ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - AQUATIC PRIMARY PRODUCTION pdf

113
Primary productivity in aquatic systems, like the same
process in terrestrial environments, provides the base of
the food web upon which all higher levels of an ecosystem
depend. Biological productivity is the increase in organic
material per unit of area or volume with time. This addi-
tion of organic matter is the material from which the various
plant and animal communities of an ecosystem are made,
and is dependent on the conversion of inorganic matter
into organic matter. Conversion is accomplished by plants
through the photosynthetic process. Plants are therefore
considered to be the primary producers , and in an aquatic
ecosystem these plants include algae, bacteria, and some-
times higher plants such as water grasses and water lil-
lies. Primary productivity , the first level of productivity in
a system, can be measured as the rate of photosynthesis,
addition of biomass per unit of time (yield), or indirectly
by nutrient loss or a measure of respiration of the aquatic
community.
METHODS OF STUDY
Standing crop refers to the part of biological production
per unit area or per unit volume that is physically present
as biomass and that is not lost in respiration. Standing crop
measurements over a period of time give an indirect mea-
sure of productivity in terms of yield. Plankton, microscopic
floating plants and animals, can be collected in a plankton net
and may be counted under a microscope or weighed. Aquatic
biologists have used standing crop measurements to estimate
productivity longer than any other method (e.g. Lohman,
1908). This method is still also used for periphyton (attached
algae) or rooted plants.

Only within the past few decades have biologists pro-
gressed from merely counting numbers of organisms to
calculating biomass, and more recently, to expressing bio-
mass yield. Fishery biologists, like farmers, for many years
have measured fish productivity in terms of tons produced
per acre of water surface per year. Calculating biomass and
biomass yield is an important step forward since changes in
standing crop reflect the net effect of many biological and
physical events and therefore are not directly proportional
to productivity. For example, the standing crop of a phyto-
plankton community may be greatly diminished by preda-
tion and water movement, while photosynthetic rates of the
survivors may remain high.
The measurement of plant pigments such as chlorophyll a
is also a standing crop measurement that is frequently used
and may now be done through remote sensing by aircraft or
satellites.
UPTAKE OF NUTRIENTS
Another early attempt at measuring the rate of production in
aquatic ecosystems was to measure the inorganic nutrients
taken up in a given system and to calculate the amount of
biological production required to absorb this amount. Atkins
(1922, 1923) studied the decrease in carbon dioxide and
phosphate in measuring production in the North Sea, and
Steel (1956), also working in the North Sea, estimated the
annual cycle of plant production by considering changes in
the inorganic phosphate in relation to vertical mixing of the
water mass. Many biologists consider phosphorus to be a
difficult element to study in this respect because organisms
often store it in excess of their requirements for optimum

growth.
Measuring nutrient uptake in an indirect method of deter-
mining the rate of productivity in an aquatic ecosystem and is
influenced by various other biological activities. Nevertheless,
it has been important in the development toward more precise
measurements of the dynamic aquatic ecosystem.
MEASUREMENTS OF OXYGEN AND CARBON
DIOXIDE
The net rate at which the phytoplankton community of a
given ecosystem incorporates carbon dioxide may be esti-
mated in moderately to highly productive aquatic environ-
ments by direct measurement of the short-term fluctuations
in the dissolved oxygen it produces. The calculations are
based on the assumption that a mole of oxygen is released
into the environment for each mole of carbon dioxide reduced
in photosynthesis. This method precludes the necessity of
enclosing the phytoplankton in a bottle. If measurements are
made at regular hourly intervals over a 24-hour period, the
average hourly decrease in oxygen during times of darkness
when no photosynthesis is occurring can be determined. It is
assumed that respiration removes this amount of oxygen each
hour throughout the day thus giving a measure of the gross
rate at which the community incorporates carbon dioxide.
AQUATIC PRIMARY PRODUCTION
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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 amounts
of
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 ECOSYSTEMS
Lindeman (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, and
© 2006 by Taylor & Francis Group, LLC
AQUATIC PRIMARY PRODUCTION 115
ecology require such a complex model of energy flow that it
is difficult to relate it to the natural world. In an imaginary
world or model of a system in which the function units are
discrete trophic levels, it is not only possible but stimulating
to describe the flow of energy through an ecosystem. But
when the functional units of the system being investigated
are conceived of as macromolecules it is difficult to translate
biomass accumulation into energy units.
Besides requiring a portion of their autotrophic produc-
tion for respiration, phytoplankton communities must also
reserve a portion for the maintenance of community struc-
ture. In terms of information theory, energy expended for
community maintenance is referred to as “ information .”
Energy information cost has never been measured directly
but there is indirect evidence that it must be paid. For exam-
ple, when an aquatic ecosystem is altered artificially with

