114 AQUATIC PRIMARY PRODUCTION
An analogous method exists for recording fluctuations in
carbon dioxide.
The pH meter, which measures acidity, has been suc-
cessfully employed to measure these carbon dioxide changes
in the aquatic ecosystem since the removal of carbon dioxide
from the water for photosynthesis is accompanied by a pro-
portional rise in pH. This pH shift has been used to estimate
both photosynthesis and respiration. The sea and some fresh
waters are too buffered against changes in pH to make this
method useful in all environments, but it has been employed
with success in lakes and for continuously monitoring the
growth of cultures. Carbon dioxide may also be directly
measured by standard volumetric or gasometric techniques.
Although carbon dioxide and oxygen can be measured
with relative precision, the overall precision of productiv-
ity measurements made by these techniques is not generally
great because of uncertainties in the corrections for diffu-
sion, water movements, or extended enclosure time. Some
of the oxygen produced by higher aquatic plants may not be
immediately released thus causing a lag period in the evolu-
tion of oxygen into the environment. The primary advantage
this method has over the more sensitive^14 C method is the
added benefit of an estimate of community respiration.
Some of the uncertainties of the previous method can
be reduced by enclosing phytoplankton samples just long
enough in glass bottles for measurable changes in the con-
centration of oxygen and carbon dioxide to occur, but not
long enough for depletion of nutrients or the growth of bac-
teria on the inside bottle surface. This method is called the
light and dark bottle method. The name is derived from the
fact that identical samples are placed in a transparent “light
bottle” and an opaque “dark bottle.” Gross and net produc-
tivity of the plankton community from which the samples
were taken can be estimated by calculating the difference in
the oxygen content between the two bottles after a predeter-
mined period of incubation and with that present initially.
Productivity determinations that are dependent on mea-
surements of oxygen are based on some estimated photosyn-
thetic quotient (moles O 2 liberated/moles CO 2 incorporated).
For the photosynthesis of carbohydrates the ratio is unity.
For the synthesis of an algal cell, however, the expected ratio
is higher, and presumably varies with the physiological state
of the algae and the nutrients available.
Oxygen methods in general have rather poor sensitiv-
ity and are of no use if the gross incorporation of inorganic
carbon during the test period is less than about 20 mg of
carbon per cubic meter. Several days may be required in
many of the less productive aquatic environments for this
much photosynthesis to occur and bacteria may develop on
the insides of the container during this time, invalidating the
results.
Photosynthetic rates can be measured in light and dark
bottles also by determining the amount of carbon fixed in
particulate form after a short incubation. This can be done by
inoculating the bottles with radioactive carbon (Na 2 14 CO 3 ).
Sensitivities with this method are much greater than the
standard method and much shorter periods of incubation are
possible. It is possible to obtain easily measurable amountsof^14 C in particulate form after only two hours by adjusting
the specific activity of the inoculums. However, unlike the
oxygen method, the dark bottle results do not provide an
estimate of community respiration thus giving the ecologist
less information with which to work.
The^14 C method has been widely used because it is sensi-
tive and rapid. One outcome of its popularity is that a great
deal of scrutiny has been devoted to the method itself. After
18 years of use, however, it is still not clear whether the^14 C
is measuring gross productivity, net productivity, or some-
thing in between. The results probably most closely estimate
net productivity, but it may be that this method applies only
to a particular set of experimental conditions.
Already mentioned is the evidence that some of the^14 C
that is fixed during incubation may seep out of the algal cells in
the form of water-soluble organic compounds. This material is
presumably utilized by bacteria rather than passed on directly
to the next higher trophic level as is the remainder of the con-
sumed primary productivity. The amount of primary production
liberated extracellularly is large enough to be measured with
precision and a number of workers are now routinely including
quantitative studies of extracellular products of photosynthesis
as part of the measurements of primary productivity.
Calibration of radioactive sources and instruments for
measuring radioactivity pose a serious technical problem for
the^14 C method. In order to calculate productivity in terms of
carbon uptake it is necessary to know accurately the amount
of^14 C added in microcuries and the number of microcuries
recovered in particulate form by filtering the sample through
a membrane filter.
Further it has been found that phytoplankton cells may
become damaged during filtration and calculations based on
these conditions will show lower productivity rates than are
actually the case.
A point deserving emphasis is that those of us measuring
primary productivity are still attempting to determine more
precisely what is being measured, and generalizations about
the transfer of energy through aquatic food-webs should be
made continuously. Neither this nor any other practical tech-
nique adequately measures the change in oxidation state of
the carbon that is fixed. The subsequent ecological role of
newly fixed carbon is even more difficult to measure because
of the various ways the photosynthate may be used.USE OF PRIMARY PRODUCTIVITY
MEASUREMENTS IN AQUATIC ECOSYSTEMSLindeman (1942) developed a trophic-dynamic model of an
aquatic ecosystem and introduced the concept of “energy
flow,” or the efficiency of energy transfer from one trophic
level to the next, to describe its operation. A certain value
derived from the measured primary productivity represented
the input of energy into the next grazing level, and so forth
up the food chain. It was consistent with Lindeman’s purpose
to express his data as energy units (calories). Subsequent
workers have continued to probe the concept of energy
flow. However, advances in biochemistry, physiology, andC001_007_r03.indd 114C001_007_r03.indd 114 11/18/2005 10:14:56 AM11/18/2005 10:14:56 AM