period. The net plankton 



188 



