PLANKTON IN NITROGEN AND PHOSPHORUS CYCLES
113
could be argued that these low values may have reflected the fact that
the plants had been cultured in the laboratory, certain data obtained
for natural populations of phytoplankton have also been inconsistent
with Fleming's ratios. Thus, coastal phytoplankton studied by Antia
et al. (1963) were found to have C :N :P ratios varying from 85 : 16.5 : 1
to 52 : 13.5 : 1, the former when nitrate was depleted and the latter when
nitrate was present in excess. Furthermore, an oceanic population
analysed by McAllister et al. (1960) gave ratios of 86 : 17 : 1 when nitrate
was plentiful: for this the oxidative ratio d0:dP would be only
Later in this review (Section IX) it will be seen that the atomic ratio
nitrogen : phosphorus in zooplankton can also vary considerably ; but
the data obtained by Beers (1966) for mixed zooplankton collected
from the Sargasso Sea are worth quoting in the present context.
Copepods and chaetognaths, which together comprised the greater
proportion of the total population, had nitrogen : phosphorus ratios
greater than 27 :1 and, on the basis of these data, Holm-Hanscn et al.
(1966) suggested that realistic average C :N :P ratios for zooplankton
(allowing for a preponderance of copepods) would be 117 :23 : l .
Why many values for apparent oxygen utilization (AOU) are
consistent with Fleming's (1 940) atomic ratios, but certain data for
C :N :P in plankton are not, is a question that deserves further study.
-240:l.
IV. UPTAKE OF NITROGEN COMPOUNDS BY PHYTOPLANKTON
For much of the year in temperate regions, and almost permanently
in tropical sca areas, the water above the thermocline contains low
levels of dissolved nitrogen and phosphorus compounds. In this
situation, plant production is dependent upon the nutrients supplied by
regeneration and vertical transport, and the growth of those species
which can most efficiently assimilate and utilize the low concentrations
which become available should be favoured. For this reason, them is
currently a great interest in understanding the relationship between
phytoplankton growth and nutrient levels, as this is likely to be an
important factor in determining species succession and geographical
and temporal variations in natural populations.
A. Inorganic forms of nitrogen
In general, organic forms of nitrogen such as amino acids do not
appear to be satisfactory nitrogen sources for phytoplankton growth
(Harvey, 1940; Guillard, 1963); and although urea and uric acid are
utilized by some coastal and estuarine species, these compounds are
113
could be argued that these low values may have reflected the fact that
the plants had been cultured in the laboratory, certain data obtained
for natural populations of phytoplankton have also been inconsistent
with Fleming's ratios. Thus, coastal phytoplankton studied by Antia
et al. (1963) were found to have C :N :P ratios varying from 85 : 16.5 : 1
to 52 : 13.5 : 1, the former when nitrate was depleted and the latter when
nitrate was present in excess. Furthermore, an oceanic population
analysed by McAllister et al. (1960) gave ratios of 86 : 17 : 1 when nitrate
was plentiful: for this the oxidative ratio d0:dP would be only
Later in this review (Section IX) it will be seen that the atomic ratio
nitrogen : phosphorus in zooplankton can also vary considerably ; but
the data obtained by Beers (1966) for mixed zooplankton collected
from the Sargasso Sea are worth quoting in the present context.
Copepods and chaetognaths, which together comprised the greater
proportion of the total population, had nitrogen : phosphorus ratios
greater than 27 :1 and, on the basis of these data, Holm-Hanscn et al.
(1966) suggested that realistic average C :N :P ratios for zooplankton
(allowing for a preponderance of copepods) would be 117 :23 : l .
Why many values for apparent oxygen utilization (AOU) are
consistent with Fleming's (1 940) atomic ratios, but certain data for
C :N :P in plankton are not, is a question that deserves further study.
-240:l.
IV. UPTAKE OF NITROGEN COMPOUNDS BY PHYTOPLANKTON
For much of the year in temperate regions, and almost permanently
in tropical sca areas, the water above the thermocline contains low
levels of dissolved nitrogen and phosphorus compounds. In this
situation, plant production is dependent upon the nutrients supplied by
regeneration and vertical transport, and the growth of those species
which can most efficiently assimilate and utilize the low concentrations
which become available should be favoured. For this reason, them is
currently a great interest in understanding the relationship between
phytoplankton growth and nutrient levels, as this is likely to be an
important factor in determining species succession and geographical
and temporal variations in natural populations.
A. Inorganic forms of nitrogen
In general, organic forms of nitrogen such as amino acids do not
appear to be satisfactory nitrogen sources for phytoplankton growth
(Harvey, 1940; Guillard, 1963); and although urea and uric acid are
utilized by some coastal and estuarine species, these compounds are