CIRCADIAN PRRIOI)I(IITTES JN PHYTOPLANKTON
347
n. Mechanisms
The recognition of 11 rhythm of‘ photosynthetic capcit,y is inore
than iTi years old, in the field of planktjology at least. What do we
know today about its mechanisms!
It could be assumed first that this periodicity was related in some
wily to the abundance of phytoplankton: this was indeed the more
cautious hypothesis to start with ; however, it, persisted for a long time
in spite of conflicting evidence which soon accumulated. Doty and
Oguri ( 1957) had initidly excluded the oxplanittion by vertical migrations of phytoplankters, but the effects of zooplankton migration and
grazing hitve been often implicated (eg. unpubl. paper by Hnnsen,
t9fi2; McAllister, 1963; Newhouse et al., 1967). More attention has
focused on concomitnnt changes in chlorophyll concentration-the
latter bcing thc most commonly used parameter of phytoplankton
biomass ; this point deserves some development here.
In an early die1 study, Yentsch and Ryther (1967) showed, by
chance so to speak, parallel changes of chlorophyll content and carbon
uptake, and they elevated the correlation to an explanation (see also
Yentsch and Sougel, 1968). Parallelism or similarities between chlorophyll and ciirbon uptake could also be noted by 8himada (1968),
Lorenxen (IW3), Goering et al. (1 964) arid Honjo and Hanaoka (lees),
though the :mplitude was higher for photosynthetic capacity than
for pigment concentrations ; for instance, it appears from Shimada’s
data that ‘‘ maxims and minima differed for productivity by a factor
of about five, arid for chlorophyll n by a fitctor of about two ” (Blackburn, 1966). More frequently chlorophyll variations are aperiodic or
undetectable or even opposite, to assimilation rntee which, by contrast,
always follow the general trend its defined above (Doty and Oguri, 1967 :
footnote ; Holmos and Haxo, I958 ; Angot, 1961 ; McAllister, 1963).
Thus a periodicity in the ratio photooNynttiesis/chlorophyll or “ Assimilation number ’’ or “ Assirnilationszahl ” (mg C/h/mg chl. u ) was mentioned, rather briefly or hypothetically, by some workers (Ichimura,
1958; Ohle, 1961 ; Lorenzen, 1963; McAllister, 1963; Strickland, 1965;
Newhouse et nl., 1067 ; Eppley et al., I968b) ; others clearly pointed out
the importmice and significance of such findings : the rhythm of photosynthetic capacity is paralleled by a rhythm of photosynthe& per unit
of chlorophyll a,(Fig. 4) or, in other words, it is independent, to a large
extent, of chlorophyll concentration (unpuhl. paper by Hunwn, 1962 ;
Endo, 1967, 1970; Newhouse, 1967; Sournia, 1967, 1973; Honjo and
Hanaoka, 1969). Laboratory cultures add further confirmation (Hasti n g ~ et aE., 1961 ; Yentsch and Reichert, 1063; Palmer d al., 1964;
347
n. Mechanisms
The recognition of 11 rhythm of‘ photosynthetic capcit,y is inore
than iTi years old, in the field of planktjology at least. What do we
know today about its mechanisms!
It could be assumed first that this periodicity was related in some
wily to the abundance of phytoplankton: this was indeed the more
cautious hypothesis to start with ; however, it, persisted for a long time
in spite of conflicting evidence which soon accumulated. Doty and
Oguri ( 1957) had initidly excluded the oxplanittion by vertical migrations of phytoplankters, but the effects of zooplankton migration and
grazing hitve been often implicated (eg. unpubl. paper by Hnnsen,
t9fi2; McAllister, 1963; Newhouse et al., 1967). More attention has
focused on concomitnnt changes in chlorophyll concentration-the
latter bcing thc most commonly used parameter of phytoplankton
biomass ; this point deserves some development here.
In an early die1 study, Yentsch and Ryther (1967) showed, by
chance so to speak, parallel changes of chlorophyll content and carbon
uptake, and they elevated the correlation to an explanation (see also
Yentsch and Sougel, 1968). Parallelism or similarities between chlorophyll and ciirbon uptake could also be noted by 8himada (1968),
Lorenxen (IW3), Goering et al. (1 964) arid Honjo and Hanaoka (lees),
though the :mplitude was higher for photosynthetic capacity than
for pigment concentrations ; for instance, it appears from Shimada’s
data that ‘‘ maxims and minima differed for productivity by a factor
of about five, arid for chlorophyll n by a fitctor of about two ” (Blackburn, 1966). More frequently chlorophyll variations are aperiodic or
undetectable or even opposite, to assimilation rntee which, by contrast,
always follow the general trend its defined above (Doty and Oguri, 1967 :
footnote ; Holmos and Haxo, I958 ; Angot, 1961 ; McAllister, 1963).
Thus a periodicity in the ratio photooNynttiesis/chlorophyll or “ Assimilation number ’’ or “ Assirnilationszahl ” (mg C/h/mg chl. u ) was mentioned, rather briefly or hypothetically, by some workers (Ichimura,
1958; Ohle, 1961 ; Lorenzen, 1963; McAllister, 1963; Strickland, 1965;
Newhouse et nl., 1067 ; Eppley et al., I968b) ; others clearly pointed out
the importmice and significance of such findings : the rhythm of photosynthetic capacity is paralleled by a rhythm of photosynthe& per unit
of chlorophyll a,(Fig. 4) or, in other words, it is independent, to a large
extent, of chlorophyll concentration (unpuhl. paper by Hunwn, 1962 ;
Endo, 1967, 1970; Newhouse, 1967; Sournia, 1967, 1973; Honjo and
Hanaoka, 1969). Laboratory cultures add further confirmation (Hasti n g ~ et aE., 1961 ; Yentsch and Reichert, 1063; Palmer d al., 1964;