FISHERY BULLETIN: VOL. 74, NO. 1 



the nannoplankton fraction may be selectively 

 removed from the area by horizontal advection 

 because of their low sinking rates; 2) nanno- 

 plankton may be selectively grazed; 3) environ- 

 mental conditions may favor higher net plankton 

 growth rates. 



Malone (1971c) discussed the argument for 

 selective removal of nannoplankton from upwell- 

 ing areas by horizontal advection. Briefly re- 

 stated, nannoplankton cells tend to have slower 

 sinking rates than net plankton cells (or they are 

 motile) and in convection cells they will tend to be 

 removed from the areas of upward movement and 

 concentrated in areas of downward movement 

 (Stommel 1949). In upwelling areas then, nan- 

 noplankton may be selectively removed by mass 

 transport of surface waters offshore. There is lit- 

 tle direct evidence to show that this takes place; 

 however, the advection hj^othesis is supported 

 by the observed decrease in nannoplankton 

 stocks between the Davidson Current period and 

 the upwelling period. During the Davidson Cur- 

 rent period there is a general onshore movement 

 of surface waters with water sinking along the 

 coast, while during the upwelling period the cir- 

 culation is reversed and water moved upward 

 along the coast, and the surface waters are trans- 

 ported offshore (Skogsberg 1936; Bolin and Ab- 

 bott 1963). Malone (1971c) found the level of the 

 nannoplankton stocks remained relatively con- 

 stant throughout the year; however, he reported 

 that during periods of onshore water movement 

 there was an enhancement which could be attrib- 

 uted to concentrating the nannoplankton in an 

 area of downward water movement. 



The decrease in nannoplankton stocks reported 

 in the present study may have been the result of 

 selective grazing by microzooplankton and 

 planktotrophic larvae (Thorsen 1950; Beers and 

 Stewart 1969; Parsons and LeBrasseur 1970). In 

 this area many of the benthic invertebrates have 

 their reproductive season during the spring (M. 

 Houk pers. commun.)"*; increased grazing pres- 

 sure by these larvae may have caused the de- 

 crease in nannoplankton stocks. However, the 

 extent of grazing on either fraction of the phy- 

 toplankton in Monterey Bay is not known. 

 Zooplankton samples were collected as part of 

 the routine sampling program, but gelatinous 



■•M. Houk, Department of Natural Science, University of 

 California, Santa Cruz, CA 95064. 



and colonial phytoplankton could not be sepa- 

 rated from the zooplankton for biomass estimates. 



Throughout the period of upwelling, nitrate 

 levels in the upper 10 m remained high (> 5 ^ig 

 atoms liter"^) and the chlorophyll maximum was 

 frequently located near the surface. At these 

 shallow depths light levels were in excess of in- 

 cubator light levels (0.06 langley min"M. Eppley 

 et al. (1969) have shown that the diatoms 

 Skeletonema costatum and Ditylum brightwellii 

 grow faster than Coccolithus huxleyi at high light 

 levels (0.1 langley min"^) when nitrate levels are 

 in excess of 0.8 /xg atoms liter"^, while at lower 

 light levels (0.02 langley min"^), the situation is 

 reversed and C. huxleyi will grow faster at any 

 nitrate concentration. In situ nutrient and light 

 conditions near the surface during the upwelling 

 period should favor net plankton growth. 



In the present study and in that of Malone 

 (1971c), growth rates of the net plankton were 

 lower than the growth rates of the nannoplank- 

 ton; however, the two fractions responded differ- 

 ently to increasing light as showai by the ratio of 

 the growth rates (/x net.fx nanno) increasing with 

 higher light levels (Figure 6). The regression pre- 

 dicts that net plankton growth rates would ex- 

 ceed the nannoplankton growth rates at light 

 levels similar to those where Eppley et al. (1969) 

 showed a reversal of growth rate relationships. 

 Estimated light levels in the upper part of the 

 euphotic zone are higher than the incubator light 

 levels which have been used in this study and 

 that of Malone. Since the net plankton growth 

 rates show greater enhancement with increasing 

 light than the nannoplankton, light levels in the 

 upper water column may favor the growth of the 

 net plankton fraction and lead to net plankton 

 domination of the standing stocks. 



Laws (1975) suggested that, under certain en- 

 vironmental conditions, large cells may realize a 

 higher net growth rate because of a decreasing 

 respiration rate with increasing cell size. In 

 Laws' model, when surface light levels are low or 

 the product of the attenuation coefficient and 

 mixed layer depth is large, integral productivity 

 efficiency is low and respiration losses become 

 more important. During the present study, how- 

 ever, under low light levels, the net growth rates 

 of the smaller cells (nannoplankton) exceeded 

 larger cells, and the phytoplankton populations 

 were net plankton dominated at a time when the 

 mixed layer was extremely shallow. 



Notwithstanding the possible effects of selec- 



190 



