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 EUTROPHICATIONThe 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^ ^2 ·hr^ ^1.
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 waterC001_007_r03.indd 115C001_007_r03.indd 115 11/18/2005 10:14:57 AM11/18/2005 10:14:57 AM