PLANKTON IN NITROGEN AND PHOSPHORUS CYCLES
149
plankton, and a correction was applied for the uptake of excreted
nitrogen by the food. In most instances this was neglible, and only in
experiments involving Peridinium trochoideum (Stein) Lemm. as a
food was it large enough t o invalidate the results. The average level
of nitrogen excretion during spring (April-May) was 0.71 pg/mg dry
body weightlday, compared with an autumn-winter (October-March)
level of 0.30. Nitrogen on average accounted for 6.2% of the dry body
weight and this value varied little with season. Accordingly, the
animals excreted the equivalent of 1.1% of the body nitrogen daily in
spring and 0.5% in autumn-winter. These values are much lower than
those described for other species of zooplankton and probably reflect
the large size of the animals (1-51 mg dry weight) and the low temperature of the experiments. A comparison was made between nitrogen
excretion by fed and starved animals, and the pooled data from experiments carried out in spring and summer showed that the mean value
for nitrogen excretion by animals fed on various food concentrations
(ranging from 185-500 pg N/1) was 0.70 pg N/animal/day compared
with a value of 0-58 for the starved.
The additional quantities of nitrogen excreted by feeding animals
in the experiments of Corner et al. (1965) and Conover and Corner
(1968) are relevant to the point made by Harris (1959) that some of the
ammonia released by zooplankton during feeding may not have been
excreted as an end-product of metabolism, but released in soluble form
with semi-digested faecal material. On the other hand, animals actively
feeding may increase their metabolism above the “basal” level
because of the additional work involved in digesting food.
The release of ammonia-nitrogen with faecal material has not yet
been demonstrated experimentally. However, evidence supporting
the view that metabolic activity increases when the animals feed has
been obtained from measurements of respiration rate. Thus, Corner
et al. (1965) found that the rate of oxygen consumption by Calanus
blgolandicus fed on Skeletonema was 86pl/mg dry body weightlday
compared with a value of 38 for starving animals: significantly, rates
of nitrogen excretion by the same batches of animals were in similar
proportion, the feeding Catanus excreting 7.9 pg Nfmg dry body
weightlday and the starving animals 3.6. Similarly, Conover and
Corner (1968) found values of 21.5 and 16.6 pl.O,/animal/day for the
respiration rates of feeding and starving Calanus hyperboreus, the
corresponding values for nitrogen excretion being 0.70 and 0.58
pglanimallday : again, therefore, rates of oxygen consumption for fed
and starved animals were in approximately the same ratio as the values
found for nitrogen excretion.
149
plankton, and a correction was applied for the uptake of excreted
nitrogen by the food. In most instances this was neglible, and only in
experiments involving Peridinium trochoideum (Stein) Lemm. as a
food was it large enough t o invalidate the results. The average level
of nitrogen excretion during spring (April-May) was 0.71 pg/mg dry
body weightlday, compared with an autumn-winter (October-March)
level of 0.30. Nitrogen on average accounted for 6.2% of the dry body
weight and this value varied little with season. Accordingly, the
animals excreted the equivalent of 1.1% of the body nitrogen daily in
spring and 0.5% in autumn-winter. These values are much lower than
those described for other species of zooplankton and probably reflect
the large size of the animals (1-51 mg dry weight) and the low temperature of the experiments. A comparison was made between nitrogen
excretion by fed and starved animals, and the pooled data from experiments carried out in spring and summer showed that the mean value
for nitrogen excretion by animals fed on various food concentrations
(ranging from 185-500 pg N/1) was 0.70 pg N/animal/day compared
with a value of 0-58 for the starved.
The additional quantities of nitrogen excreted by feeding animals
in the experiments of Corner et al. (1965) and Conover and Corner
(1968) are relevant to the point made by Harris (1959) that some of the
ammonia released by zooplankton during feeding may not have been
excreted as an end-product of metabolism, but released in soluble form
with semi-digested faecal material. On the other hand, animals actively
feeding may increase their metabolism above the “basal” level
because of the additional work involved in digesting food.
The release of ammonia-nitrogen with faecal material has not yet
been demonstrated experimentally. However, evidence supporting
the view that metabolic activity increases when the animals feed has
been obtained from measurements of respiration rate. Thus, Corner
et al. (1965) found that the rate of oxygen consumption by Calanus
blgolandicus fed on Skeletonema was 86pl/mg dry body weightlday
compared with a value of 38 for starving animals: significantly, rates
of nitrogen excretion by the same batches of animals were in similar
proportion, the feeding Catanus excreting 7.9 pg Nfmg dry body
weightlday and the starving animals 3.6. Similarly, Conover and
Corner (1968) found values of 21.5 and 16.6 pl.O,/animal/day for the
respiration rates of feeding and starving Calanus hyperboreus, the
corresponding values for nitrogen excretion being 0.70 and 0.58
pglanimallday : again, therefore, rates of oxygen consumption for fed
and starved animals were in approximately the same ratio as the values
found for nitrogen excretion.