Artificial Styrofoam substrates were cxposed
at the sediment surface to support a
growth of periphyton. During the spring
before these experiments, the sterile substrates
were placed randomly within both
cxperimcntal areas. Since the crayfish had
not yet migrated into shallow water and become
active, a diatom growth was able to
develop. Spcrling and Grunewald ( 1969)
found Styrofoam to provide the most reprcscntative
sampling for lotic diatom populations
under the widest range of environmental
conditions : it was unaffected over
a wide range of temperatures, had no food
value, and wet or dry did not sustain mold growth. Represcntativc subsampling was
rclativcly easy with a cork borer.
We had at each location five replicate
substrates subjected to crayfish grazing and
five controls enclosed by wire hardware
cloth (2.5 mesh cm-2), fine enough to cxelude
all crayfish. Light penetration was
reduced by the mesh and a series of experimcnts
was conducted simultaneously to assess
any effects the mesh might have on
plant productivity, measured with Goldman’s
(1963) modification of the 14C
method (Stecmann Nielsen 1952).After
a suitable algal growth had developed, subsamples
were removed both from .the experimental
and control replicates at about
2-week intervals using SCUBA. Each subsample
was placed in 150 ml of filtered lake
water with 2 ml of 5.85 @i ml-1 of 14CNa2C03
added. These were incubated in
situ for 4 h at the sampling depth with the
same light exposure as the original substrates.
Liquid scintillation counting (efficiency
of about 89% throughout) was used
to estimate the amount of 14C fixed.