The preceding experiment was also duplicatcd
in an environmental control chamber.
Six pieces of Styrofoam ( 17 x 35 cm)
were suspended at 5 m in Lake Tahoe for
about 60 days until an abundant growth of
periphyton had developed. Six 15-liter
aquaria, each containing one of the styrofoam
substrates, were filled from a single
large sample of Lake Tahoe water from 5
m and kept at 18°C on a 14-h photoperiod
from a bank of overhead “cool-white” fluorescent
lamps, at a light intensity of 5,000
lux. Each aquarium was divided into a control
and expcrimcntal (grazing) compartment
by a perforated plastic partition. A
laboratory stock of various sizes of P. kni-
UXUZUS was obtained from Lake Tahoe by
SCUBA diving, and one animal was introintroduced
into each cxpcrimental compartment
after starvation for 3 weeks to minimize the
contribution of external nutrients to the
microcosms. Subsamples were then taken
from the expcrimcntal and control compartments
weekly for 5 weeks. Primary productivity
analysis followed the procedure for
the field experiments.