220
Exercise 14
et aI., 1977). After the samples are dry, the material is resuspended in I to 5 ml of
water and 15 to 19 ml of scintillation fluid (e.g., Insta-gel, Packard Instr. Co.).
The rate of organic carbon released extracellularly in dissolved form then can be
calculated as mg C/m 3/day in similar fashion to that outlined above. These values
then can be compared for each depth to evaluate the percentage of dissolved organic
carbon released to that fixed in particulate organic matter.
Sensitivity
Theoretically there is no upper limit of sensitivity for the l4C method, but in practice
errors of time required for manipulations increase as photosynthetic rates become
exceedingly high. The lower limit is in the range of 0.01 mg C/m 3 /h, some three orders
of magnitude lower than that of the oxygen method.
Most evidence of comparative studies indicates that the l4C method measures
photosynthetic rates closer to net photosynthesis than to gross, especially when the
rate of dissolved organic matter released is added to that carbon fixed in particulate
organic matter. Organic carbon budgets for cultures of lake phytoplankton indicated
that l4C uptake overestimates net carbon production. The overestimate was slight
(10 to 20%) for rapidly increasing cultures but was higher for nutrient-poor, slowly
growing cultures (Peterson, 1978).
EXERCISES
OPTION 1. FiElD TRIPS
1. At the central depression of a lake, make a systematic comparison of the primary productivity
of the phytoplankton using the oxygen and 14C light and dark techniques. Collect water
samples, as discussed, in teams.
2. One team should obtain replicated samples for light, dark, and initial oxygen concentration
analysis at meter intervals (half-meter intervals near the surface, if productive). Part of the
team should fix the initial bottles immediately. The dissolved oxygen concentrations can
be determined by titration during the incubation period of the light and dark samples.
3. A second team should work with the instructor to collect water samples for replicated light
and dark bottles for the 14C method at the same depths as done for the oxygen technique.
Samples for alkalinity, and pH, and DIC measurements should be taken simultaneously;
these analyses can be made during the incubation period.
4. A third team should determine the temperature profile at the station and the underwater
light distribution. When instruments are available, measure the incident and reflected light
at each depth interval and, using specific filters, the distribution of spectral components
(see Exercise 2).
5. When possible, another team could collect samples of the phytoplankton at each depth
to determine simultaneously algal biomass by analyses of pigment concentrations (see
Exercise 10).
6. Proceed with all of the analyses as discussed earlier. Determine the productivity of the
phytoplankton by both the oxygen and 14C methodologies and compare your results.
7. Compare the productivity measurements with the vertical stratification patterns of
temperature and with the attenuation patterns of underwater light.
8. If phytoplanktonic biomass was determined, compare its distribution with the productivity
values.
Exercise 14
et aI., 1977). After the samples are dry, the material is resuspended in I to 5 ml of
water and 15 to 19 ml of scintillation fluid (e.g., Insta-gel, Packard Instr. Co.).
The rate of organic carbon released extracellularly in dissolved form then can be
calculated as mg C/m 3/day in similar fashion to that outlined above. These values
then can be compared for each depth to evaluate the percentage of dissolved organic
carbon released to that fixed in particulate organic matter.
Sensitivity
Theoretically there is no upper limit of sensitivity for the l4C method, but in practice
errors of time required for manipulations increase as photosynthetic rates become
exceedingly high. The lower limit is in the range of 0.01 mg C/m 3 /h, some three orders
of magnitude lower than that of the oxygen method.
Most evidence of comparative studies indicates that the l4C method measures
photosynthetic rates closer to net photosynthesis than to gross, especially when the
rate of dissolved organic matter released is added to that carbon fixed in particulate
organic matter. Organic carbon budgets for cultures of lake phytoplankton indicated
that l4C uptake overestimates net carbon production. The overestimate was slight
(10 to 20%) for rapidly increasing cultures but was higher for nutrient-poor, slowly
growing cultures (Peterson, 1978).
EXERCISES
OPTION 1. FiElD TRIPS
1. At the central depression of a lake, make a systematic comparison of the primary productivity
of the phytoplankton using the oxygen and 14C light and dark techniques. Collect water
samples, as discussed, in teams.
2. One team should obtain replicated samples for light, dark, and initial oxygen concentration
analysis at meter intervals (half-meter intervals near the surface, if productive). Part of the
team should fix the initial bottles immediately. The dissolved oxygen concentrations can
be determined by titration during the incubation period of the light and dark samples.
3. A second team should work with the instructor to collect water samples for replicated light
and dark bottles for the 14C method at the same depths as done for the oxygen technique.
Samples for alkalinity, and pH, and DIC measurements should be taken simultaneously;
these analyses can be made during the incubation period.
4. A third team should determine the temperature profile at the station and the underwater
light distribution. When instruments are available, measure the incident and reflected light
at each depth interval and, using specific filters, the distribution of spectral components
(see Exercise 2).
5. When possible, another team could collect samples of the phytoplankton at each depth
to determine simultaneously algal biomass by analyses of pigment concentrations (see
Exercise 10).
6. Proceed with all of the analyses as discussed earlier. Determine the productivity of the
phytoplankton by both the oxygen and 14C methodologies and compare your results.
7. Compare the productivity measurements with the vertical stratification patterns of
temperature and with the attenuation patterns of underwater light.
8. If phytoplanktonic biomass was determined, compare its distribution with the productivity
values.