THOMAS and OWEN; ESTIMATING PHVTOPLANKTON PRODUCTION 



in Table 1. It will be noted that confidence lim- 

 its for Ks values in given experiments are large 

 as is the confidence limit for the mean of all five 

 values which is used in subsequent calculations 

 (see Results and Discussion). This can be at- 

 tributed to lack of precision in measuring either 

 growth or uptake; even in controlled experi- 

 ments with laboratory cultures, A'., values are 

 imprecise (Eppley, Rogers, and McCarthy, 

 1969; Eppley and Thomas, 1969). 



The integrated daily growth rate, fi, can also 

 be calculated from ^^C production estimates and 

 chlorophyll concentrations using the following 

 equation: 



3'.32 [log,o(/? • chl + Prod ) - logio(/? • chl)] 



^ = 



1 day 



(2) 



as has been done for laboratory cultures by 

 Thomas (1964) and McAllister, Shah, and 

 Strickland (1964). In this equation R is the 

 carbon/chlorophyll ratio; R  chl thus is the 

 standing stock of phytoplankton carbon. The 

 constant 3.32 converts logarithms to the base 10 

 to logarithms to the base 2 and allows /x. to be 

 expressed as doublings of phytoplankton carbon 

 per day. 



In the previous paper (Thomas, 1970b), ^ 

 calculated from ammonium (equation 1) was 

 compared with /j, calculated from l^c production 

 and chlorophyll (equation 2) for the two EAS- 

 TROPAC stations where Ksand /^max were de- 

 termined from enrichment experiments. At 

 station 76.007, /x calculated from ammonium was 

 0.385 doublings/day while that calculated from 

 •^■^C uptake and chlorophyll was 0.365 doublings/ 

 day. At station 76.173 both values were iden- 

 tical — 0.276 doublings/day. For the calcula- 

 tion we used an R value of 98, that found by 

 Eppley (1968) for nitrate-free water off La 

 Jolla. 



This excellent agreement suggested that we 

 could set equation (1) equal to equation (2) and 

 solve for production as a function of ammonium 

 and chlorophyll using A's and fimax ^s constants. 

 The new equation thus derived is /q\ 



Prod = chl • 



R antilog [^max 

 L \3.32 



3.32 Ks+ S 



1 



This expression allows a direct comparison cal- 

 culated and measured ^'*C production (see Re- 

 sults and Discussion). 



METHODS 



Methods for determining A's and ^ma.x were 

 given previously (Thomas, 1970b; Maclsaac 

 and Dugdale, 1969) — see also the previous sec- 

 tion. Chlorophyll and production samples were 

 taken from the depth of the 50 % light level, 

 which was always in the upper mixed layer 

 and varied from 9 to 16 m. This depth was 

 determined by multiplying the depth at which 

 the Secchi disc disappeared by 0.38. This 

 factor employs the assumption that the Secchi 

 disc disappears at 16 % of surface light in- 

 tensity (Strickland, 1958). 



Chlorophyll was determined in these samples 

 by filtration on glass fiber filters, followed by 

 90 % acetone extraction of the filters, and mea- 

 surement of fluorescence of the extract ( Yentsch 

 and Menzel, 1963; Holm-Hansen, Lorenzen, 

 Holmes, and Strickland, 1965) using equations 

 developed by Lorenzen (1966). 



Simulated in s'itw production was measured 

 by adding 20 ^c Na^^'^COs solution to the 

 samples (Steemann Nielsen, 1952) and incu- 

 bating them in a tubular shipboard incubator 

 space in which natural light intensity was at- 

 tenuated to 50 % of that incident. Incubation 

 was started at noon and continued until sunset 

 at sea surface temperature. Following incuba- 

 tion the samples were filtered through HA Mil- 

 lipore®' filters and their radioactivity assayed 

 ashore by G-M counting of the filters. The l^c 

 solution was standardized by liquid scintillation 

 counting and the efficiency of the G-M counter 

 for these filters was determined by combusting 

 some of these and measuring the evolved ^^C02 

 with an ionization chamber. Daily uptake was 

 determined by multiplying the activity by 2; 

 we also corrected for the isotope effect by mul- 

 tiplying by 1.05. Darkened samples were incu- 

 bated with illuminated samples and dark uptake 

 was subtracted from light uptake. No cor- 



° The use of trade names is merely to facilitate de- 

 scriptions: no endorsement is implied. 



89 



