FISHERY BULLETIN: VOL. 69, NO. 1 



concentrations of 17 /.ig-at. /liter, but this latter 

 nitrogen source is probably not utilized by phy- 

 toplankton (Thomas, Renger, and Dodson, 

 in press). 



Prior to EASTROPAC (pre-1967) low ni- 

 trate/phosphate ratios in tropical Pacific poor 

 water suggested that nitrogen was a limiting 

 nutrient although ratios were increased when 

 ammonium was included along with nitrate, and 

 it was suggested that this latter nutrient alle- 

 viated N deficiency (Thomas, 1966). 



Recent EASTROPAC enrichment experi- 

 ments provided direct evidence for N limitation. 

 Phytoplankton growth occurred in experiments 

 where nutrients were added singly to sea water 

 samples only with N addition, and if N was de- 

 leted from an otherwise complete enrichment, 

 little or no growth resulted (Thomas, 1969, 

 1970b). The fact that photosynthetic assimi- 

 lation ratios were only slightly (but signifi- 

 cantly) decreased in poor water as compared 

 with rich water testified further to the allevi- 

 ation and control of deficiency by ammonium 

 (Thomas, 1970a). 



Having established which nutrient is com- 

 monly limiting, one can use a quantitative nu- 

 trient requirement in an appropriate math- 

 ematical model to estimate growth rates (pro- 

 duction) from concentration of the limiting nu- 

 trient. Recent work (Caperon, 1967; Dugdale, 

 1967) indicates that the best model is hyperbolic: 



(1) 



K, + S 

 where /j. is the phytoplankton specific growth 



rate, Mj 



is the maximum rate which is un- 



limited by low nutrient concentration, S is a 

 measured limiting nutrient concentration in sea 



water, and Kg is the "half-saturation constant" 

 — a nutrient concentration that supports a rate 

 equal to /:tma.x/2. This equation is equivalent 

 to the Michaelis-Menton formulation for enzyme 

 kinetics and was first applied to bacterial growth 

 rates by Monod (1942). Many biological pro- 

 cesses follow the hyperbolic model and since 

 growth is the result of a series of coupled en- 

 zymatic reactions, the hyperbolic model is the 

 model of choice. 



A previous paper (Thomas, 1970b) provides 

 information on /umax and Kg (for ammonium) 

 from which /i can be calculated. To obtain these 

 values we enriched samples of nutrient-poor Pa- 

 cific sea water from a depth of 10 m with a com- 

 plete mixture of non-nitrogenous nutrients to 

 which various concentrations of ammonium 

 were added. The samples were then incubated 

 in natural light approximating the intensity that 

 would be found at 10 m depth. Growth was es- 

 timated by successive daily measurements of 

 in vivo chlorophyll (Lorenzen, 1966) in each 

 treatment, and rates integrated over a daily peri- 

 od were calculated from the maximum increases 

 in chlorophyll. These rates were plotted against 

 ammonium concentrations to fit a hyperbolic 

 model and values of A',, and /xmax were obtained 

 from the plot. These values and their 95% con- 

 fidence limits are given in Table 1 for two such 

 experiments. Kg values can also be determined 

 from uptake experiments since recent work has 

 shown that A'., values for growth and uptake 

 are equivalent (Eppley and Thomas, 1969). 

 Also included in Table 1 are uptake Kg values 

 obtained by Maclsaac and Dugdale (1969) for 

 nutrient-poor tropical Pacific water. Their val- 

 ues for Vniax. the maximum uptake rate, are 

 not equivalent to /imax ^'id thus are not included 



Table 1. — Rate parameters for growth and uptake on ammonium in nutrient-poor tropical Pacific sea water. 



88 



