134
J . E . Q. RAYMONT
warmer period of the year in temperate and high latitudes. Temperature
also plays a part in the species succession of phytoplankton, especially
in temperate latitudes. Thus, Thulassiosira nordenskioldii is an important diatom a t the very beginning of the spring increase in the temperate northern Atlantic of America and Europe. Later in the spring,
species of Chuetoceros and particularly Skeletonema take over. It has long
been considered that Thulussiosira is favoured by the lower temperatures at the start of the spring increase, and that the slight rise in
temperature later on is largely responsible for the subsequent flowering
of Skeletonema. Even this succession, however, may not be entirely a
result of temperature, since Braarud (1962) has suggested that Thalassiosira has very high nutrient demands which would be satisfied only
at the beginning of the spring increase. But temperature has a part to
play in species succession. The abundance of peridinians, mainly in the
summer period, in temperate latitudes, is thought to be associated with
the generally higher temperature requirements of these algae. To some
extent this agrees with Braarud's (1961) observation that for several
dinoflagellates temperature optima are relatively high. Species succession in phytoplankton, however, is by no means limited to temperate
latitudes; it is found clearly in Arctic waters (e.g. Digby, 1953; Bursa,
1963; Holmes, 1956); it is true of Antarctic waters (Hart, 1934), and is
found also in warmer, subtropical waters (e.g. Riley, 1957; Hulburt,
Ryther and Guillard, 1960). Thus although temperature may play a part,
it is undoubtedly not the only factor in the succession of phytoplankton
forms; the rapid take over from one species to another and the fact that
one species may flower at two different periods in the year when temperature conditions are considerably different, points to the collective
action of several factors (cf. Smayda, 1963).
3. Salinity
Other factors beside light and temperature may influence primary
production. Salinity variations can be shown experimentally to have an
effect on photosynthetic rate. Curl and McLeod found Skeletonema had
an optimum rate of photosynthesis at salinities ranging between 15 and
20%,, though the process could go on over as wide a range as 11 to 40%,.
Braarud (1961) demonstrated that some species of dinoflagellates,
Ceratium, Peridinium, Prorocentrum, reproduce more actively at
lowered salinities. Provasoli and McLaughlin (1963) have shown t8hat
Peridinium balticum and Peridinium chattoni are even stenohaline
brackish water forms with an optimum range of only 8 to 12%" for
photosynthesis. By contrast, Exuviellu is a common brackish water
form, but has very wide salinity limits, occurring a t salinities of 8 t o
35%"; the optimal salinity for photosynthesis is about 20 bo 25%,.
J . E . Q. RAYMONT
warmer period of the year in temperate and high latitudes. Temperature
also plays a part in the species succession of phytoplankton, especially
in temperate latitudes. Thus, Thulassiosira nordenskioldii is an important diatom a t the very beginning of the spring increase in the temperate northern Atlantic of America and Europe. Later in the spring,
species of Chuetoceros and particularly Skeletonema take over. It has long
been considered that Thulussiosira is favoured by the lower temperatures at the start of the spring increase, and that the slight rise in
temperature later on is largely responsible for the subsequent flowering
of Skeletonema. Even this succession, however, may not be entirely a
result of temperature, since Braarud (1962) has suggested that Thalassiosira has very high nutrient demands which would be satisfied only
at the beginning of the spring increase. But temperature has a part to
play in species succession. The abundance of peridinians, mainly in the
summer period, in temperate latitudes, is thought to be associated with
the generally higher temperature requirements of these algae. To some
extent this agrees with Braarud's (1961) observation that for several
dinoflagellates temperature optima are relatively high. Species succession in phytoplankton, however, is by no means limited to temperate
latitudes; it is found clearly in Arctic waters (e.g. Digby, 1953; Bursa,
1963; Holmes, 1956); it is true of Antarctic waters (Hart, 1934), and is
found also in warmer, subtropical waters (e.g. Riley, 1957; Hulburt,
Ryther and Guillard, 1960). Thus although temperature may play a part,
it is undoubtedly not the only factor in the succession of phytoplankton
forms; the rapid take over from one species to another and the fact that
one species may flower at two different periods in the year when temperature conditions are considerably different, points to the collective
action of several factors (cf. Smayda, 1963).
3. Salinity
Other factors beside light and temperature may influence primary
production. Salinity variations can be shown experimentally to have an
effect on photosynthetic rate. Curl and McLeod found Skeletonema had
an optimum rate of photosynthesis at salinities ranging between 15 and
20%,, though the process could go on over as wide a range as 11 to 40%,.
Braarud (1961) demonstrated that some species of dinoflagellates,
Ceratium, Peridinium, Prorocentrum, reproduce more actively at
lowered salinities. Provasoli and McLaughlin (1963) have shown t8hat
Peridinium balticum and Peridinium chattoni are even stenohaline
brackish water forms with an optimum range of only 8 to 12%" for
photosynthesis. By contrast, Exuviellu is a common brackish water
form, but has very wide salinity limits, occurring a t salinities of 8 t o
35%"; the optimal salinity for photosynthesis is about 20 bo 25%,.