CCHCADIAN 1'RRTOI~IC"CKF:H I X PAYTOPLANKTON
353
the same area, according to other workers (Goering et al., l964), did
not reveal any clear die1 pattern ; in an estuary, Lorenzen (1963) found
higher coiiarnt,r.;btioiis t i t 03.00 mcl lower ones a t 12.30 h ; a nocturnril
increase WLR ~ l s o observed in tin upwclling weiL by Herhli~iitl d nl.
( 1 973) who assigned it to physical processes. Nitrite vtLriiitions, it
trppcars, have not beon investiptcd. As for ammonium, surface va1uc.s
from the Sargitsso sea (Goering et aE., 1964) are not apparently periodic :
in this areti, mean values for the euphotic layer behaved as follows
(Beers and Kelly, 1965, according to their illustration) : minimum at
midnight,, increase until 06.00 or 10.00 h, decrease until midnight.
I n lsboratory cultures of Skeletonerrm, both N-NO, and N-NH, contents
of the medium increased in the dark and declined in the light while no
viiriutions wort? c~pparent in experiments on Coccolithux (Eppley ut al.,
1971).
Meagre though this information is, one could deduce tJhat assimilation by phytoplankton occurs preferentially in the light, at least in the
case of nitrate; physiological studies (e.g. Grant and Turner, 1969)
support this idea. In this connection, Eppley's group has on two
occasions computed assimilation rates from changes in N-NO, and
N-NH, concentrations of the medium ; diel periodicity was noted in
Skdetonemn, uptakes being the higher during the first half of the light,
period and the lower in the dark (Eppley et aZ., 1971) ; a similar cycle,
with a noon milximum, occurred in shipboard cultures (Eppley et al.,
1972).
A niore dircbct approach consish in I5N-labelled techniques, first
I I H C ~
in a diel study by Goering rt al. (1 964) who stated that ". . . the
major portion of the daily nitrogen iiptttke of the cell may occur in
t h e very etwly hours of the morning " : this may be true when N-XO,
and N-NH, light uptakes minus dark uptakes are calculated relative
to particulate nitrogen (thcir Fig. l), butj the variations become somewhat, erratic fbr light uptakes or uptuke rrrten relative to sample
volume ; some oii~~ves t)eiir p r t i a l correspondence with nitrogen content
of sea water. More recent, experiments by Eppley et al. (1970) in the
Peru iiywclling resulted in an inmeusing rate from the beginning of the
iiight to the end of the day for both nitrate and ammonium. Malone
(1971), coniparing these two exikmples, considers that maximum uptirkes were obtiiintd during thr morning in oligotrophic waters and
during the afternoon in the eutrophic area, and he 8uggest.s that different
kinchtics are operating in each case according to the size compoRition
of phytoplankton.
Enzyme activities also have been studied and proved to ontlergo
circadian changes. Nitrate reductase in natural populations showed
353
the same area, according to other workers (Goering et al., l964), did
not reveal any clear die1 pattern ; in an estuary, Lorenzen (1963) found
higher coiiarnt,r.;btioiis t i t 03.00 mcl lower ones a t 12.30 h ; a nocturnril
increase WLR ~ l s o observed in tin upwclling weiL by Herhli~iitl d nl.
( 1 973) who assigned it to physical processes. Nitrite vtLriiitions, it
trppcars, have not beon investiptcd. As for ammonium, surface va1uc.s
from the Sargitsso sea (Goering et aE., 1964) are not apparently periodic :
in this areti, mean values for the euphotic layer behaved as follows
(Beers and Kelly, 1965, according to their illustration) : minimum at
midnight,, increase until 06.00 or 10.00 h, decrease until midnight.
I n lsboratory cultures of Skeletonerrm, both N-NO, and N-NH, contents
of the medium increased in the dark and declined in the light while no
viiriutions wort? c~pparent in experiments on Coccolithux (Eppley ut al.,
1971).
Meagre though this information is, one could deduce tJhat assimilation by phytoplankton occurs preferentially in the light, at least in the
case of nitrate; physiological studies (e.g. Grant and Turner, 1969)
support this idea. In this connection, Eppley's group has on two
occasions computed assimilation rates from changes in N-NO, and
N-NH, concentrations of the medium ; diel periodicity was noted in
Skdetonemn, uptakes being the higher during the first half of the light,
period and the lower in the dark (Eppley et aZ., 1971) ; a similar cycle,
with a noon milximum, occurred in shipboard cultures (Eppley et al.,
1972).
A niore dircbct approach consish in I5N-labelled techniques, first
I I H C ~
in a diel study by Goering rt al. (1 964) who stated that ". . . the
major portion of the daily nitrogen iiptttke of the cell may occur in
t h e very etwly hours of the morning " : this may be true when N-XO,
and N-NH, light uptakes minus dark uptakes are calculated relative
to particulate nitrogen (thcir Fig. l), butj the variations become somewhat, erratic fbr light uptakes or uptuke rrrten relative to sample
volume ; some oii~~ves t)eiir p r t i a l correspondence with nitrogen content
of sea water. More recent, experiments by Eppley et al. (1970) in the
Peru iiywclling resulted in an inmeusing rate from the beginning of the
iiight to the end of the day for both nitrate and ammonium. Malone
(1971), coniparing these two exikmples, considers that maximum uptirkes were obtiiintd during thr morning in oligotrophic waters and
during the afternoon in the eutrophic area, and he 8uggest.s that different
kinchtics are operating in each case according to the size compoRition
of phytoplankton.
Enzyme activities also have been studied and proved to ontlergo
circadian changes. Nitrate reductase in natural populations showed