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^ ^2 ·hr^ ^1 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^ ^2 ·hr^ ^1 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^ ^2 ·hr^ ^1 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 willtend 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
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