CONTRIBUTION OF THE NET PLANKTON AND NANNOPLANKTON 



TO THE STANDING STOCKS AND PRIMARY PRODUCTIVITY IN 

 MONTEREY BAY, CALIFORNIA DURING THE UPWELLING SEASON 



David L. Garrison^ 



ABSTRACT 



Net plankton and nannoplankton standing stocks and primary production were measured in Mon- 

 terey Bay, Calif, from January through August 1972. Throughout the period of seasonal upwelling, 

 the phytoplankton stocks were dominated by net plankton. Both fractions showed seasonal changes: 

 the net plankton concentrations increased dramatically during upwelling, while nannoplankton 

 concentrations were decreased. Nannoplankton growth rates exceeded net plankton rates at in- 

 cubator light levels; however, at higher in situ light levels near the surface, this relationship ap- 

 peared to be reversed. 



Nannoplankton decreases may have been related to their selective removal from the area of up- 

 welling by horizontal advection or selective grazing on the nannoplankton fraction. Net plankton 

 dominance during upwelling has been related to their higher growth rates when populations are 

 retained in shallow nutrient-rich nearshore waters. 



Frequently, phytoplankton are divided into two 

 size classes, depending on whether they are re- 

 tained by fine mesh nets (net plankton) or pass 

 through the mesh (nannoplankton). The inade- 

 quacy of net collections for estimating standing 

 stocks or production is clear. The standing stocks 

 of the two fractions and their relative contribu- 

 tions to primary productivity, however, are less 

 well-known. The size distribution, which may be 

 environmentally controlled (Semina 1972; Par- 

 sons and Takahashi 1973), is an important fea- 

 ture of the phytoplankton populations because 

 the size of the primary producers may affect the 

 length and efficiency of pelagic food chains 

 (Ryther 1969; Parsons and LeBrasseur 1970). The 

 purpose of this study was to determine the rela- 

 tive importance of the two fractions during the 

 upwelling season in Monterey Bay, a neritic envi- 

 ronment of the California Current system. 



Most previous studies reported that the nan- 

 noplankton fraction usually exceeds the net 

 plankton fraction, often accounting for 80 to 

 100% of the standing stocks and primary produc- 

 tion (e.g., Steeman Nielsen and Jensen 1957; 

 Holmes 1958; Yentsch and Ryther 1959; Kawa- 

 mura 1961; Holmes and Anderson 1963; Teixeira 

 1963; Gilmartin 1964; Saijo 1964; Anderson 1965; 



'Moss Landing Marine Laboratories, Moss Landing, CA 

 95039; present address: Coastal Marine Laboratory, University 

 of California, Santa Cruz, CA 95064. 



Manuscript accepted September 1975. 

 FISHERY BULLETIN: VOL. 74. NO. 1. 1976. 



Saijo and Takesue 1965; Malone 1971a, c; Parsons 

 1972; McCarthy et al. 1974). Only a few authors 

 reported net plankton dominated communities 

 (Digby 1953; Subrahmanyan and Sarma 1965). It 

 is difficult to compare these studies, however, be- 

 cause mesh sizes of 22 to 110 ^im have been vari- 

 ously used to separate the net plankton and nan- 

 noplankton fractions. 



The nannoplankton fraction may show little 

 seasonal fluctuation, while the net plankton 

 shows pronounced seasonal trends with periods of 

 abundance corresponding to increased water 

 temperatures (Yentsch and Ryther 1959), peak 

 periods of primary production (Subrahmanyan 

 and Sarma 1965), or seasonal upwelling (Malone 

 1971c). Malone (1971a) reported higher net: 

 nanno ratios for standing stocks and production 

 in neritic environments as compared with oceanic 

 areas and pronounced onshore to offshore lower- 

 ing of the ratio in the California Current region 

 during upwelling (Malone 1971c). The growth 

 rate (as indicated by the assimilation ratio 

 = mg C mg Chi a"^ h"^) of the nannoplank- 

 ton fraction is greater than that of the net plankton 

 fraction (Yentsch and Ryther 1959; Saijo and 

 Takesue 1965; Malone 1971a, c). 



Arguments presented for the predominance of 

 net plankton or nannoplankton in a given envi- 

 ronment relate cell area to volume ratios (Malone 

 1971a, c; Eppley 1972; Parsons and Takahashi 

 1973). There is a general relationship between 



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