PLANKTON IN NITROQEN AND PHOSPHORUS CYCLES
153
taining particulate material as a food. However, Marshall and Orr
(1 961), measuring phosphorus excretion by Calanus finmarchicus kept
in membrane-filtered sea water for 6-24 h, obtained a much lower value.
The animals had previously been fed on 32P-labelled unialgal cultures
and it was found that equilibrium of 32P throughout the body was only
reached after a feeding period of one week, after which the animals
excreted a quantity of 32P representing approximately 10% of the body
phosphorus daily. Much higher rates of 32P excretion were observed,
however, after the animals had fed for only 3 h on the labelled diet,
presumably because 32P had not reached equilibrium throughout the
body. Conover (1961) concluded from these data that there must be
a t least two “ pools ” of phosphorus in Calanus finmarchicus. One
pool was labile with a half-life of only 0.375 days: the other pool
was stable, with a half-life of 13 days. Calculating the relative
sizes of the two pools, he showed that by far the greater fraction
(94-99%) of the phosphorus was in the stable form, and concluded
that the mean turnover rate for phosphorus through both pools was
roughly 10% per day. Further experiments by Marshall and Orr
(196l), howevec, showed that phosphorus excretion increased when
the animals were feeding during the excretion experiments. Thus,
the mean value obtained for phosphorus excretion in all experiments with starving animals-including those in which 32P had not
yet reached equilibrium-was ISYO, compared with a value of 25%
for animals that had been fed.
Studies on phosphorus excretion by mixed populations of ZOOplankton were made by Pomeroy et al. (1963) who measured changes
in the levels of inorganic orthophosphate (estimated by the method of
Hansen and Robinson, 1953) and total phosphorus (estimated by the
method of Burton and Riley, 1956) in sea water containing animals
from the Gulf Stream and Doboy Sound, Georgia. The experiments
were made in summer a t 25°C and the amount of phosphorus excreted
in the “ organic ” form was estimated as the difference between the
levels of inorganic phosphate and total phosphorus excreted. It was
found that about half the phosphorus excreted was in the “ organic ”
form but the chemical composition of this fraction was not investigated.
Samples of mixed zooplankton from the Gulf Stream excreted an
average of 3.7 pg P/mg dry weightlday, of which 47% was “ organic ”,
and samples from the estuarine Doboy Sound excreted 9.4 pg P/mg
dry weightlday, of which 49% was “ organic ”.
Hargrave and Geen (1968) made a detailed study of phosphorus
excretion by Pseudocalanus minutus, Femora longicornis and Oithona
similis Claus from Bras d’Or Lake and Acartia tonsa from Morrison’s
153
taining particulate material as a food. However, Marshall and Orr
(1 961), measuring phosphorus excretion by Calanus finmarchicus kept
in membrane-filtered sea water for 6-24 h, obtained a much lower value.
The animals had previously been fed on 32P-labelled unialgal cultures
and it was found that equilibrium of 32P throughout the body was only
reached after a feeding period of one week, after which the animals
excreted a quantity of 32P representing approximately 10% of the body
phosphorus daily. Much higher rates of 32P excretion were observed,
however, after the animals had fed for only 3 h on the labelled diet,
presumably because 32P had not reached equilibrium throughout the
body. Conover (1961) concluded from these data that there must be
a t least two “ pools ” of phosphorus in Calanus finmarchicus. One
pool was labile with a half-life of only 0.375 days: the other pool
was stable, with a half-life of 13 days. Calculating the relative
sizes of the two pools, he showed that by far the greater fraction
(94-99%) of the phosphorus was in the stable form, and concluded
that the mean turnover rate for phosphorus through both pools was
roughly 10% per day. Further experiments by Marshall and Orr
(196l), howevec, showed that phosphorus excretion increased when
the animals were feeding during the excretion experiments. Thus,
the mean value obtained for phosphorus excretion in all experiments with starving animals-including those in which 32P had not
yet reached equilibrium-was ISYO, compared with a value of 25%
for animals that had been fed.
Studies on phosphorus excretion by mixed populations of ZOOplankton were made by Pomeroy et al. (1963) who measured changes
in the levels of inorganic orthophosphate (estimated by the method of
Hansen and Robinson, 1953) and total phosphorus (estimated by the
method of Burton and Riley, 1956) in sea water containing animals
from the Gulf Stream and Doboy Sound, Georgia. The experiments
were made in summer a t 25°C and the amount of phosphorus excreted
in the “ organic ” form was estimated as the difference between the
levels of inorganic phosphate and total phosphorus excreted. It was
found that about half the phosphorus excreted was in the “ organic ”
form but the chemical composition of this fraction was not investigated.
Samples of mixed zooplankton from the Gulf Stream excreted an
average of 3.7 pg P/mg dry weightlday, of which 47% was “ organic ”,
and samples from the estuarine Doboy Sound excreted 9.4 pg P/mg
dry weightlday, of which 49% was “ organic ”.
Hargrave and Geen (1968) made a detailed study of phosphorus
excretion by Pseudocalanus minutus, Femora longicornis and Oithona
similis Claus from Bras d’Or Lake and Acartia tonsa from Morrison’s