BIOTIC INFLUENCES AFFECTING POPULATION GROWTH OF 



PLANKTONIC ALGAE 



By Theodore R. Rice, Fishery Research Biologist 



The enormous fluctuation in abundance of 

 phytoplankton in oceans, lakes, and ponds has at- 

 tracted the attention of oceanographers and lim- 

 nologists for over half a century. Many hy- 

 potheses have been advanced to explain this phe- 

 nomenon. The common belief has been that the 

 abundance of each species, the size of the total 

 population, and the succession of species during 

 the season are controlled by changes in the phys- 

 ical factors or by a lack of the necessary nutrients, 

 or by a combination of the two influences. Atten- 

 tion lias also been called to the action of filter- 

 feeding animals in reducing the numbers of the 

 phytoplankton. 



The further suggestion has been made that the 

 intensive growth, or bloom, 1 of one species might 

 affect the growth of another species, thus exert- 

 ing an influence on the seasonal succession of 

 species in the body of water concerned. Akehurst 

 (1931) made a study of phytoplankton in fresh- 

 water ponds over a period of 4 years. All at- 

 tempts to correlate fluctuations with chemical and 

 physical factors failed. Therefore, he reasoned 

 that other factors such as the complicated action 

 of toxins were very important. These toxins were 

 believed to originate from the phytoplankton and 

 were defined as ''excretion products which serve 

 as accessory nutrients," inhibiting the growth of 

 some species of phytoplankton and stimulating the 

 growth of others. He further believed that the 

 toxins of a particular species not only inhibited 



Note. — This paper is m revision of a thesis that was submitted 

 tn Harvard University in 1:14:1 in partial fulfillment of thi 

 guirements fur the degree of doctor of philosophy. The author 

 acknowledges his. sincere appreciation to Dr. George L. Clarke, 

 fur stimulating criticism and kindly encouragement which con- 

 tributed in mil to the completion of this work. 



1 The terms "bloom," "flowering," "outburst," and "pulse" 

 refer to a rapid increase in the numbers of phytoplankton above 

 the level of iihunihinee previously existing in the area. No nu 



merlca] value has n agreed upon which defines a bloom, but 



population densities above 1 mlllio lis per liter have hera 



commonly referred to as blooms. A bloom is often ac panled 



bj a coloring of the water. 



its own further growth after a certain length of 

 time, but that the toxins of the "oil producing 

 group" of algae inhibited the growth of some 

 species and stimulated the growth of other species 

 in this group while all members of the "starch 

 group" were stimulated. The same general ac- 

 tion of toxins of the "starch producing group" 

 were also suggested. While Akehurst made very 

 detailed studies in the field, it is believed by the 

 author that his conclusions which treated all al- 

 gae as either oil or starch producers are too general. 

 A study of this complicated type of interaction 

 should be made between species and preferably 

 under conl rolled laboratory conditions. 



.More recently it has been shown in laboratory 

 cultures of planktonic algae that metabolites, 

 or other biological influences, of a species tend to 

 inhibit the growth of that species. Pratt and 

 Fong ( 1940) grew cultures of Chlorella vulgaris 

 in inorganic medium for different periods of time 

 and until different population sizes had been ob- 

 tained. The cells were then removed by filtering 

 the culture medium. This filtered culture me- 

 dium, which shall be referred to as "conditioned 

 medium," was used in different proportions with 

 fresh medium titter the pi I had been adjusted to 

 prepare new cultures of Chlorella. The growth 

 of Chlort lln in this medium was found to be slow- 

 er than in culture medium to which no "con- 

 ditioned medium" had been added. The con- 

 clusions Pratt drew from these experiments tire 

 as follows: (1) That the growth of Chlorella is 

 inhibited by the presence of the "conditioned me- 

 dium" in the culture medium; (2) that the de- 

 pression of growth increases as the percentage 

 uf -conditioned medium" increases; and (•$) that 

 for a given concentration of "conditioned medi- 

 um" in the culture medium, the depression of 

 growth varies inversely with the size of Chlorella 

 population in the "conditioned medium*' prior to 



filtering. 



227 



