The interaction of light and turbulence was not significant nor was the effect 
of ammonia enrichment. However the ammonia addition brought the 
concentration in the tanks from ~3/iM to so that the plankton were 
never seriously nutrient limited. It was also interesting that there was no 
response of the phytoplankton in the unmixed microcosms to increased light, 
while there was a clear increase in the stirred tank populations with higher light 
levels (Figure 25-4). The numbers of zooplankton, again dominated by A. 
clausi, were very low throughout the experiment (Nauplii ^10/L; juveniles 
«5/L) and no dramatic differences among treatments developed. However, the 
mean numbers of nauplii and juveniles observed during the experiment were 
higher in the tanks with no paddle and lowest in the tanks with one paddle. 
Analysis of the data showed that this difference in the means was statistically 
significant (a:0.05) and that there was no significant interaction of nauplii, 
juveniles, or adults with light intensity. There was no statistically significant 
difference in the mean number of adults in the different turbulence levels. 
In order to find out if turbulence had a direct stimulating effect on 
phytoplankton, two experiments were carried out during January and 
February in which an attempt was made to remove zooplankton from some of 
the microcosms by filtering the water through a #20 (80 ju) net. This was 
effective in reducing the zooplankton levels by about 70 percent in the first 
experiment and by about 90 percent in the second. In addition, light levels 
were increased from 6 ly/day during the January run to 16 ly/day in February 
and ammonia was added to all tanks at the start of the second experiment in an 
attempt to stimulate vigorous phytoplankton growth. Temperatures ranged 
from 0-0.5°C during the first experiment and from 0-3°C during the second. 
The results of the first experiment showed no significant effect of 
turbulence on the numbers of phytoplankton or zooplankton in the 
microcosms (Figures 25-5 and 25-6). The lack of turbulence effect on the 
phytoplankton was observed in tanks with and essentially without zooplankton 
(Figure 25-5). It is interesting to note that the variation in zooplankton 
numbers by a factor of about 3.5 had no significant effect on the levels of 
phytoplankton, probably due to low temperatures and therefore reduced 
grazing rates. 
When the experiment was repeated a month later with higher light and 
nutrients, a phytoplankton bloom was produced during the first week in all of 
the microcosms (Figure 25-7). During this period there did not appear to be 
any effect of the turbulence on phytoplankton growth either with or without 
zooplankton. Moreover, the grazing pressure of the small number of A. clausi 
in the unfiltered water (~15 animals/L) at these low temperatures had little or 
no effect on the bloom. However, the bloom declined much more slowly in the 
unstirred microcosms, so that after 10-15 days the standing crops in the 
398 
