THE PRODUCTION O F MARINE FLANKTON
161
(1939) also believed that over the productive periods of phytoplankton
growth in temperate latitudes, the zooplankton was largely responsible
for regulating the size of plant population. The importance of grazing
activity had been noted in Arctic waters by Braarud (1935), by Bigelow
et al. (1940) for the Gulf of Maine, by Wimpeiiny (1936, 1938) for the
North Sea, by Holmes (1956) in the Labrador Sea, by Hart (1942) for
the Antarctic, and in other areas. Grazing intensity of course may
vary, especially at high latitudes. Thus Halldal (1953) considers that in
the Norwegian Sea it was much more intensive in spring than later in
the year. In very productive areas, such as Long Island Sound (Riley,
1956; 1959), Block Island Sound (Riley, 1952) and Tisbury Great Pond
(Deevey, 1948), the phytoplankton crop may be rich, and grazing
appears to exert little influence on the density of the algae (cf. also
Gross et al., 1947). Despite these exceptions, the effect of the herbivorous
zooplankton - appendicularians and salps, a majority of copepod
species, many euphausids, the shelled pteropods, cladocerans and many
meroplanktonic larvae - on the phytoplankton crop can be remarkable.
The rapid reduction in phytoplankton crop seen in temperate and high
latitudes may therefore not be a result of nutrient lack but of grazing
activity.
Cushing et al. (1963) deal with the relations between a Calanus
population feeding on a phytoplankton burst in the North Sea over a
period from March to June. From a comparison of the crop of phytoplankton present and the estimated rate of algal production, he believes
that grazing is the dominant cause of algal mortality; there was no
evidence of a lack of nutrients during the erdy decline in the algal
population. Grazing is believed to be the niajor controlling factor,
according to Cushing, in the temperate spring outburst of phytoplankton. With intensive grazing there is considerable regeneration of
nutrients in the euphotic zone, and only when the rate of regeneration
decreases does the reduction in concentration of nutrients due to the
thermocline becoming a limiting factor.
Harvey et al. (1935) obtained a minimal estimate of overall production
of phytoplankton from the reduction in nutrier.t level over the period of
the spring increase. For 1933, density was estimated at 85 000 plant
pigment units/m3,* whereas the actual value of the standing crop was
only 2 500 units/m3. For the following year, Harvey found that the
standing crop was only 2-3% of the estimated total production. Hart
(1 942), following Harvey’s calculations, suggested that the standing
crop of phytoplankton in the highly productive South Georgia region
of the Antarctic was only 2% of the calculated production; for oceanic
* Arbitrary pigment units were used as a measuremimt of chlorophyll before pure
chlorophyll was used aa the standard method of estimation.
161
(1939) also believed that over the productive periods of phytoplankton
growth in temperate latitudes, the zooplankton was largely responsible
for regulating the size of plant population. The importance of grazing
activity had been noted in Arctic waters by Braarud (1935), by Bigelow
et al. (1940) for the Gulf of Maine, by Wimpeiiny (1936, 1938) for the
North Sea, by Holmes (1956) in the Labrador Sea, by Hart (1942) for
the Antarctic, and in other areas. Grazing intensity of course may
vary, especially at high latitudes. Thus Halldal (1953) considers that in
the Norwegian Sea it was much more intensive in spring than later in
the year. In very productive areas, such as Long Island Sound (Riley,
1956; 1959), Block Island Sound (Riley, 1952) and Tisbury Great Pond
(Deevey, 1948), the phytoplankton crop may be rich, and grazing
appears to exert little influence on the density of the algae (cf. also
Gross et al., 1947). Despite these exceptions, the effect of the herbivorous
zooplankton - appendicularians and salps, a majority of copepod
species, many euphausids, the shelled pteropods, cladocerans and many
meroplanktonic larvae - on the phytoplankton crop can be remarkable.
The rapid reduction in phytoplankton crop seen in temperate and high
latitudes may therefore not be a result of nutrient lack but of grazing
activity.
Cushing et al. (1963) deal with the relations between a Calanus
population feeding on a phytoplankton burst in the North Sea over a
period from March to June. From a comparison of the crop of phytoplankton present and the estimated rate of algal production, he believes
that grazing is the dominant cause of algal mortality; there was no
evidence of a lack of nutrients during the erdy decline in the algal
population. Grazing is believed to be the niajor controlling factor,
according to Cushing, in the temperate spring outburst of phytoplankton. With intensive grazing there is considerable regeneration of
nutrients in the euphotic zone, and only when the rate of regeneration
decreases does the reduction in concentration of nutrients due to the
thermocline becoming a limiting factor.
Harvey et al. (1935) obtained a minimal estimate of overall production
of phytoplankton from the reduction in nutrier.t level over the period of
the spring increase. For 1933, density was estimated at 85 000 plant
pigment units/m3,* whereas the actual value of the standing crop was
only 2 500 units/m3. For the following year, Harvey found that the
standing crop was only 2-3% of the estimated total production. Hart
(1 942), following Harvey’s calculations, suggested that the standing
crop of phytoplankton in the highly productive South Georgia region
of the Antarctic was only 2% of the calculated production; for oceanic
* Arbitrary pigment units were used as a measuremimt of chlorophyll before pure
chlorophyll was used aa the standard method of estimation.