turbulent tanks were markedly lower both with and without zooplankton 
(Figure 25-7). Statistical analysis of the mean numbers of cells in each 
treatment during the experiment showed that the average standing crop of 
phytoplankton was significantly higher (a = 0.05) in the unstirred tanks. Again, 
this is clearly the reverse of the pattern found in the first three experiments, 
but repeats the trend suggested by the January run (Figure 25-5). There were 
no significant differences in the new numbers of zooplankton between the two 
turbulence levels during this experiment, with both showing small populations 
that fluctuated between about 5-15 animals per liter. This is the first 
experiment in which the phytoplankton showed a significant response to 
turbulence (albeit opposite to that found previously) but zooplankton numbers 
did not. Again, it is interesting to note that an almost 9 fold increase in 
zooplankton numbers did not result in any statistically significant decline in 
the numbers of phytoplankton. 
We attempted to repeat the zooplankton removal experiment during July of 
the following summer with water temperatures between 19-20.5°C. However, 
the hatching and development rate of zooplankton eggs and nauplii is so rapid 
at the higher temperatures that it was virtually impossible to reduce the 
numbers of zooplankton very much by the filtration method used. 
Nevertheless, the results were interesting. The pattern found in the first three 
experiments emerged once again, with phytoplankton growth clearly enhanced 
by the turbulent mixing and zooplankton surpressed (Figures 25-8 and 25-9). 
A final experiment was carried out during August in which the interaction 
of turbulence and water turnover rate in the microcosms was explored. Water 
temperatures varied between 19-21°C. While there was no significant effect of 
turnover rate on the plankton, the same statistically significant stimulation of 
phytoplankton growth was found in the stirred microcosms where zooplankton 
significantly declined by a factor of 2-3 (Figure 25-10). The ten-fold increase in 
phytoplankton associated with somewhat more than a halving of the 
zooplankton in the turbulent microcosms may reflect the tight coupling of 
these two compartments that has been suggested in numerical simulations of 
the summer plankton (Rremer and Nixon 1978). This result contrasts with our 
earlier experiments carried out at lower temperatures in which significant 
reductions in zooplankton numbers had no significant effect on the mean 
phytoplankton standing crop. 
DISCUSSION 
The Importance of Turbulence 
It seems clear that the presence or absence of turbulent mixing in the 
microcosms had a significant influence on the abundance of phytoplankton 
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