208
Exercise 14
tions, pH, or specific conductance. The resultant
evaluation is a measure of community metabolism, and a number of critical assumptions
must be made when assessing which components
of the overall biological communities are causing
the observed changes in environmental parameters over short periods of time. This approach
will be undertaken in Exercise 24.
Finally, autotrophic productivity of lake ecosystems that are sufficiently large to stratify
thermally can be estimated indirectly by measuring long-term changes in biomass, reductions
in certain nutrients, or hypo lim netic oxygen
deficits or accumulations of CO 2 . This systems
approach will be treated separately in Exercise
29.
IN SITU SAMPLING
To demonstrate the methodology with in situ sampling and its limitations, it is
recommended that the primary productivity of a single vertical profile be determined
for the deepest part of a representative lake(s). Research applications, in which accurate
evaluations of phytoplanktonic productivity are necessary, would normally require
some horizontal analysis of the spatial heterogeneity in phytoplankton distribution
and productivity.
The water within a vertical profile should be collected, beginning at the surface and
working downward, with an opaque, nonmetallic water sampler. The Van Dorn
sampler discussed earlier (p. 82) is well suited. Sufficient metal ions leach from metallic
samplers, even upon brief exposure to the sample, to affect rates of photosynthesis
significantly in subsequent incubations.
The objective is to measure variations in the depth profiles of photosynthetic
activity per unit volume of water. From the depth profile, the photosynthetic activity
below a given area of lake surface can be calculated by integration. The number of
samples required is governed by the gradient and depth of the euphotic zone. Photosynthesis is generally, but not always, reduced to zero at the depth where ambient
light intensity is reduced to ca. 0.5% of the surface value. When possible, replicate
samples should be taken at each meter of depth in low to moderately productive
waters, and at 0.5-m intervals in shallow, productive waters where the light is
attenuated rapidly. The narrower the depth intervals, the more accurate will be the
profile of productivity. A minimum of five depths should be sampled through the
euphotic zone. Bottles then are filled as rapidly as possible under shaded conditions
and then held in a light-proof box until samples from all depths have been collected.
CHANGES IN DISSOLVED OXYGEN
Water samples from the various depths are enclosed in both transparent ("light")
and in completely opaque ("dark") bottles. The bottles should be of high-quality
glass, e.g., Pyrex, with the tips of the ground-glass stoppers tapered to permit sealing
without including any air bubbles. While quartz bottles are preferred because they
do not absorb short-wave radiation as normal silica glass does (Findenegg, 1966),
quartz bottles are simply too expensive for most purposes. Ultraviolet light usually
is absorbed rapidly in water (Exercise 2), and therefore its inhibitory effect would be
most pronounced in the upper layers (1 to 2 m) of the lake. Dark bottles can be made
by covering them with a double layer of black, plastic electrician's tape to completely
exclude light.
Exercise 14
tions, pH, or specific conductance. The resultant
evaluation is a measure of community metabolism, and a number of critical assumptions
must be made when assessing which components
of the overall biological communities are causing
the observed changes in environmental parameters over short periods of time. This approach
will be undertaken in Exercise 24.
Finally, autotrophic productivity of lake ecosystems that are sufficiently large to stratify
thermally can be estimated indirectly by measuring long-term changes in biomass, reductions
in certain nutrients, or hypo lim netic oxygen
deficits or accumulations of CO 2 . This systems
approach will be treated separately in Exercise
29.
IN SITU SAMPLING
To demonstrate the methodology with in situ sampling and its limitations, it is
recommended that the primary productivity of a single vertical profile be determined
for the deepest part of a representative lake(s). Research applications, in which accurate
evaluations of phytoplanktonic productivity are necessary, would normally require
some horizontal analysis of the spatial heterogeneity in phytoplankton distribution
and productivity.
The water within a vertical profile should be collected, beginning at the surface and
working downward, with an opaque, nonmetallic water sampler. The Van Dorn
sampler discussed earlier (p. 82) is well suited. Sufficient metal ions leach from metallic
samplers, even upon brief exposure to the sample, to affect rates of photosynthesis
significantly in subsequent incubations.
The objective is to measure variations in the depth profiles of photosynthetic
activity per unit volume of water. From the depth profile, the photosynthetic activity
below a given area of lake surface can be calculated by integration. The number of
samples required is governed by the gradient and depth of the euphotic zone. Photosynthesis is generally, but not always, reduced to zero at the depth where ambient
light intensity is reduced to ca. 0.5% of the surface value. When possible, replicate
samples should be taken at each meter of depth in low to moderately productive
waters, and at 0.5-m intervals in shallow, productive waters where the light is
attenuated rapidly. The narrower the depth intervals, the more accurate will be the
profile of productivity. A minimum of five depths should be sampled through the
euphotic zone. Bottles then are filled as rapidly as possible under shaded conditions
and then held in a light-proof box until samples from all depths have been collected.
CHANGES IN DISSOLVED OXYGEN
Water samples from the various depths are enclosed in both transparent ("light")
and in completely opaque ("dark") bottles. The bottles should be of high-quality
glass, e.g., Pyrex, with the tips of the ground-glass stoppers tapered to permit sealing
without including any air bubbles. While quartz bottles are preferred because they
do not absorb short-wave radiation as normal silica glass does (Findenegg, 1966),
quartz bottles are simply too expensive for most purposes. Ultraviolet light usually
is absorbed rapidly in water (Exercise 2), and therefore its inhibitory effect would be
most pronounced in the upper layers (1 to 2 m) of the lake. Dark bottles can be made
by covering them with a double layer of black, plastic electrician's tape to completely
exclude light.