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)uM to ~6/uM, so that the plankton were 

 never seriously nutrient Umited. It was also interesting that there was no 

 response of the phytoplankton in the unmixed microcosms to increased hght, 

 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 '^lO/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 /j) 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, hght levels 

 were increased from 6 ly/day during the January run to 1 6 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 grov^h. 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 vdthout 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 exp..riment 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 



400 



