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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



rapidly decomposed. Unfortunately, we have no 

 information as yet on these important points. 



In the experiment run with pond water which 

 had supported a large growth of Pandorina for 2 

 weeks preceding the experiment, the growth rate 

 of Chlorella was inhibited 19 percent and Nitzschia 

 30 percent in Berkefeld-filtered pond water when 

 compared with growth rates in pond water, which 

 in addition was washed with Norit A and auto- 

 claved. 



In natural bodies of water, one species of alga 

 seldom if ever exists completely alone, but blooms 

 are generally dominated by one species. How- 

 ever, during the blooms studied in the pond in 

 these experiments, Pandorina became so dominant 

 it is believed that the observed influence of the 

 pond water on the growth rate of Chlorella and 

 Nitzschia can be attributed to antagonistic sub- 

 stances produced by that dominant species. An- 

 other difficulty in the study of the effect of water 

 from ponds, lakes, and the ocean on the growth 

 of phytoplankton is that the water contains, in 

 addition to the inorganic nutrients, organic mate- 

 rials derived not only from the phytoplankton but 

 also from the soil, from animals, and from other 

 plants. 



From information obtained in this study and by 

 other investigators, the author suggests that sub- 

 stances originating from phytoplankton may have 

 one of the following effects upon the growth rate 

 of some species of phytoplankton: (1) They may 

 be necessary for any growth, (2) they may stimu- 

 late growth, or (3) they may inhibit growth. If 

 these assumptions are correct, it can be seen that 

 the seasonal fluctuations in total phytoplankton 

 numbers and in the numbers of each species, as 

 well as a definite succession of species, may in part 

 be dependent upon the phytoplankton itself. 



SUMMARY 



1. The growth rates of both Chlorella and 

 Nitzschia were less when the species were grown 



together in mixed cultures than when they were 

 grown in pure culture, depending upon the size of 

 the populations used. 



2. It was demonstrated that an increase in the 

 inhibition of growth rate of Chlorella populations 

 of the same size occurred with an increase in the 

 initial concentration of Nitzschia. Similarly, an 

 increase in inhibition of the growth rate of small 

 populations of Nitzschia was brought about by in- 

 creasing the initial concentration of Chlorella. 



3. If culture medium in which either Chlorella 

 or Nitzschia had been growing was Berkefekl 

 filtered to remove the cells and nutrients added 

 to the medium and the pH adjusted, the growth 

 rates of both Chlorella and Nitzschia were in- 

 hibited when the species were again grown in this 

 medium. A portion of the same conditioned me- 

 dium which in addition was washed with Norit A 

 and autoclaved did not inhibit the growth rates 

 of either Chlorella, or Nitzschia. It was thus con- 

 cluded that the antagonistic substance was either 

 removed or destroyed by the latter treatment. 



4. When Chlorella was grown on agar the antag- 

 onistic substance produced by it was absorbed by 

 the agar and created an area on which the 

 Nitzschia cells would not grow. 



5. The growth rates of both Chlorella and 

 Nitzschia were inhibited in culture medium pre- 

 pared with pond water which had supported a 

 large growth of Pandorina for a period of 2 weeks. 



6. It is concluded that antagonistic substances 

 arising from the metabolism of phytoplankton are 

 important, at least in fresh-water ponds, in in- 

 fluencing the seasonal fluctuations in total phyto- 

 plankton numbers and in the numbers of each 

 species, as well as in causing a definite succession 

 of species. 



7. It was found that by transferring a small 

 piece of agar on which Nitzschia was growing to 

 the fresh agar, as much growth resulted in 1 week 

 as was previously obtained in from 4 to 6 weeks 

 with transfers made by the conventional bac- 

 teriological streaking method. 