the aim of increasing the production of fish, zooplankton
and fish may increase in greater proportion than the phy-
toplankton (McConnell, 1965; Goldman, 1968). Perhaps a
large amount of primary production remains with the phy-
toplankton as information necessary for the maintenance
or development of community structure. Grazers then have
access only to the production in excess of this threshold
level. If the magnitude of the information cost is high rela-
tive to primary production, then a small increase in the rate
of growth of the primary producers will provide a relatively
larger increase in the food supply of grazers and in turn the
fish that consume them.
There are difficulties that must be met in the course of
fitting measurements of primary productivity to the trophic-
dynamic model. A highly variable yet often significant
portion of primary production, as measured by
14
C light-
and-dark bottle experiments, is not retained by the produc-
ers but instead moves into the environment in soluble form.
It is difficult to measure the absolute magnitude of such
excretion by a community of natural plankton because the
excreta can rapidly serve as a substrate for bacterial growth
and thus find its way back to particulate or inorganic form
during the incubation period. Although this excrement is
part of the primary productivity and eventually serves as
an energy source for organisms at the higher trophic levels,
the pathway along which this energy flows does not follow
the usual linear sequence modeled for the transfer of energy
from phytoplankton to herbivorous zooplankton. There is

evidence that the amount of energy involved may some-
times be of the same order of magnitude as that recovered in
particulate form in routine
14
C productivity studies.
The role of allochthonous material (material brought in
from outside the system) in supporting the energy require-
ments of consumer organisms must also be considered
in studies of energy flow. No natural aquatic ecosystem is
entirely closed. Potential energy enters in the form of organic
solutes and debris. Organic solutes undergo conversion to
particulate matter through bacterial action. Sorokin (1965) in
Russia found this type of production of particulate matter to
be the most important in producing food for crustacean filter-
feeders. Particulate and dissolved organic matter may also
arise in the aquatic environment through chemosynthesis.
This is a form of primary production not usually considered
and therefore not usually measured. Although its magnitude
may not be great in many systems, Sorokin found it to be very
important in the Rybinsk reservoir and in the Black Sea.
PRIMARY PRODUCTION AND EUTROPHICATION
The process of increasing productivity of a body of water is
known as eutrophication and in the idealized succession of
lakes, a lake would start as oligotrophic (low productivity),
becoming mesotrophic (medium productivity) eventually
eutrophic (highly productive) and finally dystrophic, a bog
stage in which the lake has almost been filled in by weeds and
the productivity has been greatly decreased. The concept of
eutrophic and oligotrophic lake types is not a new one. It was
used by Naumann (1919) to indicate the difference between

the more productive lakes of the cultivated lowlands and the
less productive mountain lakes. The trophic state of five dif-
ferent aquatic environments will be discussed below.
The general progression from an oligotrophic to an eutro-
phic and finally to a dystrophic lake (lake succession) is as
much a result of the original basin shape, climate, and such
edaphic factors as soil, as it is of geologic age. It is unlikely
that some shallow lakes ever passed through a stage that
could be considered oligotrophic, and it is just as unlikely
that the first lake to be considered here, Lake Vanda, will
ever become eutrophic. It is also possible that the “progres-
sion” may be halted or reversed.
Lake Vanda, located in “dry” Wright Valley near
McMurdo Sound in Antarctica, is one of the least productive
lakes in the world. The lake is permanently sealed under 3 to
4 meters of very clear ice which transmits 14 to 20% of the
incident radiation to the water below. This provides enough
light to power the photosynthesis of a sparse phytoplankton
population to a depth of 60 meters (Goldman et al. , 1967).
Lake Vanda can be classified as ultraoligotrophic, since its
mean productivity is only about 1 mg C·m
Ϫ2
·hr
Ϫ1
.
Lake Tahoe in the Sierra Nevada of California and
Nevada is an alpine lake long esteemed for its remarkable
clarity. Although it is more productive than Lake Vanda, it is
still oligotrophic. The lake is characterized by a deep eupho-
tic (lighted) zone, with photosynthesis occurring in the phy-

toplankton and attached plants to a depth of about 100 m.
Although the production under a unit of surface area is not
small, the intensity of productivity per unit of volume is
extremely low. Lake Tahoe’s low fertility (as inferred from
its productivity per unit volume) is the result of a restricted
watershed, whose granitic rocks provide a minimum of
nutrient salts. This situation is rapidly being altered by
human activity in the Tahoe Basin. The cultural eutrophica-
tion of the lake is accelerated by sewage disposal in the basin
and by the exposure of mineral soils through road build-
ing and other construction activities. Since Lake Tahoe’s
water is saturated with oxygen all the way down the water
column, the decomposition of dead plankton sinking slowly
towards the bottom is essentially complete. This means that
nutrients are returned to the system and because of a water
© 2006 by Taylor & Francis Group, LLC
116 AQUATIC PRIMARY PRODUCTION
retention time of over 600 years the increase in fertility will
be cumulative.
Castle Lake, located at an elevation of 5600 feet in the
Klamath Mountains of northern California, shows some of
the characteristics of Lake Tahoe as well as those of more
productive environments. It, therefore, is best classified as
mesotrophic. Although it has a mean productivity of about
70 mg C·m
Ϫ2
·hr
Ϫ1
during the growing season, it shows a
depletion in oxygen in its deep water during summer stratifi-

