460 ^- T. Edmondson 



comparable to Winokur's (1948, 1949) work on eight species of Chlorella, and that 

 stops short of a study of comparative nutrition. 



Considerable effort was expended in trying to discern such qualitative sources of 

 variation in the present data. As one approach, the fraction of the volume of phyto- 

 plankton material made up by diatoms was computed. This fraction varied from less 

 than 1 % to 97 % by volume, with the other organisms being a great variety of kinds 

 including dinoflagellates and Chlorella, but there was no discernible tendency for 

 periods of large diatom content to be low or high in the amount of photosynthesis 

 per unit light and chlorophyll. The periods during which the population was com- 

 posed of less than 50% diatoms are indicated by lines attached to the points of Fig. 3. 

 Thus, large variations in the representation of diatoms did not alter the relationships 

 discussed. Likewise there was no relation with the fraction of the population made 

 up by non-photosynthetic protista. This is not to say that many aspects of popula- 

 tion biology will not be altered by taxonomic changes of this kind, or that such pheno- 

 mena did not exist during the tank experiments, but merely that the relation of 

 photosynthesis to chlorophyll and light depended more on the sheer quantity of 

 chlorophyll than on the way the chlorophyll was distributed among the major taxono- 

 mic groups present. 



PHOTOSYNTHETIC POPULATION EFFICIENCY 



Having measurements of the rate of energy input into the tanks and the gross rate 

 of photosynthesis, we can calculate the gross population efficiency, or fraction of 

 energy used in photosynthesis. In Experiment 1, gross efficiency for the four tanks 

 together was 0-34% when computed on the basis of surface intensity of visible Hght, 

 and 0-95 % on the basis of the intensity of the depth of the bottle. The figures for each 

 tank are in order, for surface intensity, 0-23, 0-31, 0-41. and 0-43%. For intensity 

 at the depth of the bottles, the corresponding figures are 0-51, 0-67, 1-29, and 1-56%. 



Since these figures are based on the intensity of visible light, they should be multi- 

 plied by 2 before comparing them with figures in the literature which are based on 

 total radiation. Efficiencies of natural, unfertihzed populations have been reported 

 between 0-02 and 0-40 %. Nelson and Edmondson (in press) discuss a lake the effici- 

 ency of which was greatly increased by fertihzation. That the heavily fertihzed tanks 

 are not much more efficient relative to natural populations may be surprising, but is 

 easily explained by the fact that the tanks were so shallow that much of the available 

 fight must have been wasted by absorption into the sides and bottom and by reflection 

 back out of the water. That fertihzation increased efficiency can best be seen by com- 

 paring each tank with Tank 1, bearing in mind that Tank 1 was inadvertently fertilized 

 lightly. 



ASSESSMENT OF CONDITION BY ENRICHMENT 



As shown in previous sections, the responsiveness of different populations to fer- 

 tilization was different, and varied with the amount of fertiUzer. For full interpreta- 

 tion of events in natural populations, it is necessary to obtain information on respon- 

 siveness in order to identify limiting and controlling factors. The work of Lund on 

 lakes is the most successful to date, basing interpretation of seasonal events on labora- 

 tory analysis of the physiological condition of the population and its response to 

 changed conditions (1949, 1950). 



