160
J . E . G . R A Y M O N T
1960; 1961). Hulburt, Ryther, and Guillard (1960) found in the seas off
Bermuda that the phytoplankton at 50m depth was always richer than
at the surface; usually different species were represented also at the two
levels. Several recent investigations indicate, indeed, that the phytoplankton maximum in warmer waters may be near the bottom of the
euphotic zone. In temperate latitudes during winter the photosynthetic
zone is extremely shallow; there may be surface algal maxima temporarily, but mixing is usually active. In spring and summer the
phytoplankton may by no means be equally distributed, and with
surface inhibition, there may be a maximum somewhat below the
surface (cf. Anderson, 1964). Sometimes, where a marked thermocline
occurs, maximum density of algae may be associated with this layer.
For example, in the Black Sea a thermocline occurs at a depth of about
20m and at this level there is a maximum of phytoplankton although
the total depth of the euphotic zone approaches 60m (Sorokin, 1964a).
Steemann Nielsen (1964a) has drawn attention to the importance of the
vertical distribution of phytoplankton throughout the euphotic zone in
relation to production under a unit of surface. In tropical latitudes, or in
temperate latitudes during bright summer days, overall production
may be markedly increased by a greater density of phytoplankton at
somewhat deeper levels. By contrast, at temperate latitudes with
average or poor light conditions, production may be increased by a
more or less regular distribution of phytoplankton or even by having
algae massed nearer the surface. In the tropics, if plankton were equally
distributed from the surface throughout the euphotic zone, the rate of
production would be maximal at some intermediate depth (ca. 20m).
Only with a reduction of incident light to 50% throughout the day
would maximal production approach the surface.
V. GRAZING BY ZOOPLANKTON
Although the standing crop of phytoplankton is closely related to
overall production beneath a square metre of surface, the size of standing crop may be at times misleading since the quantity of algae removed by grazing, essentially by the zooplankton, is not taken into
consideration. Harvey et al. (1935) drew attention to the remarkable
reduction in phytoplankton crop immediately following the spring
outburst in English Channel waters and noted that this was accompanied by marked grazing activity of the increasing zooplankton
population. Over the summer, Harvey et al. concluded that although
replenishment of nutrients from below the thermocline limited production, the generally rather low, but fluctuating, phytoplankton crop waa
largely determined by the rate of grazing of the zooplankton. Regeneration of nutrients within the euphotic zone was also important. Clarke
J . E . G . R A Y M O N T
1960; 1961). Hulburt, Ryther, and Guillard (1960) found in the seas off
Bermuda that the phytoplankton at 50m depth was always richer than
at the surface; usually different species were represented also at the two
levels. Several recent investigations indicate, indeed, that the phytoplankton maximum in warmer waters may be near the bottom of the
euphotic zone. In temperate latitudes during winter the photosynthetic
zone is extremely shallow; there may be surface algal maxima temporarily, but mixing is usually active. In spring and summer the
phytoplankton may by no means be equally distributed, and with
surface inhibition, there may be a maximum somewhat below the
surface (cf. Anderson, 1964). Sometimes, where a marked thermocline
occurs, maximum density of algae may be associated with this layer.
For example, in the Black Sea a thermocline occurs at a depth of about
20m and at this level there is a maximum of phytoplankton although
the total depth of the euphotic zone approaches 60m (Sorokin, 1964a).
Steemann Nielsen (1964a) has drawn attention to the importance of the
vertical distribution of phytoplankton throughout the euphotic zone in
relation to production under a unit of surface. In tropical latitudes, or in
temperate latitudes during bright summer days, overall production
may be markedly increased by a greater density of phytoplankton at
somewhat deeper levels. By contrast, at temperate latitudes with
average or poor light conditions, production may be increased by a
more or less regular distribution of phytoplankton or even by having
algae massed nearer the surface. In the tropics, if plankton were equally
distributed from the surface throughout the euphotic zone, the rate of
production would be maximal at some intermediate depth (ca. 20m).
Only with a reduction of incident light to 50% throughout the day
would maximal production approach the surface.
V. GRAZING BY ZOOPLANKTON
Although the standing crop of phytoplankton is closely related to
overall production beneath a square metre of surface, the size of standing crop may be at times misleading since the quantity of algae removed by grazing, essentially by the zooplankton, is not taken into
consideration. Harvey et al. (1935) drew attention to the remarkable
reduction in phytoplankton crop immediately following the spring
outburst in English Channel waters and noted that this was accompanied by marked grazing activity of the increasing zooplankton
population. Over the summer, Harvey et al. concluded that although
replenishment of nutrients from below the thermocline limited production, the generally rather low, but fluctuating, phytoplankton crop waa
largely determined by the rate of grazing of the zooplankton. Regeneration of nutrients within the euphotic zone was also important. Clarke