MALONE: RELATIVE IMPORTANCE OF NANNOPLANKTON AND NETPLANKTON 



March, the iietplankton maximum moved pro- 

 grressively toward the surface while the chloro- 

 phyll concentration of the maximum and in the 

 water column steadily decreased. If this change 

 in depth was due solely to the upward movement 

 of the nitrate-rich layer in the photic zone, some 

 increase in netplankton would have been ob- 

 served during the time taken for the maximum 

 to move from a depth of 75 m to 5 m. In addi- 

 tion, measurements made in the Peru Current, 

 where vertical advection was not in evidence and 

 the photic zone was well stratified (Malone, 

 in press a), support the hypothesis that upward 

 water movements, in addition to high nitrate 

 concentrations, are necessary prerequisites for 

 netijlankton productivity to approach or exceed 

 that of the nannoplankton. Netplankton pro- 

 ductivity and the net/nanno productivity ratio 

 were low despite high nitrate concentrations 

 (Figure 13). 



Two lines of evidence indicate that the net- 

 plankton and nannoplankton respond differently 

 to varying nitrate concentrations. The first is 



Figure 13. — Mean netplankton ( A ) and nannoplankton 

 ( G ) productivity as a function of mean NO3-N concen- 

 trations with 95':'i confidence limits: 0.1 ^M, offshore 

 oceanic region; 0..3 ju.M, CalCOFI 3, Oceanic Period; 

 1.5 fiM, CalCOFI 3, Mixed Period; 9.2 ^M, inshore up- 

 welling. 



based on the relationship between productivity 

 and nitrate concentrations encountered in dif- 

 ferent environments (Figui'elS). Nannoplank- 

 ton productivity increased rapidly as NO3-N in- 

 creased from about 0.0 to 0.5 (jlU. Above 0.5 

 fjLU nannoplankton productivity increased a- 

 symptotically. In contrast, netplankton produc- 

 tivity increased slowly over concentrations of 

 0.0 to 1.5 /AM and then increased rapidly with 

 concentrations in excess of 1.5 /uM (California 

 Current system). The netplankton, therefore, 

 tend to have higher half-saturation constants 

 and maximum uptake rates for nitrate than the 

 nannoplankton, so that NO3-N concentrations 

 above 1 to 3 /xu are necessary before the net- 

 plankton can effectively compete with the nan- 

 noplankton. This agrees with the results of 

 Maclsaac and Dugdale (1969) and Eppley et al. 

 (1969), which indicate that small-celled oceanic 

 species in oligotrophic waters have Ks values for 

 nitrate uptake of less than 0.5 jj-M while large- 

 celled neritic species in eutrophic waters have Ks 

 values greater than 1.0 ^iM. 



The observed inshore vertical distributions of 

 netplankton and nannoplankton chlorophyll 

 were also consistent with these observations. 

 The netplankton chlorophyll maximum was al- 

 ways found at depths where NO3-N concentra- 

 tions were greater than 2 /xm, while during non- 

 upwelling periods (when concentrations less 

 than 2 /aM were found in the photic zone) the 

 nannoplankton maximum occuri'ed at depths 

 where the NO.rN concentration was between 0.2 

 and 2.0 fiM. Similar observations were made by 

 Eppley (1970) who found that diatoms were as- 

 sociated with relatively high nitrate concentra- 

 tions at depths where light intensities were high 

 enough for growth to occur. 



Based on these observations, netplankton pro- 

 ductivity and standing crop will increase relative 

 to the nannoplankton only when NOa-N concen- 

 trations above 1 to 3 ;u,M are found in the upper 

 half of the photic zone and when the netplankton 

 standing crop is supported in the photic zone by 

 vertical advection, i.e., upwelling. 



Decreases in netplankton standing crop and 

 net/nanno ratios were related to influxes of 

 oceanic water and increases in grazing pressure 

 in Monterey Bay. Variations in phytoplankton 



815 



