T H E P R O D U C T I O N O F M A R I N E P L A N K T O N
177
The relationship between the phytoplankton and the zooplankton therefore approaches a steady state; continuous grazing releases excretory
material which is rapidly remineralized, and the nutrient concentration
remains very low as it is continuously utilized. The opposite condition
is seen in colder waters where typically an enormous early burst of
phytoplankton is followed by very heavy grazing, but frequently there
is a considerable time lag between the increase in phytoplankton and
the rise in the zooplankton herbivores. Only when the crop of phytoplankton has been sufficiently reduced does new regeneration take
place. This concept of the very efficient use of primary production in
warmer waters has been supported more recently by the work of other
investigators. For example Menzel and Ryther (1961) have found that
off Bermuda the zooplankton uses almost 100% of the production of
algae. Grice and Hart (1962) suggest a remarkably efficient utilization
of the phytoplankton in Sargasso waters, the herbivores showing far
greater diversity of species than in neritic and. in boreal waters. Presumably the herbivores are more specifically adapted to utilize the
various types of food efficiently; they perhaps have very specialized
food requirements. Grice and Hart also call actention to the balance
between herbivores and carnivorous groups in the zooplankton. In
warm subtropical waters about half the zooplankton is herbivorous as
against 65 yo in neritic boreal areas. Two predominantly carnivorous
groups (the chaetognaths and siphonophores) were also much more
important in warm tropical waters in respect to the zooplankton as a
whole than in the coastal areas. In tropical waters therefore, not only is
the herbivorous plankton utilizing the phytopbnkton as efficiently as
possible, but it would appear that the herbivore population is kept in
delicate balance with the carnivorous forms. In coastal and boreal
waters there may be an excessive production of herbivorous plankton
following a phytoplankton outburst. This may account in part for the
sudden great swarms of salps which Grice and Hart observed in the
more neritic areas. Steemann Nielsen (196213, 1963) has pointed out that
in warm oligotrophic waters the herbivores mush search out their food
and the greater depth of the euphotic zone will make greater energy
demands on the zooplankton for the food obtained. Since less of the food
aseimilated is presumably available for growth and reproduction, it is
likely that the zooplankton will be longer-lived. The ratio between the
standing stock and the rate of production for both herbivorous and
carnivorous plankton animals will vary, but it will tend to be higher in
oligotrophic areas than in the nutrient-richer wihxs of temperate and
higher latitudes. In these typically richer regions any factors which
allow phytoplankton growth to start more slowly, and so allow the zooplankton to begin grazing down the phytoplankton crop, will prevent
177
The relationship between the phytoplankton and the zooplankton therefore approaches a steady state; continuous grazing releases excretory
material which is rapidly remineralized, and the nutrient concentration
remains very low as it is continuously utilized. The opposite condition
is seen in colder waters where typically an enormous early burst of
phytoplankton is followed by very heavy grazing, but frequently there
is a considerable time lag between the increase in phytoplankton and
the rise in the zooplankton herbivores. Only when the crop of phytoplankton has been sufficiently reduced does new regeneration take
place. This concept of the very efficient use of primary production in
warmer waters has been supported more recently by the work of other
investigators. For example Menzel and Ryther (1961) have found that
off Bermuda the zooplankton uses almost 100% of the production of
algae. Grice and Hart (1962) suggest a remarkably efficient utilization
of the phytoplankton in Sargasso waters, the herbivores showing far
greater diversity of species than in neritic and. in boreal waters. Presumably the herbivores are more specifically adapted to utilize the
various types of food efficiently; they perhaps have very specialized
food requirements. Grice and Hart also call actention to the balance
between herbivores and carnivorous groups in the zooplankton. In
warm subtropical waters about half the zooplankton is herbivorous as
against 65 yo in neritic boreal areas. Two predominantly carnivorous
groups (the chaetognaths and siphonophores) were also much more
important in warm tropical waters in respect to the zooplankton as a
whole than in the coastal areas. In tropical waters therefore, not only is
the herbivorous plankton utilizing the phytopbnkton as efficiently as
possible, but it would appear that the herbivore population is kept in
delicate balance with the carnivorous forms. In coastal and boreal
waters there may be an excessive production of herbivorous plankton
following a phytoplankton outburst. This may account in part for the
sudden great swarms of salps which Grice and Hart observed in the
more neritic areas. Steemann Nielsen (196213, 1963) has pointed out that
in warm oligotrophic waters the herbivores mush search out their food
and the greater depth of the euphotic zone will make greater energy
demands on the zooplankton for the food obtained. Since less of the food
aseimilated is presumably available for growth and reproduction, it is
likely that the zooplankton will be longer-lived. The ratio between the
standing stock and the rate of production for both herbivorous and
carnivorous plankton animals will vary, but it will tend to be higher in
oligotrophic areas than in the nutrient-richer wihxs of temperate and
higher latitudes. In these typically richer regions any factors which
allow phytoplankton growth to start more slowly, and so allow the zooplankton to begin grazing down the phytoplankton crop, will prevent