GARRISON: NET PLANKTON AND NANNOPLANKTON IN MONTEREY BAY 



tive grazing on the nannoplankton or their selec- 

 tive removal by horizontal advection, the de- 

 velopment of the upwelling bloom in Monterey 

 Bay is largely a result of the increase in the net 

 plankton fraction and may be explained in terms 

 of conditions which are favorable for net plankton 

 growth. High nutrient concentrations can be 

 maintained in the euphotic zone by downward 

 mixing from the surface which extends below the 

 pycnocline or by a continual input of nutrients to 

 the surface waters by upwelling. Optimal light 

 levels, however, are found only in the upper part 

 of the euphotic zone. The combination of these 

 conditions that constitute optimal growth condi- 

 tions for the net plankton fraction occur when the 

 phytoplankton stocks are restricted to a shallow 

 mixed layer above the pycnocline which has been 

 "pushed up" by upwelling water. Optimal growth 

 conditions vary spatially and seasonally and may 

 be primarily responsible for the net plankton and 

 nannoplankton relationship observed in Mon- 

 terey Bay. 



Nutrients do not appear to limit the growth 

 rates of either fraction as correlation coefficients 

 of nutrient levels with growth rates were not sig- 

 nificant and, although nutrient levels change 

 seasonally, Malone (1971c) reported little sea- 

 sonal variation in assimilation rates. Light 

 levels, however, are potentially limiting a short 

 distance from the surface and can influence the 

 ratio of net:nanno growth rates. 



An increase in the depth of the mixed layer 

 results in a decrease in the average light expo- 

 sure for phytoplankton cells in the mixed layer 

 (Parsons and Takahashi 1973). The net plankton 

 fraction will be more strongly influenced than the 

 nannoplankton because their optimal growth 

 rates occur at light levels near the surface, and 

 their vertical distribution is strongly controlled 

 by water movement. Upwelling water move- 

 ments result in a shallow pycnocline and shallow 

 mixed layer; with a slack in the upwelling rate, 

 the pycnocline sinks and there is a deeper mixed 

 layer. In the present study, net plankton maxima 

 were concentrated above the pycnocline, whereas 

 no particularly strong relationship between the 

 nannoplankton maxima were observed (the nan- 

 noplankton maxima were often not well defined). 

 Malone (1971c) showed that the net plankton 

 maxima were located below the nannoplankton 

 maxima during periods when upwelling was 

 slack or that both were located at the surface dur- 

 ing periods of upwelling, and he emphasized the 



role of upward movement in controlling the verti- 

 cal distribution of the net plankton fraction. 



Malone (1971c) showed an onshore to offshore 

 decrease in the ratio of net:nanno standing 

 stocks. Yoshida (1967) showed the potential for a 

 narrow zone of stronger upwelling associated 

 with the edge of the continental shelf where the 

 effects of upwelling are maximal at the edge of 

 the shelf and decrease exponentially shoreward 

 and seaward. A decrease in the upwelling rate 

 away from the continental shelf would result in 

 reduced suspension of sinking cells, a deeper 

 mixed layer, and lower average light levels for 

 phytoplankton cells in the mixed layer and could 

 reduce the net:nanno growth rate ratio. Malone's 

 data showed shallow mixed layers during periods 

 of strong upwelling at inshore stations and a 

 trend for an increasing mixed layer depth 

 offshore. In Monterey Bay during the upwelling 

 season, the mixed layer is frequently shallow or 

 the pycnocline intersects the surface. There are 

 considerable amounts of hydrographic data which 

 show this characteristic distribution (Broenkow 

 and Benz 1973) and corresponding phytoplankton 

 standing stock data which show significant strat- 

 ification of the phytoplankton standing stocks 

 above the shallow pycnocline (Silver see footnote 

 3). 



The depth of the pycnocline and mixed layer 

 vary seasonally in response to the upward move- 

 ment of isotherms during upwelling and the sink- 

 ing of isotherms when upwelling ceases. Upwell- 

 ing, however, is not a continuous process and 

 may be particularly sporadic near the end of the 

 upwelling season (Bolin and Abbott 1963; 

 Smethie 1973). Malone (1971c) reported net 

 plankton dominated stocks only during periods of 

 strong upwelling, which suggests that in deep 

 water continual upwelling is necessary to main- 

 tain optimal growth conditions for the net 

 plankton fraction. During the present study the 

 net plankton fraction dominated the phytoplank- 

 ton populations in shallow water throughout the 

 upwelling season. This evidence and previous 

 evidence for an offshore decrease in the netrnanno 

 ratios (Malone 1971c) suggest that physical pro- 

 cesses in shallow water are sufficient to maintain 

 net plankton populations and mitigate the lack of 

 continual upwelling. 



The physical processes in shallow water that 

 could serve to maintain favorable growth condi- 

 tions for the net plankton fraction or maintain 

 the population between periods of favorable con- 



191 



