FISHERY BULLETIN: VOL. 69. NO. 4 



productivity and grazing conformed to what 

 Gushing (1959) has referred to as an unbal- 

 anced seasonal cycle of primary production and 

 primary consumption. Neritic regions in tem- 

 perate waters are generally characterized by 

 about a 2-month time interval between peaks 

 in phytoiilankton and zooplankton biomass 

 (Gushing, 1959; Heinrich, 1962), with a time 

 lag of about 1 month between the onset of the 

 spring bloom and the increase in zooplankton 

 standing crop. Martin (1965) found a 2-month 

 lag between the maximum phytoplankton stand- 

 ing crop and the increase in zooplankton stand- 

 ing stock. 



In Monterey Bay, about 2 months elapsed be- 

 tween the March-April phytoplankton bloom and 

 the rapid increase in grazing pressure observed 

 during June and July (Figure 3) . Although up- 

 welling was in progress (NO.i-N concentrations 

 were greater than 5 fj.u throughout the photic 

 zone and the netplankton chlorophyll maximum 

 was in the upper 10 m), the phytoplankton chlo- 

 rophyll content of the water column declined as 

 grazing pi'essure increased. The netplankton 

 fraction fell continuously while the nannoplank- 

 ton dropped at first and then increased (Figure 

 5). The reduction in standing crop was accom- 

 panied by a steady decline in the ratio of net- 

 plankton-to-nannoplankton chloro])hyll in the 

 water column, from 1.1 near the beginning of 

 the increase in grazing pressure to 0.1 at its 

 peak. Thus, it appears that (1) the phyto- 

 plankton bloom was ultimately limited by graz- 

 ing; (2) the netplankton fraction, dominated by 

 Nitzschia spp. and Rhizosolenia spp. {&0'/c of 

 the netplankton by number), was selectively 

 grazed; and (3) the cycle of netplankton pro- 

 duction and animal grazing was unbalanced. 



Variations in the net/nanno chlorophyll m~^ 

 ratio were significantly related to concurrent 

 changes in the nitrate content of the photic zone 

 (an indicator of upwelling) and to grazing pres- 

 sure (F = 5.56, P = 0.05) by the multiple re- 

 gression equation: 



net/nanno = 1.76 + 0.003 (NO3-N) 

 — 2.53 (Phaeo/Chl-a). 



This equation is based on 20 sets of data (Cal- 

 COFI 3), and the partial correlation coefficients 



for the interactions between the net/nanno ratio 

 and nitrate concentration (r = + 0.51) and be- 

 tween the ratio and grazing pressure (r = 

 — 0.56) are significant at the 0.05 level. The 

 evidence suggests, therefore, that upwelling is 

 a necessary precondition for netplankton pro- 

 ductivity and standing crop to approach or ex- 

 ceed that of the nannoplankton in marine en- 

 vironments where water depth greatly exceeds 

 the maximum depth of wind-driven turbulent 

 mixing. 



The relative constancy of the nannoplankton 

 relative to the netplankton fraction, in spite of 

 marked changes in the concentration of inor- 

 ganic nitrogen, the intensity and direction of 

 vertical water movements, and grazing pressure, 

 is puzzling. The assimilation ratios of both 

 fractions exhibited little variability, but on the 

 average nannoplankton ratios were twice as 

 great as those of the netplankton. Since this 

 ratio is an index of growth rate (cf. Eppley and 

 Strickland, 1968), the nannoplankton must have 

 been limited primarily by "cropping" factors 

 (Dicknian, 1969), at least during those periods 

 when netplankton productivity was increasing 

 relative to nannoplankton productivity. This is 

 supported by the observation that the chloro- 

 phyll content of nannoplankton and neti:)lankton 

 cells also exhibited little variability during the 

 period of study. During upwelling, two pro- 

 cesses could selectively remove nannoplankton 

 cells from upwelling regions: (1) grazing and 

 (2) horizontal advection. 



If nannoplankton grazers were predominantly 

 protozoans (Beers and Stewart, 1969) with 

 short generation times and netplankton grazers 

 were crustaceans and fishes with long genera- 

 tion times, the coupling between primary pro- 

 ductivity and grazing would be much closer for 

 nannoplankton-based food chains than for net- 

 plaiikton-based food chains. The cycle of nan- 

 noplankton productivity and animal grazing 

 would be balanced (Gushing, 1959; Heinrich, 

 1962) in contrast to the unbalanced character 

 of netplankton-based food chains. This would 

 tend to dami)en fluctuations in the nannoplank- 

 ton fraction relative to the netplankton fraction. 



Similarly, if nannoplankton cells were selec- 

 tively removed from sites of upwelling by mass 



816 



