138
J . E. 0. RAYMONT
Under culture conditions it is possible by varying the amounts of
nitrate and phosphate in the medium to affect to some extent the composition of the phytoplankton crop. However, Redfield (1934) showed
early on that the N:P ratio in the sea approximates by atoms t o
16: 1, and though there are considerable variations in parts of the ocean,
the average phytoplankton in the oceans maintains the same ratio. In a
recent review Redfield, Ketchum, and Richards (1963) have shown that
despite low N: P ratios which may occur in Long Island Sound and south
of New York, the ratio in the phytoplankton still keeps fairly constant
until extremely low nitrogen values are reached. Jeffries (1 962) has also
called attention to remarkable variations in the N:P ratio in polluted
estuary waters, but phytoplankton composition remained probably
near normal. In nature therefore it seems that the algae assimilate only
as fast as the limiting nutrient is regenerated, and marked variations in
the N:P ratio of the phytoplankton are unlikely.
The remarkable growth of marine phytoplankton at such great dilutions of all nutrient requirements as normally occur in the sea is partly
explained by the microscopic size of the phytoplankton cells, which
allows better diffusion of nutrients and also confers a vastly greater
surface to volume ratio, thus promoting absorption. It is likely, however,
that to some extent the variations in the size of phytoplankton species,
and even within a species, are related to nutrient requirements. Braarud
(1962) suggests that Rhizosolenia styliformis and Thalassiothrix
longissim are mostly oceanic species of relative large cell size which
require favourable nutrient conditions. With the more coastal diatom
Skeletonem costatum, under satisfactory nutrient conditions there is
rapid cell division, followed by auxospore formation, which maintains a
relatively large cell size. As nutrients are used up, however, the cells
diminish in size and tend to sink; spore formation then commonly
follows. Margalef (1 958) has also studied the succession of phytoplankton
forms with particular reference to average cell size. He believes that in
a spring burst, growth commences mainly with small cell diatoms which
are capable of rapid division, but these are succeeded by medium-sized
species and finally by an increasing proportion of the motile phytoplankton forms with a lower rate of division. One factor in this “size succession” is believed to be changing nutrient concentration.
5. Miw Nutrients
So far only what are frequently called the major phytoplankton
nutrients, nitrate and phosphate, have been considered. Largely as the
result of laboratory experiment, it has become increasingly obvious that
a number of other elements normally present in trace concentrations in
sea water are also easential to healthy plant growth. Iron, manganese,
J . E. 0. RAYMONT
Under culture conditions it is possible by varying the amounts of
nitrate and phosphate in the medium to affect to some extent the composition of the phytoplankton crop. However, Redfield (1934) showed
early on that the N:P ratio in the sea approximates by atoms t o
16: 1, and though there are considerable variations in parts of the ocean,
the average phytoplankton in the oceans maintains the same ratio. In a
recent review Redfield, Ketchum, and Richards (1963) have shown that
despite low N: P ratios which may occur in Long Island Sound and south
of New York, the ratio in the phytoplankton still keeps fairly constant
until extremely low nitrogen values are reached. Jeffries (1 962) has also
called attention to remarkable variations in the N:P ratio in polluted
estuary waters, but phytoplankton composition remained probably
near normal. In nature therefore it seems that the algae assimilate only
as fast as the limiting nutrient is regenerated, and marked variations in
the N:P ratio of the phytoplankton are unlikely.
The remarkable growth of marine phytoplankton at such great dilutions of all nutrient requirements as normally occur in the sea is partly
explained by the microscopic size of the phytoplankton cells, which
allows better diffusion of nutrients and also confers a vastly greater
surface to volume ratio, thus promoting absorption. It is likely, however,
that to some extent the variations in the size of phytoplankton species,
and even within a species, are related to nutrient requirements. Braarud
(1962) suggests that Rhizosolenia styliformis and Thalassiothrix
longissim are mostly oceanic species of relative large cell size which
require favourable nutrient conditions. With the more coastal diatom
Skeletonem costatum, under satisfactory nutrient conditions there is
rapid cell division, followed by auxospore formation, which maintains a
relatively large cell size. As nutrients are used up, however, the cells
diminish in size and tend to sink; spore formation then commonly
follows. Margalef (1 958) has also studied the succession of phytoplankton
forms with particular reference to average cell size. He believes that in
a spring burst, growth commences mainly with small cell diatoms which
are capable of rapid division, but these are succeeded by medium-sized
species and finally by an increasing proportion of the motile phytoplankton forms with a lower rate of division. One factor in this “size succession” is believed to be changing nutrient concentration.
5. Miw Nutrients
So far only what are frequently called the major phytoplankton
nutrients, nitrate and phosphate, have been considered. Largely as the
result of laboratory experiment, it has become increasingly obvious that
a number of other elements normally present in trace concentrations in
sea water are also easential to healthy plant growth. Iron, manganese,