FISHERY BULLETIN: VOL. 69. NO. 4 



Increases in netplankton productivity and stand- 

 ing crop were closely related to upwelling, both 

 as a consequence of the positive vertical advec- 

 tion and the entrainment of nitrate into the 

 upper half of the photic zone. The requirement 

 for positive vertical advection was probably re- 

 lated to both cell size and motility so that the 

 vertical distribution of nannoplankters was more 

 independent of vertical water movements. The 

 relationship between cell size and A/V ratios 

 probably accounted for the higher nitrate re- 

 quirements of the netplankton. Decreases in the 

 netplankton were primarily due to grazing and 

 to removal from the photic zone by downward 

 water movements. 



The stability of the nannoplankton compared 

 to the variability of the netplankton is inter- 

 esting, especially in light of the marked changes 

 observed in the concentration of inorganic nitro- 

 gen compounds and the direction and intensity 

 of vertical water movements. Since nanno- 

 plankton assimilation ratios were consistently 

 high and twice as great as netplankton assimi- 

 lation ratios, the nannoplankton must have been 

 limited primarily by cropping factors during up- 

 welling periods when netplankton standing crop 

 was increasing relative to that of the nanno- 

 plankton. Under these conditions increases in 

 the nannoplankton fraction will be dampened by 

 selective removal from u])welling sites by mass 

 transport away from the coast and grazing by 

 organisms with short generation times (e.g., 

 protozoans). Decreases in nannoplankton 

 standing crop due to "excessive" grazing or re- 

 moval from the photic zone by downward water 

 movements will be limited by the motility and 

 short generation times of nannoplankton species. 

 The motility of nannoplankters in combination 

 with onshore mass transport and downward 

 water movements will also favor an offshore- 

 inshore increase in nannoplankton productivity 

 and standing crop. 



Finally, it is clear that the nannoplankton and 

 netplankton components of i)h.vtoplankton com- 

 munities respond differently to changes in their 

 environment; that cell size, surface-to-volume 

 ratios, and motility play important roles in me- 

 diating these resi)onses; and that changes in 

 netplankton and nannoplankton productivity rel- 



ative to each other have definite consequences 

 with respect to energy flow through phytoplank- 

 ton-based food chains. 



ACKNOWLEDGMENTS 



I am grateful to M. Gilmartin, D. P. Abbott, 

 and B. Robison for their criticisms and encour- 

 agement, and to M. S. Doty for his timely advice 

 during the preparation of this paper. I thank 

 Peter Davoll and David Bracher for the hydro- 

 graphic and nutrient chemistry data, and M. 

 Youngbluth for the zooplankton data. 



This research was supported in part by NSF 

 Grants GB 8408 Al, GB 8374 A2, GD 27254, 

 and in pai't by NIH Predoctoral Fellowship 5 

 FOl GM 44364-02, and was submitted in partial 

 fulfillment of the Ph.D. dissertation requirement, 

 Stanford University. 



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