PLANKTON IN NITROGEN AND PHOSPHORUS CYCLES
171
Mullin and Brooks (1967) have successfully reared Calanus helgohndicus and Rhinculanus nasutus in the laboratory and have now
measured K , values under different experimental conditions (Mullin
and Brooks, 1970). They found that neither temperature nor quality
of food affected K,, and there was no regular decrease in K , with
increasing age. The latter finding seems to conflict with those of Petipa
(1967) and Pavlova (1967) but may be due to beliavioural differences
between animals in the sea and those reared in the laboratory. Thus,
Petipa (19G6) has drawn attention to the amount of energy used by
animals in the sea when searching for food and when undergoing
diurnal vertical migration. Using her earlier data (Petipa, 1964a ; l9G4b)
she has calculated (Petipa, 1967) that the energy expended by older
stages of Calanus helgolandicus migrating vertically over a distance of
50-100 m in the sea is 31-35 times as great as that of animals in the
laboratory. By contrast, the smaller species Acartia clausi, which
migrates over a much smaller distance (10-15 m), apparently maintains a constant level of metabolism. Petipa concludes that this is
why K , values for the older stages of Calanus helgolandicus are lower
than those for the older stages of Acartia clausi. Petipa’s claim that
vertical migration can increase the metabolic rate of some zooplankton
by as much as a factor of 35 seems difficult to reconcile with the view
that migration between warm surface water and deeper cooler water
provides an energy “ bonus ’’ for the animals (McLaren, 1963).
It would be useful to know whether rates of excretion of nitrogen and
phosphorus by animals vertically migrating are changed by this process
sufficiently to affect K, and K, values, but so far this problem has not
been studied.
Relevant to our discussion of growth efficiencies is thc fact that
values for K , have been used by Shushkina (1968) to calculate rates of
production of various stages of a zooplankton population. From data in
the literature relating respiration rate to body weight he calculated the
oxygen consumption, T, and hence calories expended ; and from pubK2T .
lished values for K 2 , he estimated growth for each stage as -
Combining these data with numbers and weights of animals in every
stage of a population of Haloptilus longicornis (Claus) from the Fiji Sea,
he calculated growth rate per day, P, as a fraction of biomass, B, for
the various stages as 0.30 (copepodite 11), 0.06 (copepodite 111), 0.07
(copepodite IV), 0.03 (copepodite V) and 1.15 (egg production). There
seems no reason why this procedure, described by Shushkina as “ t h e
physiological method ”, should not be applied in calculations of rates of
nitrogen and phosphorus production by a population of a particular
1 - K2
171
Mullin and Brooks (1967) have successfully reared Calanus helgohndicus and Rhinculanus nasutus in the laboratory and have now
measured K , values under different experimental conditions (Mullin
and Brooks, 1970). They found that neither temperature nor quality
of food affected K,, and there was no regular decrease in K , with
increasing age. The latter finding seems to conflict with those of Petipa
(1967) and Pavlova (1967) but may be due to beliavioural differences
between animals in the sea and those reared in the laboratory. Thus,
Petipa (19G6) has drawn attention to the amount of energy used by
animals in the sea when searching for food and when undergoing
diurnal vertical migration. Using her earlier data (Petipa, 1964a ; l9G4b)
she has calculated (Petipa, 1967) that the energy expended by older
stages of Calanus helgolandicus migrating vertically over a distance of
50-100 m in the sea is 31-35 times as great as that of animals in the
laboratory. By contrast, the smaller species Acartia clausi, which
migrates over a much smaller distance (10-15 m), apparently maintains a constant level of metabolism. Petipa concludes that this is
why K , values for the older stages of Calanus helgolandicus are lower
than those for the older stages of Acartia clausi. Petipa’s claim that
vertical migration can increase the metabolic rate of some zooplankton
by as much as a factor of 35 seems difficult to reconcile with the view
that migration between warm surface water and deeper cooler water
provides an energy “ bonus ’’ for the animals (McLaren, 1963).
It would be useful to know whether rates of excretion of nitrogen and
phosphorus by animals vertically migrating are changed by this process
sufficiently to affect K, and K, values, but so far this problem has not
been studied.
Relevant to our discussion of growth efficiencies is thc fact that
values for K , have been used by Shushkina (1968) to calculate rates of
production of various stages of a zooplankton population. From data in
the literature relating respiration rate to body weight he calculated the
oxygen consumption, T, and hence calories expended ; and from pubK2T .
lished values for K 2 , he estimated growth for each stage as -
Combining these data with numbers and weights of animals in every
stage of a population of Haloptilus longicornis (Claus) from the Fiji Sea,
he calculated growth rate per day, P, as a fraction of biomass, B, for
the various stages as 0.30 (copepodite 11), 0.06 (copepodite 111), 0.07
(copepodite IV), 0.03 (copepodite V) and 1.15 (egg production). There
seems no reason why this procedure, described by Shushkina as “ t h e
physiological method ”, should not be applied in calculations of rates of
nitrogen and phosphorus production by a population of a particular
1 - K2