PLANKTON I N NITROGEN AND PHOSPHORUS CYCLES
111
phate and oxygen appeared to have behaved conservatively) : the AOP
was simply the negative value of the apparent oxygen utilization. It
was found that during the spring, most of the APU values agreed, within
f 0.1 pg-atoms/l, with the value expected from the AOP : APU ratio
of -276 :l. The APU values for offshore water in early summer were
considerably lower than would have been expected, on the basis of the
same ratio, from the AOP values; and Stefhnsson and Richards
suggested that loss of the oxygen to the atmosphere accounted for this.
There was, however, at the same time, a reasonable correlation between
the APU and the AOP for inshore waters: indicating that photosynthesis was the predominating factor regulating the nutrient concentration in this region.
The measured nutrient levels in the sea represent the sum of the
preformed and the regenerated concentrations so that, using the
oxidative ratios given earlier, one may write for phosphate (P) and
nitrate (N) : P,,,, = Pprer + Po, = Ppref + 0.0036 AOU and N,,,, =
N,,,, + No, = N,,,, + 0.058 AOU.
Thus, a plot of measured nutrient concentrations against AOU
values should be linear, and the intercept at zero AOU would correspond to the preformed nutrient concentration (at the depth appropriate
to AOU = 0).
It has been pointed out (Pytkowicz, 1968) that, hidden in these
equations, is the assumption that the oxygen concentration in a mass of
water is equivalent to the sum of the fractional contributions to the
total oxygen content by each water type which mixed to form the mass,
and that this assumption is true only if the water types are similar in
temperature and salinity. In practice, however, the error introduced,
even if the water types have different characteristics, is usually small
and Park (1967) showed that the nutrient and oxygen concentrations
present in the sea off the Oregon coast were in good accord with these
equations.
Broenkow (1956) reported measurements of AOU values and
nutrient concentrations in the Costa Rica dome, a region of upwelling
in the eastern tropical Pacific Ocean where deep waters with higher
than normal salinities and nutrient levels but lower oxygen concentrations are brought to the surface. The AN :LIP value of 16.5 :1 was
normal and independent of depth, but the AAOU :AP ratio gave two
values, namely -274 :1 for water obtained above 120 m and -114 :1
for water below this depth. Broenkow was able to show that loss of
oxygen to the atmosphere was significant only in the upper 20 m and
the points even for these depths were within the confidence limits of the
calculated regression line, The lower ratio found for the deeper waters
111
phate and oxygen appeared to have behaved conservatively) : the AOP
was simply the negative value of the apparent oxygen utilization. It
was found that during the spring, most of the APU values agreed, within
f 0.1 pg-atoms/l, with the value expected from the AOP : APU ratio
of -276 :l. The APU values for offshore water in early summer were
considerably lower than would have been expected, on the basis of the
same ratio, from the AOP values; and Stefhnsson and Richards
suggested that loss of the oxygen to the atmosphere accounted for this.
There was, however, at the same time, a reasonable correlation between
the APU and the AOP for inshore waters: indicating that photosynthesis was the predominating factor regulating the nutrient concentration in this region.
The measured nutrient levels in the sea represent the sum of the
preformed and the regenerated concentrations so that, using the
oxidative ratios given earlier, one may write for phosphate (P) and
nitrate (N) : P,,,, = Pprer + Po, = Ppref + 0.0036 AOU and N,,,, =
N,,,, + No, = N,,,, + 0.058 AOU.
Thus, a plot of measured nutrient concentrations against AOU
values should be linear, and the intercept at zero AOU would correspond to the preformed nutrient concentration (at the depth appropriate
to AOU = 0).
It has been pointed out (Pytkowicz, 1968) that, hidden in these
equations, is the assumption that the oxygen concentration in a mass of
water is equivalent to the sum of the fractional contributions to the
total oxygen content by each water type which mixed to form the mass,
and that this assumption is true only if the water types are similar in
temperature and salinity. In practice, however, the error introduced,
even if the water types have different characteristics, is usually small
and Park (1967) showed that the nutrient and oxygen concentrations
present in the sea off the Oregon coast were in good accord with these
equations.
Broenkow (1956) reported measurements of AOU values and
nutrient concentrations in the Costa Rica dome, a region of upwelling
in the eastern tropical Pacific Ocean where deep waters with higher
than normal salinities and nutrient levels but lower oxygen concentrations are brought to the surface. The AN :LIP value of 16.5 :1 was
normal and independent of depth, but the AAOU :AP ratio gave two
values, namely -274 :1 for water obtained above 120 m and -114 :1
for water below this depth. Broenkow was able to show that loss of
oxygen to the atmosphere was significant only in the upper 20 m and
the points even for these depths were within the confidence limits of the
calculated regression line, The lower ratio found for the deeper waters