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 higlier (a = 0.05) in the unstirred tanks. Again, 

 tliis is clearly the reverse of tlie pattern tound 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. Tliis is the tust 

 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 higlier temperatures that it was virtually impossible to reduce the 

 numbers of zooplankton very much by the filtration method used. 

 Nevertheless, the results were interesting. Tlie pattern found in the tlrst three 

 experiments emerged once again, v^th phytoplankton growth clearly enlianced 

 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*^0. Wliile 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 tiglit coupling of 

 these two compartments that has been suggested in numerical simulations of 

 the summer plankton (Kremer and Nixon 1978). This result contrasts with our 

 earUer 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 



404 



