CIRCADIAN PEHIO1)IClTIES I N PHYTOPLANKTON
367
are to be investigated and evltliiatcd in relation to taxonomic position
and environrncntitl factors (the prencnt review has pointed out some
cvident gaps).
MoNt of thc prcscnt Itnowledge comcs from isolltted data, in other
words from studics dovotcd to i t single or a few parameters. Synthetic
approtichen, in which a number of physical, chemical und biological
pitramet8crs are measured, will open new views, since all the variations
are obviously interrelated. It is in such a synecological perspective
that phyt>oplctnkton ciiii be expected to contribute the most to the
study of biologicnl rhythms.
XV 11. SUMMARY
The deit'ii on circ:idinti (did) variations and rhythms in natural
populations of marine (occasionally freshwater) phytoplankton are
reported ctnti discussed with reference to laboratory work and, more
generally, to itigtt1 physiology nnd chronobiology.
(:enereition times of phytoplankters range from a few hours to
scveral days ; i i 24-hour lifetime can be retained as an average or an
order of magnitude. Some degree of synchrony is commonly obtained
in cultures and itpparently occurs in nature, though no generalization
about the timing of cell division can be made in either case. The
general trend of vertioal migrations consists of a morning or diurnal
upward movenient and an afternoon or nocturnal downward one ;
mainly phototttxis is involved but other factors interfere, such as
pitssivc settling (the rtth of which seems to pmallel the migration
pattern). L;iborntor.y tts well as field studies display R variety of die1
curves for chlorophyll conccntrat ion which happens to be, as suggested
frotn physiologi(:iil data, one of the worst indicators of phytoplankton
periodicity ; ncvcrtheless, for a given sct of conditions, a definite trend
is observed and i t circiidittn corrwtion may be worked out. The rhythm
of phot,osynthetic cspwity, or I )oty arid Oguri's rhythm, is identifiod
with the physiologists's " afternoon depression " ; fundamentally, it
reflects n periodic photosynthesizing capability of a given hiomaw, in
spibe of t lit! doggcd assumption that chlorophyll variations were rcsponsible : its implications on the measurement of primary production
are discussed. Surprisingly, such an afternoon depression is but seldom
exhibited undctr naturttl illumination ; environmental factors, light
adaptation and taxonomic composition add further complexity to the
ntlturcil cmrse of the photosynthetic periodicity. Little or nothing is
known about raitlier respiration or excretion. Phosphorus and nitrogen consumpt,ion tnkes plnc!r primarily during daylight ; simultaneously,
nutrient concentrations in seawater decrease from sunrise to sunset
367
are to be investigated and evltliiatcd in relation to taxonomic position
and environrncntitl factors (the prencnt review has pointed out some
cvident gaps).
MoNt of thc prcscnt Itnowledge comcs from isolltted data, in other
words from studics dovotcd to i t single or a few parameters. Synthetic
approtichen, in which a number of physical, chemical und biological
pitramet8crs are measured, will open new views, since all the variations
are obviously interrelated. It is in such a synecological perspective
that phyt>oplctnkton ciiii be expected to contribute the most to the
study of biologicnl rhythms.
XV 11. SUMMARY
The deit'ii on circ:idinti (did) variations and rhythms in natural
populations of marine (occasionally freshwater) phytoplankton are
reported ctnti discussed with reference to laboratory work and, more
generally, to itigtt1 physiology nnd chronobiology.
(:enereition times of phytoplankters range from a few hours to
scveral days ; i i 24-hour lifetime can be retained as an average or an
order of magnitude. Some degree of synchrony is commonly obtained
in cultures and itpparently occurs in nature, though no generalization
about the timing of cell division can be made in either case. The
general trend of vertioal migrations consists of a morning or diurnal
upward movenient and an afternoon or nocturnal downward one ;
mainly phototttxis is involved but other factors interfere, such as
pitssivc settling (the rtth of which seems to pmallel the migration
pattern). L;iborntor.y tts well as field studies display R variety of die1
curves for chlorophyll conccntrat ion which happens to be, as suggested
frotn physiologi(:iil data, one of the worst indicators of phytoplankton
periodicity ; ncvcrtheless, for a given sct of conditions, a definite trend
is observed and i t circiidittn corrwtion may be worked out. The rhythm
of phot,osynthetic cspwity, or I )oty arid Oguri's rhythm, is identifiod
with the physiologists's " afternoon depression " ; fundamentally, it
reflects n periodic photosynthesizing capability of a given hiomaw, in
spibe of t lit! doggcd assumption that chlorophyll variations were rcsponsible : its implications on the measurement of primary production
are discussed. Surprisingly, such an afternoon depression is but seldom
exhibited undctr naturttl illumination ; environmental factors, light
adaptation and taxonomic composition add further complexity to the
ntlturcil cmrse of the photosynthetic periodicity. Little or nothing is
known about raitlier respiration or excretion. Phosphorus and nitrogen consumpt,ion tnkes plnc!r primarily during daylight ; simultaneously,
nutrient concentrations in seawater decrease from sunrise to sunset