36 



PROBLEMS OF LAKE BIOLOGY 



the surface the illumination has sometimes 

 been found sufficiently strong to inhibit 

 photosynthesis, although the exact explana- 

 tion for this is wanting (Clarke 1936a and 

 Riley 1938). Below the point of maxi- 

 minn photosynthesis the rate drops olf to a 

 depth at which no appreciable reaction 

 takes place, but respiration continues at all 

 levels to which living cells may be dis- 

 tributed. The intensity of light at which 

 photosynthesis just balances respiration is 

 termed the compensation intensity, the value 

 of which has been reported in two recent 

 investigations as 500 lux and 350 lux (or 

 0.3 per cent of noon sunlight) respectively 

 (Pettersson, Hogiund, and Landberg 1934, 

 Pettersson 1938, and Jenkin 1937). Since 

 the compensation intensity will naturally 

 vary from species to species, further de- 

 terminations for types important ecologi- 

 cally are seriously needed. The depth at 

 which the compensation intensity is found, 

 knoAvn as the compensation point, has been 

 located (for short intervals) in a number of 

 cases:* Sargasso Sea, greater than 100 

 meters; English Channel, 45 meters; Gulf 

 of Maine, 24-30 meters; East Sound (AV. 

 coast of Canada) , 10-19 meters ; Trout Lake, 

 Wis., 8-16 meters; Woods Hole Harbor, 7 

 meters. The depth of the compensation 

 point over an average 24 hour period is, 

 however, the significant value from the point 

 of view of the continued growth and 

 ecological role of each species. Marshall 

 and Orr (1928), working in the Clyde Sea 

 area, found that on this basis the depth 

 varied from 2 meters to 30 meters according 

 to the season. Further observations from 

 this angle in other areas are obviously of the 

 greatest imx^ortance. There appear to be no 

 determinations of the compensation point 

 for 24 hours for lakes ! 



The productivity of any body of water 

 depends ultimately upon the efficiency with 

 which the plants can transform radiant 

 energy into the potential energy of organic 

 tissue. Kozminski (1938) attempted to ap- 

 proach this problem by measuring directly 

 the quantity and distribution of chloro- 



* From Pettersson, Hogiund and Landberg 

 (1934), Clarke (1936a), Jenkin (1937), and 

 Manning, Juday and Wolf (1938). 



phyll in lakes, but found that the measure- 

 ments failed to give an exact index of the 

 mass of the phytoplankton or of the in- 

 tensity of photosynthesis because of the 

 presence of inactive chlorophyll. He states, 

 however, that "when it is possible to mea- 

 sure photosynthetically active chlorophyll 

 only, this will be the best measure of pro- 

 ductivity." 



Another method of approach to this prob- 

 lem is the calculation of the quantity of 

 carbohydrate produced on the basis of the 

 amount of oxygen evolved by plant cells 

 suspended in hanging bottles. Of the vari- 

 ous experiments conducted by Manning et 

 al. (1938) in Wisconsin lakes the highest 

 quantum efficiency observed was .05. Since 

 this value is so much lower than the value 

 of .25 obtained by Warburg and Negelein 

 (1923) in the laboratory, investigation of 

 this discrepancy is demanded. Elsewhere 

 Manning* has calculated on the basis of 

 the average abundance of chlorophyll in a 

 series of lakes that from 1.6 per cent to 14 

 per cent of the light incident on the lake is 

 absorbed by the chloroplasts of the phyto- 

 plankton. An average value for the quan- 

 tum efficiency under natural conditions ac- 

 cording to Manning is .012 which is equi- 

 valent to an efficiencj^ for photosynthesis of 

 2.7 per cent. When the two factors are 

 taken toa'ether, we obtain : 



Incident light 

 absorbed 



Eflficiency of 

 photosynthesis 



Efficiency of 

 production 



min. 1.6% 

 max. 14 " 



X 



X 



2.7% 



.043% 

 .38 " 



Tlie efficiency with which aquatic algae 

 manufacture carbohydrate is, therefore, 

 only a small fraction of a per cent. And 

 not all of this product would be found in 

 the tissues of the plant because a large part 

 of it is consumed concomitanth^ in respira- 

 tion and other life processes. 



It thus appears that most of the light 



* ' ' Photosynthesis in relation to light penetration 

 into lake waters. ' ' Symposium of Hydrobiology. 

 Joint meeting of the Limnological Society of Amer- 

 ica and the American Society of Zoologists, Indian- 

 apolis, Ind., December 30, 1937. 



