io8 Roosevelt Wild Life Bulletin 



Although hght i- essential for the growth of algae, very intense 

 light may not be favorable. When attempts are made to grow algae 

 under cultural conditions it is frequently necessary to place them in 

 diffuse light, since they are killed by the direct rays of the aun. 

 This direct killing effect of the sun"s rays may be of much greater 

 importance than has been suspected, and ma}- account for the fact 

 that the surface of a lake frequently contains fewer algae than the 

 region just below the surface. This effect of the intensity of the 

 sun's rays has been noted by Fritsch ( '07; in a discussion of the 

 algal flora of Ceylon, an-d he suggests that the great predominance 

 of blue-green algae in the tropics may be due to a protection of the 

 green coloring matter from the intense light by the blue coloring 

 matter. 



The amount of light falling upon any lake and passing some 

 distance into the water is generally sufficient to cause a great develop- 

 ment of algae, so that the amount of raw food materials present is 

 the primary factor that determines the abundance or scarcit\- of 

 development of the phytoplankton. Carbon dioxid is consumed in 

 the process of photosynthesis, and the scarcity or abundance of 

 plankton organisms is directly proportional to the amount of this 

 compound that is available for carbohydrate food manufacture. 

 Carbon dioxid exists in lake water in a dissolved state, and when 

 it has been withdrawn by the activit\- of the plant it must be 

 replenished either from the air, by surface waters flowing into the 

 lake, by respiration of living organisms within the lake, by decom- 

 position of organic matter, or from the dissolved bicarbonates of 

 calcium and magnesium. The first mentioned sources are of but 

 little importance, and the major portion of carbon dioxid comes from 

 the last two sources mentioned. The amount of carbon dioxid present 

 in solution varies in different lakes. It may be present in the form 

 of free carbon dioxid that is readily soluble in water, or it may 

 exist in union with other substances in t^vo different states : either 

 as the normal carbonate of calcium or magnesium (CaCO., or 

 MgCO;.), which is known as the fixed or combined carbonate: or 

 in the form of a bicarbonate ( CaCOs-HoCOg j where a part of it 

 is in such a loose chemical union that it may be utilized by algae 

 in their photosynthetic work. The extensive work of Birge and 

 Juday ( 11 j on the dissolved gases of lake waters shows that under 

 midsummer conditions the free carbon dioxid of the upper zone, 

 the zone of photosynthesis, is frequently entirely used up, and as 

 there is no source aside from the free carbon dioxid, algal growth 

 ceases. This presence or absence of free carbon dioxid can be 

 determined by phenolpthalin. an acid reaction indicating its presence 

 an-d a neutral or an alkaline reaction indicating a deficiency. There 

 is, as has been indicated above, a potential source of raw food 

 material in the supply of half-bound carbonates in the water. This 

 supply is not constant in all lake waters but is directly proportional 

 to the hardness of the water since, at the time when the algal growths 

 draw upon it, the amounts of the fixed and half -bound carbon dioxid 

 in the water are equal. An index of the amount of potential supply 



