Primary Productivity of Phytoplankton
211
oxygen production and consumption must be multiplied by the ratio of moles of
carbon to moles of oxygen (12 mg Cj32 mg O 2 = 0.375). Then,
G
h
h . ( Cj 3jh) [(02,LB) - (02, DB)] (1000)(0.375)
ross p otosynt eSlS mg m
= - - - - - - - - - - - - - -
(PQ)t)
where t = hours of incubation and O 2 = oxygen in mgjl;
N
h
h · ( Cj 3jh) [(02,LB)-(02,LB)](1000)(0.375)
et p otosynt eSlS mg m
= ~-=----'-----'---=---'-=--'------'--'---'(PQ)(t)
R . . ( Cj 3jh) [(02, IB) -(02, DB)] (RQ)(1000)(0.375)
esplratlOn mg m
= - - - -
t
To estimate the rate of photosynthetic productivity through the water column of
the euphotic zone below one square meter of water surface, the values of mg Cjm 3 jh
are plotted against depth. The area of the curve is integrated with an electronic
digitizer or by planimetry (see Exercise 1, p. 10) and then compared to a known area
on the same graph of known mgCjm 3 jh versus known depth (as in Fig. 1.5).
Assumptions and Errors
It must be remembered that the oxygen change method measures community
metabolism. The respiration measured is not only that of the phytoplankton, but
also includes respiration of bacteria and zooplankton in the sample. Filtration of the
sample through nets to remove larger zooplankton is not recommended because the
phytoplankton community may be altered in the process.
It is further assumed in this calculation that respiration is not affected by
illumination; this assumption is not valid. Respiration results from mitochondrial
activity, which has been shown to be altered by light, and of photorespiration, i.e.,
CO 2 generated from glycolate metabolism, which is influenced by and proportional
to dissolved oxygen concentration, light intensity, and temperature. These factors
may change markedly, and their variations are not taken into account by this method.
The rates of photosynthesis and of community respiration fluctuate throughout
the course of a day. Rates of net photosynthesis often have been observed to be
greater in the early hours of daylight and then to decrease markedly in the afternoon
period of high light, increased concentrations of dissolved oxygen, and higher pH
values. Incubations are made commonly from mid-morning to mid-afternoon (e.g.,
from 1000 to 1400 hours) to compensate to some extent for these variations and to
obtain an average value. Expansion of the productivity values to the daylight period
can be approximated by introducing a diurnal factor, whereby the proportion of
insolation received during the incubation period is expanded to a total for the whole
day (see calculations of the 14C method, pp. 216). This transformation assumes that
photosynthetic rates are proportional to light intensity i.e., that photosynthesis is not
subject to light saturation. Although photosynthetic rates at most depths of a water
column are undersaturated with light because of the exponential attenuation of light
with increasing depth, saturation with light may occur near the surface.
Under ideal conditions of sampling, fixation, and titration, dissolved oxygen determinations can be as precise as ±0.02 mgjl (Exercise 6). However, these conditions
are met rarely in experimental situations, and precision deteriorates as a result.
Assuming a PQ of 1.2 and a statistical probability limit of 0.05, the smallest amount
211
oxygen production and consumption must be multiplied by the ratio of moles of
carbon to moles of oxygen (12 mg Cj32 mg O 2 = 0.375). Then,
G
h
h . ( Cj 3jh) [(02,LB) - (02, DB)] (1000)(0.375)
ross p otosynt eSlS mg m
= - - - - - - - - - - - - - -
(PQ)t)
where t = hours of incubation and O 2 = oxygen in mgjl;
N
h
h · ( Cj 3jh) [(02,LB)-(02,LB)](1000)(0.375)
et p otosynt eSlS mg m
= ~-=----'-----'---=---'-=--'------'--'---'(PQ)(t)
R . . ( Cj 3jh) [(02, IB) -(02, DB)] (RQ)(1000)(0.375)
esplratlOn mg m
= - - - -
t
To estimate the rate of photosynthetic productivity through the water column of
the euphotic zone below one square meter of water surface, the values of mg Cjm 3 jh
are plotted against depth. The area of the curve is integrated with an electronic
digitizer or by planimetry (see Exercise 1, p. 10) and then compared to a known area
on the same graph of known mgCjm 3 jh versus known depth (as in Fig. 1.5).
Assumptions and Errors
It must be remembered that the oxygen change method measures community
metabolism. The respiration measured is not only that of the phytoplankton, but
also includes respiration of bacteria and zooplankton in the sample. Filtration of the
sample through nets to remove larger zooplankton is not recommended because the
phytoplankton community may be altered in the process.
It is further assumed in this calculation that respiration is not affected by
illumination; this assumption is not valid. Respiration results from mitochondrial
activity, which has been shown to be altered by light, and of photorespiration, i.e.,
CO 2 generated from glycolate metabolism, which is influenced by and proportional
to dissolved oxygen concentration, light intensity, and temperature. These factors
may change markedly, and their variations are not taken into account by this method.
The rates of photosynthesis and of community respiration fluctuate throughout
the course of a day. Rates of net photosynthesis often have been observed to be
greater in the early hours of daylight and then to decrease markedly in the afternoon
period of high light, increased concentrations of dissolved oxygen, and higher pH
values. Incubations are made commonly from mid-morning to mid-afternoon (e.g.,
from 1000 to 1400 hours) to compensate to some extent for these variations and to
obtain an average value. Expansion of the productivity values to the daylight period
can be approximated by introducing a diurnal factor, whereby the proportion of
insolation received during the incubation period is expanded to a total for the whole
day (see calculations of the 14C method, pp. 216). This transformation assumes that
photosynthetic rates are proportional to light intensity i.e., that photosynthesis is not
subject to light saturation. Although photosynthetic rates at most depths of a water
column are undersaturated with light because of the exponential attenuation of light
with increasing depth, saturation with light may occur near the surface.
Under ideal conditions of sampling, fixation, and titration, dissolved oxygen determinations can be as precise as ±0.02 mgjl (Exercise 6). However, these conditions
are met rarely in experimental situations, and precision deteriorates as a result.
Assuming a PQ of 1.2 and a statistical probability limit of 0.05, the smallest amount