PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest

est adapted organism vacillates betwee n
species with time as the environment changes .
The level of nutrients in a lake ultimatel y
determines the level of phytoplankton whic h
the lake can support . Nutrients are utilized b y
plants as components of structural molecules ,
such as DNA, RNA and enzymes . If nutrien t
levels are in excess of structural demands an d
other conditions are favorable for photosynthesis,
then reproduction can occur . Nutrient s
are rare elements in lake ecosystems, very
eutrophicated lakes being exceptions . Aquati c
communities have evolved many mechanisms
fOr their conservation (Pomeroy 1970) . Algae
and littoral plants can absorb nutrients almos t
instantaneously from the water . Aquatic plants
can take up nutrients as they are available and
store them until conditions become more suit -
able for growth when the nutrients are utilized .
Light and temperature conditions necessary for
photosynthesis are much more predictable ,
with definite seasonal and diurnal patterns ,
than nutrient levels, which the algae cells en -
counter more or less randomly . Then, from the
viewpoint of algae, a good strategy to evolve
would be a means of acquiring nutrient s
whenever they occur and wa iting to make use
of them until sufficient light and temperature
conditions (these are predictable) prevail .
In addition to being able to utilize nutrient s
as they occur, the phytoplankton-zooplankto n
community recycles nutrients rapidly . As th e
algae die and sink to the bottom, nutrients are
released in two ways . Autolytical release occurs
by simple diffusion. Mechanical release occurs
when the cell membrane ruptures . Betwee n
25 and 70 percent of the nutrients containe d
in a sinking, dead organism are released i n
these ways (Johannes 1968) .
Zooplankton eat substantial proportions o f
the algae . Herbivores are large in relationship
to their food supply . Since zooplankton are
eating a rich source of essential nutrients ,
they consume many more nutrients than they
need for structural components. The excess is
simply excreted into the water .
The classical role of bacteria, as the principal
agents of nutrient regeneration is questioned
by Johannes (1968) and Pomeroy
(1970). Bacteria accumulate nutrients ove r
the level required in growth, as algae do .
Johannes (1968) believes that protozoa, ofte n
neglected as important organisms in lak e
systems, are responsible for regeneratin g
nutrients by eating bacteria and excreting th e
excess nutrients .
In addition to biological means of nutrient
cycling, many purely physical processes affec t
nutrient distribution in lake systems . Nutrients
enter the lake through the inflow (both surface
flow and seepage) and precipitation . Nutrients
leave the lake through outflow (both surface
flow and seepage) . Sediments are very important
in the nutrient budgets of lakes . They may
act as a trap for essential nutrients, such a s
phosphorous. The phosphorous is tied up b y
iron and precipitated out at high redox potentials.
Decreasing redox potential usually ac -
companies decreasing oxygen concentration .
When the redox potential falls below a certain
level the phosphorous goes into solution, there -
by becoming available to photosynthesizers i n
the waters above . In oligotropic lakes th e
redox potential is always high and phosphorous
bound in falling detritus is lost to th e
system when it reaches the sediments . Th e
cycling of nitrogen is more complicated an d
even more intimately tied in with the redo x
potential . The rate at which essential nutrients
are recycled through the community, interchanged
with the sediments, and lost and
replenished through inflow and outflow deter -
mines the productivity of the lake . The
response of this system to perturbation, measured
not only by the level of productivity but
by changes in community structure, is th e
central theme of our research effort .
Production is usually defined as the total
elaboration of organic matter by photosynthetic
organisms in a specified time period ,
while productivity is the production per unit
time. Photosynthetic rate depends principall y
upon light intensity, temperature, and essential
nutrient concentration (Riley 1946 ,
Rhyther 1956, Tailing 1961) . Photosynthetic
rate is different for different species of alga e
(Talling 1955) . The production under a uni t
area of lake surface is the sum over species o f
the integral with respect to time and depth o f
the algae densities times their respectiv e
photosynthetic rates . The above generalization
may appear overly simple ; however, the
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