156
E. D. 9. CORNER AND ANTHONY 0. DAVIES
C. Sea-sonul surveys of nitrogen and phosphorus excretion
Data were obtained by Conover and Corner (1968) for nitrogen
excretion by zooplankton at different times of year, but the first attempt
to study both nitrogen and phosphorus excretion in relation to the
seasonal production of phytoplankton was that of Martin (1968)
using mixed zooplankton collected from Narragansett Bay. Martin
found that the rate of nitrogen excretion (measured as ammonia by
the Witting-Buch method : Barnes, 1959) by the animals was inversely
related to the level of plant food available, the mean value in spring
being 3.5 pg N/mg dry body weightlday (average plant population
17 x lo6 cells/l) whereas that in autumn was nearly ten times as high,
namely 34 (average plant population 0.54 x lo6 cells/l). A similar
trend was found in phosphorus excretion (measured as inorganic
phosphate by the Deniges-Atkins method : Wattenberg, 1937), the
mean spring level being 2.4 pg P/mg dry body weightlday compared
with an average autumn value of 6.6.
These data appear to conflict with the results of laboratory studies
showing a positive correlation between nitrogen and phosphorus
excretion on the one hand, and food level on the other. They also
conflict with the observations of Butler et al. (1969) who, using Calanw
Jinmarchicus collected from the Clyde, showed that the mean value for
nitrogen excretion (measured as total nitrogen, both inorganic and
organic, by the UV-irradiation method of Armstrong and Tibbitts,
1968) by females and stage Vs during spring was 13.4 pg N/mg dry body
weightlday compared with an average autumn level of only 2.74 ; and
that phosphate excretion (again measured as total phosphorus after
UV-irradiation) averaged 2.21 pg P/mg dry body weightlday in spring
compared with a mean value of 0.56 in autumn.
In a further study (Butler et al., 1970), nitrogen and phosphorus
excretion by Calanus in the Clyde was measured at all seasons, together
with levels of the plant population as chlorophyll a. A summary of
the data is shown in Fig. 6 from which it is clear that the levels of plant
food and those of nitrogen and phosphorus excretion by the animals
followed a similar pattern throughout the year. Corroborative evidence
was provided by laboratory experiments in which animals fed on
natural concentrations of phytoplankton at the time of the spring
diatom flowering excreted more nitrogen and phosphorus than did
starving controls.
More detailed data from studies made during the early part of a
spring diatom flowering are shown in Fig. 7. Although the peak levels
of nitrogen and phosphorus excretion do not exactly coincide with
E. D. 9. CORNER AND ANTHONY 0. DAVIES
C. Sea-sonul surveys of nitrogen and phosphorus excretion
Data were obtained by Conover and Corner (1968) for nitrogen
excretion by zooplankton at different times of year, but the first attempt
to study both nitrogen and phosphorus excretion in relation to the
seasonal production of phytoplankton was that of Martin (1968)
using mixed zooplankton collected from Narragansett Bay. Martin
found that the rate of nitrogen excretion (measured as ammonia by
the Witting-Buch method : Barnes, 1959) by the animals was inversely
related to the level of plant food available, the mean value in spring
being 3.5 pg N/mg dry body weightlday (average plant population
17 x lo6 cells/l) whereas that in autumn was nearly ten times as high,
namely 34 (average plant population 0.54 x lo6 cells/l). A similar
trend was found in phosphorus excretion (measured as inorganic
phosphate by the Deniges-Atkins method : Wattenberg, 1937), the
mean spring level being 2.4 pg P/mg dry body weightlday compared
with an average autumn value of 6.6.
These data appear to conflict with the results of laboratory studies
showing a positive correlation between nitrogen and phosphorus
excretion on the one hand, and food level on the other. They also
conflict with the observations of Butler et al. (1969) who, using Calanw
Jinmarchicus collected from the Clyde, showed that the mean value for
nitrogen excretion (measured as total nitrogen, both inorganic and
organic, by the UV-irradiation method of Armstrong and Tibbitts,
1968) by females and stage Vs during spring was 13.4 pg N/mg dry body
weightlday compared with an average autumn level of only 2.74 ; and
that phosphate excretion (again measured as total phosphorus after
UV-irradiation) averaged 2.21 pg P/mg dry body weightlday in spring
compared with a mean value of 0.56 in autumn.
In a further study (Butler et al., 1970), nitrogen and phosphorus
excretion by Calanus in the Clyde was measured at all seasons, together
with levels of the plant population as chlorophyll a. A summary of
the data is shown in Fig. 6 from which it is clear that the levels of plant
food and those of nitrogen and phosphorus excretion by the animals
followed a similar pattern throughout the year. Corroborative evidence
was provided by laboratory experiments in which animals fed on
natural concentrations of phytoplankton at the time of the spring
diatom flowering excreted more nitrogen and phosphorus than did
starving controls.
More detailed data from studies made during the early part of a
spring diatom flowering are shown in Fig. 7. Although the peak levels
of nitrogen and phosphorus excretion do not exactly coincide with