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

complexity of lake productivity will become
apparent.
The exponential decay of light intensity is
a function of depth . Temperature varies with
depth . Although factors leading to lake stratification
are well understood, the construction
of predictive models for temperature distributions
in lakes has only recently begun . Light
intensity varies with time, because of seasonal
and diurnal patterns plus changing weathe r
conditions . There are vertical currents due t o
advection and eddy diffusion in lakes (Rile y
et al . 1949) . These currents cause temperature,
nutrient concentrations, and the population
densities to vary with time . We must als o
consider feedback mechanisms such as selfshading
(Tailing 1960), and local depletion of
nutrients by algae .
Another interesting feedback mechanis m
involves the relation between phytoplankto n
and zooplankton densities . The rate of grazing
on phytoplankton is determined by zooplankton
density and its relative grazing rate (Riley
1947) . On the other hand, the relative grazing
rate of zooplankton is a function of phytoplankton
density ; a saturated grazing rate is
reached at high algal population densities
(Holling 1959, Riley 1947) . Zooplankton also
influence nutrient availability by excretin g
soluble nutrients into the water .
In addition to the limnetic plankton communities,
there are, in lake ecosystems, littoral
communities and, in areas deeper tha n
maximal light penetration, benthic communities.
The processes that occur in these communities
are somewhat different than in th e
limnetic community . Littoral communities
are similar to terrestrial communities in many
respects . Emergent and floating forms have
access to atmospheric carbon dioxide . Attached
forms have direct access to nutrients
held in the sediments ; whereas, limnetic algae
are free floating and must depend upon larg e
ratios of surface area to volume and upo n
passive sinking for efficient nutrient absorption.
In littoral areas, herbivores are small ,
relative to plant size . They consume little of
the plant biomass. Littoral plants show seasonal
die back, with much material being de -
composed . As in terrestrial systems, bacteri a
and detritus feeders are the primary agents of
nutrient regeneration .
Hutchinson and Bowen (1950), in a radiophosphorous
study, showed that littoral
plants can soak up and hold a large quantity
of nutrients . Thus limnetic and littoral plant s
do compete for nutrients . The littoral plants
are less efficient than algae in absorbing nutrients
because of their smaller ratio of surfac e
area to volume . The littoral system's ability to
hold nutrients (because of slow nutrient re -
generation by bacterial decomposition an d
relatively small grazing pressure) offsets inefficient
nutrient absorption. Phytoplankton ,
because of their short life span and hig h
predation rate, cannot hold nutrients as long
as littoral plants, but they are very efficient in
absorbing nutrients .
Deep benthic communities are made up o f
detritus feeders that feed upon seston rainin g
down from the productive epilimnion . They
may be important in nutrient regeneration
(Johannes 1968) . These animals constitute
important food sources for higher orde r
consumers .
A promising theory which may be of great
help in generating and answering question s
about higher consumers, is that of feeding
strategies. For a review of a substantial literature
on the subject, see Schoener (1971) . The
essence of the theory of feeding strategies i s
that selective processes tend to produce populations
of organisms which strive to maximiz e
their energy (essential nutriment) intake .
Predators may accomplish this end in severa l
ways; however, it appears reasonable that i t
would pay to eat the largest prey or the easiest
to capture whenever they are encountered ,
chasing relatively small difficult-to-catch prey
only as a last resort . It is currently being
demonstrated that this theory applies to a
large variety of animals. The consequence is
that one is able not only to predict the foo d
habits of individual species but also to estimate
their growth dynamics .
Acknowledgments
The work reported in this paper was supported
by National Science Foundation Grant
GB-20963 to the Coniferous Forest Biome ,
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