218
Exercise 14
thought generally. The strongest influence is exerted by the drop of the photosynthetic
activity during the course of the day, whereas neither the subsurface light inhibition
nor the degree of adaptation to low light intensity will produce serious errors. The
best period for obtaining experimental data from a day divided into five equal period
is generally the second one; the production rate of that period can be expected to be
in the order of 30% of the total day rate, provided that nutrient depletion, or its
effect on photosynthesis, remains within moderate limits.
When, instead, no nutrient depletion occurs at all (giving a symmetrical trend of
surface rates), about 25% of the total day rate will be measured during the second,
and 30% during the third, period. In any case, during both the second and third
periods about 55 to 60% of the total day rate is produced. Accordingly, the error
introduced in estimating day rate integrals of photosynthesis from exposures during
the second and third periods will be of the order of ± 10%, or less.
An extension of the latter method may be used to estimate the productivity on
days between closely measured values. If the productivity were measured, for example,
at 5-day intervals, a fair estimate of the productivity of the intervening days would
be derived from the actual measurements, the light energy on the days of productivity
measurements, and light values for days between the actual measurements.
For example, at Lake Hypothetical, the measurement of productivity on August
3, 1990, was 753 mg e/m 3 /day, and the total light value was 638 gcal/cm 2 /day. The
next productivity measurement was not made until August 8, 1990, but light was
measured continuously at the lake with a recording pyrheliometer. Hence, the
estimated values must be viewed as approximate, and extrapolation for more than
a few days is probably not justifiable because of changing densities, species composition, and other factors.
Productivity
Light
Date
(mg Cjm 3/day) (gcal/cm2/day)
Remarks
Aug. 3
753
638
Measured productivity
Aug. 4
662
561
Estimated from productivity of Aug. 3
Aug. 5
851
721
Estimated from productivity of Aug. 3
Aug. 6
388
321
Estimated from productivity of Aug. 8
Aug. 7
152
126
Estimated from productivity of Aug. 8
Aug. 8
521
431
Measured productivity
Expression on an Annual Basis
The daily productivity values then are plotted on an annual scale (Fig. 14.3), and this
annual curve is integrated by planimetry and compared to a standard area of the
graph (e.g., 300 mg e/m 3 /day versus 30 days). From these data, an annual mean
productivity (mean mg elm 3 Iday) may be calculated by dividing the total productivity (g Cjm 3 /year) by 365. The annual mean value is an excellent basis for comparison
of productivity between lake ecosystems [see Wetzel (1983)].
An areal estimate of the primary productivity by phytoplankton for the entire
pelagial zone of a lake may be made by several methods. The most accurate method
is done by determining the annual productivity at each depth interval (0 to 0.5 m, 0.5
to 1.5 m, 1.5 to 2.5 m, and so on) and multiplying these values by the actual volume
of water in that layer (Wetzel, 1964). The annual productivity of each of these strata,
weighted for volume differences, then is summed and divided by 365 to give the
g C/lake/day (Fig. 14.4).
Exercise 14
thought generally. The strongest influence is exerted by the drop of the photosynthetic
activity during the course of the day, whereas neither the subsurface light inhibition
nor the degree of adaptation to low light intensity will produce serious errors. The
best period for obtaining experimental data from a day divided into five equal period
is generally the second one; the production rate of that period can be expected to be
in the order of 30% of the total day rate, provided that nutrient depletion, or its
effect on photosynthesis, remains within moderate limits.
When, instead, no nutrient depletion occurs at all (giving a symmetrical trend of
surface rates), about 25% of the total day rate will be measured during the second,
and 30% during the third, period. In any case, during both the second and third
periods about 55 to 60% of the total day rate is produced. Accordingly, the error
introduced in estimating day rate integrals of photosynthesis from exposures during
the second and third periods will be of the order of ± 10%, or less.
An extension of the latter method may be used to estimate the productivity on
days between closely measured values. If the productivity were measured, for example,
at 5-day intervals, a fair estimate of the productivity of the intervening days would
be derived from the actual measurements, the light energy on the days of productivity
measurements, and light values for days between the actual measurements.
For example, at Lake Hypothetical, the measurement of productivity on August
3, 1990, was 753 mg e/m 3 /day, and the total light value was 638 gcal/cm 2 /day. The
next productivity measurement was not made until August 8, 1990, but light was
measured continuously at the lake with a recording pyrheliometer. Hence, the
estimated values must be viewed as approximate, and extrapolation for more than
a few days is probably not justifiable because of changing densities, species composition, and other factors.
Productivity
Light
Date
(mg Cjm 3/day) (gcal/cm2/day)
Remarks
Aug. 3
753
638
Measured productivity
Aug. 4
662
561
Estimated from productivity of Aug. 3
Aug. 5
851
721
Estimated from productivity of Aug. 3
Aug. 6
388
321
Estimated from productivity of Aug. 8
Aug. 7
152
126
Estimated from productivity of Aug. 8
Aug. 8
521
431
Measured productivity
Expression on an Annual Basis
The daily productivity values then are plotted on an annual scale (Fig. 14.3), and this
annual curve is integrated by planimetry and compared to a standard area of the
graph (e.g., 300 mg e/m 3 /day versus 30 days). From these data, an annual mean
productivity (mean mg elm 3 Iday) may be calculated by dividing the total productivity (g Cjm 3 /year) by 365. The annual mean value is an excellent basis for comparison
of productivity between lake ecosystems [see Wetzel (1983)].
An areal estimate of the primary productivity by phytoplankton for the entire
pelagial zone of a lake may be made by several methods. The most accurate method
is done by determining the annual productivity at each depth interval (0 to 0.5 m, 0.5
to 1.5 m, 1.5 to 2.5 m, and so on) and multiplying these values by the actual volume
of water in that layer (Wetzel, 1964). The annual productivity of each of these strata,
weighted for volume differences, then is summed and divided by 365 to give the
g C/lake/day (Fig. 14.4).