FISHERY BLXLETIN: VOL. 70, NO. 4 



gant work in such problems is that of Lund and 

 colleagues on diatom succession in the English 

 lakes. Artificially mixing a lake in summer, 

 when it would normally be stratified, permitted 

 a bloom of Melosira italica, a diatom which usu- 

 ally sinks out of the water column upon the for- 

 mation of a thermocline in late spring (Lund, 

 1971). 



Another factor which tends to select against 

 large-celled species in low-nutrient waters re- 

 sults from a low surface 'volume ratio and a con- 

 sequent inability to absorb nutrients from low 

 concentration (Munk and Riley, 1952). This 

 generalization has been confirmed in laboratory 

 experiments on the kinetics of nutrient absorp- 

 tion where large-celled species showed higher 

 half-saturation constants (Ks) for nitrate and 

 ammonium uptake than small-celled species 

 (Eppley, Rogers, and McCarthy, 1969). Simi- 

 larly, the Ks for assimilation of vitamin B12 by 

 phytoplankton depends on cell size (Carlucci, 

 1972)." 



The argument with respect to netplankton vs. 

 nanoplankton and the expected seasonal changes 

 in assimilation number with temperature can be 

 summarized as follows: (1) Nanoplankton 

 show higher assimilation numbers (and growth 

 rates) than do netplankton. This generalization 

 results both from observations of natural phy- 

 toplankton and from studies of variations with 

 cell size in laboratory cultures. (2) Increasing 

 insolation in the spring results in increased 

 water temperatures, and often in stratification 

 and seasonal thermoclines. Nutrients in the 

 mixed layer then tend to be depleted and often 

 rather quickly, except in very shallow water 

 where regenerative activities of microorganisms 

 in sediments can maintain adequate nutrient 

 levels for rapid phytoplankton growth. (3) 

 Stratification of the water column tends to dis- 

 courage the growth of large-celled species and 

 long chain diatoms, because (a) reduced vertical 

 mixing may result in their sinking out of the 

 water column and (b) they are less effective in 



^ Carlucci, A. F. 1972. Saturation constants for 

 vitamin assimilation by phytoplankton. (Unpubl. 

 manuscr.) 



absorbing nutrients from low ambient concen- 

 trations than are nanoplankton. (4) Both sea- 

 sonal increase in temperature and in the ratio 

 of nanoplankton /'netplankton should increase 

 assimilation numbers for photosynthesis except 

 where growth and i^hotosynthetic rates are re- 

 duced by nutrient limitation. 



Nanoplankton would be expected to be more 

 abundant, relative to netplankton, in oligotro- 

 phic waters because of their lower sinking rates 

 and lower Ks values for nutrient absorption. 

 Hence, phytoplankton of warm, oligotrophic 

 tropical waters would be expected to show high 

 assimilation numbers (and growth rates) except 

 for effects of nutrient limitation. One can begin 

 to understand from all this why a graph of as- 

 similation number vs. temperature for observa- 

 tion of natural phytoplankton usually fails to 

 show the relationship expected from Figure 9, 

 and why so much current work emphasizes the 

 role of nutrient concentrations in phytoplankton 

 growth in the sea. 



Some exceptional marine waters which do 

 show the expected relationship betw^een assim- 

 ilation number and temperature are shallow 

 coastal estuaries where nutrient regeneration 

 on the bottom maintains a high nutrient input 

 to the overlying water. Examples reported for 

 the east coast of the United States are Barlow, 

 Lorenzen, and Myren (1963), Williams and 

 Murdoch (1966), and Mandelli et al. (1970). 

 Both of the latter papers show graphs of assim- 

 ilation number vs. temperature which match 

 beautifully the relation expected in Figure 9. 

 Williams and Murdoch's data fall between the 

 C/Chl. a 30 and 60 lines, with an indication of 

 higher C/Chl. a ratio in winter, as expected. 

 Mandelli et al. present two graphs, one for di- 

 atoms and the other for dinoflagellates. Assim- 

 ilation numbers of the latter are higher than 

 those for diatoms and fall on the line in Figure 9 

 for C/Chl. a = 30. They also show the seasonal 

 change in relative numbers of diatoms and dino- 

 flagellates; the latter are more abundant at high- 

 er temperatures. 



Williams and Murdoch (1966) cite several 

 other studies which show parallels between phy- 

 toplankton production in shallow marine waters 

 and temperature over the seasons. The Danish 



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