154
J. E. a. RAYMONT
areas the small variation in light and temperature throughout the year
tends to maintain phytoplankton production at a relatively constant
level. Normally in tropical regions, the thermocline is permanent, and
therefore there can be little effective nutrient replenishment and little
variation in nutrient level in the euphotic zone throughout the year.
At all times, nutrient level is low and regeneration inside the euphotic
zone is a most important factor in the maintenance of primary production. Regions of upwelling or of current divergences are, of course, exceptional in that nutrients are brought near the surface and production
tends to be high. Apart from these regions, however, there may be
areas in warmer seas where the relatively small differences between
“summer” and “winter” temperatures are sufficient to cause further
stratification of the upper layers and the establishment of a seasonal
thermocline. If the cooling of the uppermost strata is sufficient and
there are also strong wind effects, some mixing may occur in the cooler
months leading to a small winter flowering of plankton. Menzel and
Ryther (1960; 1961) found areas of the Sargasso Sea near Bermuda
where such a winter blooming might occur depending on the degree of
vertical mixing. Maximum production occurred when the upper water .
was practically isothermal to ca. 400m, approximately the depth of the
permanent thermocline. When over the summer months a seasonal
thermocline was evident at about 100m. depth, production was reduced
(cf. Steele and Menzel, 1962). Hulburt et al. (1960) suggest that the
April flowering near Bermuda may be three or four times that of the
autumnlwinter level of production; summer production is even lower.
Farther south, in tropical waters, the stratification of the uppermost
layers is permanent; mixing cannot occur appreciably even throughout
the euphotic zone and so a winter blooming does not normally occur.
There is a tendency therefore in tropical areas for there to be relatively
little change in phytoplankton density over the year. Corcoran and
Alexander (1963) have suggested that there is little seasonal cycle in
Florida current waters, though they have recorded occasional high
levels of production and crops reaching even 0.5 mg chlorophyll alms.
Cushing (1959a and b) has pointed out from the results of earlier
workers such as Riley et al. (1949) and Bernard (1939) that for the
Sargasso and Mediterranean a slight seasonal fluctuation in phytoplankton crop with a more productive season in winter may be evident.
But close to the Equator the productive season is perhaps even earlier
and the seasonal fluctuations are less. Cushing suggests a seasonal amplitude of 5 times for the Sargasso Sea, whereas a fluctuatior, of some
50 times might be true for higher latitudes.
Zenkevitch (1963) has pointed out that although very high crops of
phytoplankton can occasionally be found in the coldest Arctic waters,
J. E. a. RAYMONT
areas the small variation in light and temperature throughout the year
tends to maintain phytoplankton production at a relatively constant
level. Normally in tropical regions, the thermocline is permanent, and
therefore there can be little effective nutrient replenishment and little
variation in nutrient level in the euphotic zone throughout the year.
At all times, nutrient level is low and regeneration inside the euphotic
zone is a most important factor in the maintenance of primary production. Regions of upwelling or of current divergences are, of course, exceptional in that nutrients are brought near the surface and production
tends to be high. Apart from these regions, however, there may be
areas in warmer seas where the relatively small differences between
“summer” and “winter” temperatures are sufficient to cause further
stratification of the upper layers and the establishment of a seasonal
thermocline. If the cooling of the uppermost strata is sufficient and
there are also strong wind effects, some mixing may occur in the cooler
months leading to a small winter flowering of plankton. Menzel and
Ryther (1960; 1961) found areas of the Sargasso Sea near Bermuda
where such a winter blooming might occur depending on the degree of
vertical mixing. Maximum production occurred when the upper water .
was practically isothermal to ca. 400m, approximately the depth of the
permanent thermocline. When over the summer months a seasonal
thermocline was evident at about 100m. depth, production was reduced
(cf. Steele and Menzel, 1962). Hulburt et al. (1960) suggest that the
April flowering near Bermuda may be three or four times that of the
autumnlwinter level of production; summer production is even lower.
Farther south, in tropical waters, the stratification of the uppermost
layers is permanent; mixing cannot occur appreciably even throughout
the euphotic zone and so a winter blooming does not normally occur.
There is a tendency therefore in tropical areas for there to be relatively
little change in phytoplankton density over the year. Corcoran and
Alexander (1963) have suggested that there is little seasonal cycle in
Florida current waters, though they have recorded occasional high
levels of production and crops reaching even 0.5 mg chlorophyll alms.
Cushing (1959a and b) has pointed out from the results of earlier
workers such as Riley et al. (1949) and Bernard (1939) that for the
Sargasso and Mediterranean a slight seasonal fluctuation in phytoplankton crop with a more productive season in winter may be evident.
But close to the Equator the productive season is perhaps even earlier
and the seasonal fluctuations are less. Cushing suggests a seasonal amplitude of 5 times for the Sargasso Sea, whereas a fluctuatior, of some
50 times might be true for higher latitudes.
Zenkevitch (1963) has pointed out that although very high crops of
phytoplankton can occasionally be found in the coldest Arctic waters,