PLANKTON IN NITROGEN AND PHOSPHORUS CYCLES
137
the amount of plant food in the sea rises above a certain level. He
observed that in various studies on the feeding of zooplankton a certain
threshold level of algal food was reached beyond which neither growth
nor reproduction was limited by the amount of food available. He
calculated that this threshold value lay between 3 and 20 g biomass/m3,
and proposed that superfluous feeding took place in the sea when the
phytoplankton population rose above the lesscr of the two values,
recalculated as corresponding to 390 mg C/m3 or 107-109 cells/m3.
From an examination of phytoplankton data in various sea areas he
concluded that superfluous feeding should be of widespread occurrence
during one or two months in the spring. Among the consequences of
this would be ( 1 ) a greater nutritive value of faecal pellets and (2) an
increased rate of nutrient regeneration. Evidence of heightened
nutrient excretion at a time when food is plentiful has been found in
seasonal surveys of nitrogen excretion by Calanus hyperboreus in the
Gulf of Maine (Conover and Corner, 1968) and of nitrogen and phosphorus excretion by C.$nmarchicus in the Clyde sea-area (Butler et al.,
1969, 1970). However, Beklemishev’s view that assimilation is poor at
times of year when plant food is abundant is based on the field observations of Harvey et al. (1935) for the English Channel and of Riley
(1946, 1947) for Gcorges Bank off Cape Cod. A valuable and stimulating
reappraisal of these-and
other-field
measurements of secondary
production has recently been published by Mullin (1969) : accordingly,
we need deal with them only in outline.
The collecting of field data concerned with the production of ZOOplankton in the sea poses several problems. One is that the nets used
to capture the animals may not provide a truly representative sample,
larger members of the zooplankton tending to avoid capture and smaller
members passing through the nets. Another difficulty is that factors
other than grazing by the animals may reduce the plant population,
part of which may sink out of the cuphotic zone or become dispersed
over a wide area by water moverncnts. For these and other reasons
Harvcy et al. (1935) and Riley (1947) emphasized the speculative
nature of their conclusions.
Briefly, Harvey et al. estimated the total production of phytoplankton during spring at Station L4 (English Channel) from the decrease in the phosphate content of the sea water. This fell by 7 mg
P/m3 over a period of 60 days, giving an average daily production of
phytoplankton phosphorus of 0.1 1 mg. The zooplankton present on
any one day during the same period (mid-February to mid-April)
contained an average of 0.29 mg P, roughly two and a half times that
of the average daily plant production. Harvey et al. could find no
137
the amount of plant food in the sea rises above a certain level. He
observed that in various studies on the feeding of zooplankton a certain
threshold level of algal food was reached beyond which neither growth
nor reproduction was limited by the amount of food available. He
calculated that this threshold value lay between 3 and 20 g biomass/m3,
and proposed that superfluous feeding took place in the sea when the
phytoplankton population rose above the lesscr of the two values,
recalculated as corresponding to 390 mg C/m3 or 107-109 cells/m3.
From an examination of phytoplankton data in various sea areas he
concluded that superfluous feeding should be of widespread occurrence
during one or two months in the spring. Among the consequences of
this would be ( 1 ) a greater nutritive value of faecal pellets and (2) an
increased rate of nutrient regeneration. Evidence of heightened
nutrient excretion at a time when food is plentiful has been found in
seasonal surveys of nitrogen excretion by Calanus hyperboreus in the
Gulf of Maine (Conover and Corner, 1968) and of nitrogen and phosphorus excretion by C.$nmarchicus in the Clyde sea-area (Butler et al.,
1969, 1970). However, Beklemishev’s view that assimilation is poor at
times of year when plant food is abundant is based on the field observations of Harvey et al. (1935) for the English Channel and of Riley
(1946, 1947) for Gcorges Bank off Cape Cod. A valuable and stimulating
reappraisal of these-and
other-field
measurements of secondary
production has recently been published by Mullin (1969) : accordingly,
we need deal with them only in outline.
The collecting of field data concerned with the production of ZOOplankton in the sea poses several problems. One is that the nets used
to capture the animals may not provide a truly representative sample,
larger members of the zooplankton tending to avoid capture and smaller
members passing through the nets. Another difficulty is that factors
other than grazing by the animals may reduce the plant population,
part of which may sink out of the cuphotic zone or become dispersed
over a wide area by water moverncnts. For these and other reasons
Harvcy et al. (1935) and Riley (1947) emphasized the speculative
nature of their conclusions.
Briefly, Harvey et al. estimated the total production of phytoplankton during spring at Station L4 (English Channel) from the decrease in the phosphate content of the sea water. This fell by 7 mg
P/m3 over a period of 60 days, giving an average daily production of
phytoplankton phosphorus of 0.1 1 mg. The zooplankton present on
any one day during the same period (mid-February to mid-April)
contained an average of 0.29 mg P, roughly two and a half times that
of the average daily plant production. Harvey et al. could find no