cation and also under ice cover during late winter.
Clear lake is an extremely eutrophic shallow lake with
periodic blooms of such bluegreen algae as Aphanizomenon
and Microcystis and inorganic turbidity greatly reducing the
transparency of the water. The photosynthetic zone is thus
limited to the upper four meters with a high intensity of
productivity per unit volume yielding an average of about
300 mg C·m
Ϫ2
·hr
Ϫ1
during the growing season. Because
Clear Lake is shallow, it does not stratify for more than a few
hours at a time during the summer, and the phytoplankton
which sink below the light zone are continuously returned
to it by mixing.
Cedar Lake lies near Castle Lake in the Klamath
Mountains. Its shallow basin is nearly filled with sediment
as it nears the end of its existence as a lake. Numerous scars
of similar lakes to be found in the area are prophetic of Cedar
Lake’s future. Terrestrial plants are already invading the lake,
and higher aquatic plants reach the surface in many places.
The photosynthesis beneath a unit of surface area amounts
to only about 6.0 mg C·m
Ϫ2
·hr
Ϫ1
during the growing season
as the lake is now only about four meters in depth and may
be considered a dystrophic lake. Some lakes of this type pass

to a bog condition before extinction; in others, their shallow
basins may go completely dry during summer and their flora
and fauna become those of vernal ponds.
In examining some aspects of the productivity of these
five lakes, the variation in both the intensity of photosyn-
thesis and the depth to which it occurs is evident. The great
importance of the total available light can scarcely be over-
emphasized. This was first made apparent to the author
during studies of primary productivity and limiting factors
in three oligotrophic lakes of the Alaskan Peninsula, where
weather conditions imposed severe light limitations on the
phytoplankton productivity. The average photosynthesis on
both a cloudy and a bright day was within 10% of being
proportional to the available light energy.
Nutrient limiting factors have been reviewed by Lund
(1965) and examined by the author in a number of lakes. In
Brooks Lake, Alaska a sequence of the most limiting factors
ranged from magnesium in the spring through nitrogen in the
summer to phosphorous in the fall (Goldman, 1960). In Castle
Lake potassium, sulfur, and the trace element molybdenum
were found to be the most limiting. In Lake Tahoe iron and
nitrogen gave greatest photosynthetic response with nitrogen
of particular importance. Trace elements, either singly or in
combination, have been found to stimulate photosynthesis
in quite a variety of lakes. In general, some component of the
phytoplankton population will respond positively to almost
any nutrient addition, but the community as a whole will
tend to share some common deficiencies. Justus von Liebig
did not intend to apply his law of the minimum as rigidly as
some have interpreted it, and we can best envision nutrient

limitation from the standpoint of the balance and interac-
tions of the whole nutrient medium with the community of
organisms present at any given time. Much about the nutri-
ent requirements of phytoplankton can be gleaned from the
excellent treatise of Hutchinson (1967).
It must be borne in mind that the primary productivity
of a given lake may vary greatly from place to place, and
measurements made at any one location may not provide a
very good estimate for the lake as a whole.
Variability in productivity beneath a unit of surface area
is particularly evident in Lake Tahoe, where attached algae
are already becoming a nuisance in the shallow water and trans-
parency is often markedly reduced near streams which drain
disturbed watersheds. In July, 1962, the productivity of Lake
Tahoe showed great increase near areas of high nutrient inflow
(Goldman and Carter, 1965). This condition was even more
evident in the summer of 1967 when Crystal Bay at the north
end of the lake and the southern end of the lake showed differ-
ent periods of high productivity. This variability in productivity
may be influenced by sewage discharge and land disturbance.
Were it not for the great volume of the lake (155 km
3
), it would
already be showing more severe signs of eutrophication.
In the foregoing paper I have attempted to sketch my
impressions of aquatic primary productivity treating the sub-
ject both as a research task and as a body of information to
be interpreted. I believe that biological productivity can no
longer be considered a matter of simple academic interest, but
of unquestioned importance for survival. The productivity and

harvest of most of the world’s terrestrial and aquatic environ-
ments must be increased if the world population is to have any
real hope of having enough to eat. This increase is not possible
unless we gain a much better understanding of both aquatic and
terrestrial productivity. Only with a more sound understanding
of the processes which control productivity at the level of the
primary producers can we have any real hope of understanding
the intricate pathways that energy moves and biomass accumu-
lates in various links of the food chain. With this information in
hand the productivity of aquatic environments can be increased
or decreased for the benefit of mankind.
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biological relation, J. Mar. Biol. Assoc. UK , 12, 717–771.
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and Reservoirs , F. B. Taub, Ed., Elsevier, Amsterdam.
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C for eliminating serious
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CHARLES R. GOLDMAN

University of California, Davis
ATMOSPHERIC: see also AIR—various titles
© 2006 by Taylor & Francis Group, LLC