PLANKTON IN NITROGEN AND PHOSPHORUS CYCLES
159
The excretion experiments carried out by Butler et al. were begun as
soon as possible after the animals had been taken from the sea and
sorted in the laboratory. During this period the guts were emptied and,
as no food was given to the animals during the four hour period of the
experiments, the nitrogen and phosphorus excreted represented endproducts of metabolism and did not include unassimilated foodstuffs
released in soluble form together with faecal material. The absence of
food for this short period did not reduce excretion rates below those of
feeding animals, Corner et al. (1965) having found that female Calanus
actively feeding on Skeletonemu in the laboratory excreted 64-1 1.7 pg
N/mg dry body weightlday, which compares closely with the range
7.11-10-50 obtained by Butler et al. (1970) with animals collected at a
time when peak concentrations of diatoms, including Skeletonemu,
were present in the Clyde.
The question arises why the data of Martin (1968) conflict with
those of Butler et al. (1969, 1970). Martin’s measurements of excretion
rate were made with animals placed in unfiltered sea water containing
natural phytoplankton, particulate material and microzooplankton.
Thus, as the animals were feeding on diets normally available in the sea
the excretion rates were more likely to represent those occurring in
nature. However, no corrections were made for the possible uptake of
excreted ammonia and phosphate by the phytoplankton, although the
excretion experiments lasted 24 h. This uptake would have been
greatest during spring when the food concentration was 17 x lo8
cells/l and nutrient levels in the sea were minimal (see Martin, 1965)
and could have contributed to the low values found for nitrogen and
phosphorus excretion at that time. The effect in autumn, however,
when the plant population was only 0.54 x lo6 celIs/l and nutrient
levels in the sea were higher, would have been much less, perhaps
negligible. Another possibility is that whereas the species studied by
Butler et al. was primarily herbivorous, the mixture of zooplankton
used by Martin was dominated by Acartia tonsa during summer and
autumn and this species can use both plant and animal diets (Anraku
and Omori, 1963). Accordingly, the high levels of nitrogen and phosphorus excretion observed by Martin during the autumn could have
been the result of animals feeding on microzooplankton.
A summary of all the various data on nitrogen and phosphorus
excretion by zooplankton is given in Table VIII and illustrates the very
large degree of variation found. Such variation doubtless reflects the
many different factors (e.g. food supply, body size, season) that can
influence the final result. In every case where both nitrogen and phosphorus excretion have been measured under the same conditions the
159
The excretion experiments carried out by Butler et al. were begun as
soon as possible after the animals had been taken from the sea and
sorted in the laboratory. During this period the guts were emptied and,
as no food was given to the animals during the four hour period of the
experiments, the nitrogen and phosphorus excreted represented endproducts of metabolism and did not include unassimilated foodstuffs
released in soluble form together with faecal material. The absence of
food for this short period did not reduce excretion rates below those of
feeding animals, Corner et al. (1965) having found that female Calanus
actively feeding on Skeletonemu in the laboratory excreted 64-1 1.7 pg
N/mg dry body weightlday, which compares closely with the range
7.11-10-50 obtained by Butler et al. (1970) with animals collected at a
time when peak concentrations of diatoms, including Skeletonemu,
were present in the Clyde.
The question arises why the data of Martin (1968) conflict with
those of Butler et al. (1969, 1970). Martin’s measurements of excretion
rate were made with animals placed in unfiltered sea water containing
natural phytoplankton, particulate material and microzooplankton.
Thus, as the animals were feeding on diets normally available in the sea
the excretion rates were more likely to represent those occurring in
nature. However, no corrections were made for the possible uptake of
excreted ammonia and phosphate by the phytoplankton, although the
excretion experiments lasted 24 h. This uptake would have been
greatest during spring when the food concentration was 17 x lo8
cells/l and nutrient levels in the sea were minimal (see Martin, 1965)
and could have contributed to the low values found for nitrogen and
phosphorus excretion at that time. The effect in autumn, however,
when the plant population was only 0.54 x lo6 celIs/l and nutrient
levels in the sea were higher, would have been much less, perhaps
negligible. Another possibility is that whereas the species studied by
Butler et al. was primarily herbivorous, the mixture of zooplankton
used by Martin was dominated by Acartia tonsa during summer and
autumn and this species can use both plant and animal diets (Anraku
and Omori, 1963). Accordingly, the high levels of nitrogen and phosphorus excretion observed by Martin during the autumn could have
been the result of animals feeding on microzooplankton.
A summary of all the various data on nitrogen and phosphorus
excretion by zooplankton is given in Table VIII and illustrates the very
large degree of variation found. Such variation doubtless reflects the
many different factors (e.g. food supply, body size, season) that can
influence the final result. In every case where both nitrogen and phosphorus excretion have been measured under the same conditions the