American Fisheries SDcicti/ 239 



AvatcT. to lie witlidrawii from circulation. Least l'a\()i'alil\- situ- 

 ated is the deep and steep-sided lake, wliose cold (U'ptlis are con- 

 tinually s\vallo\viu<i- almost all of the pi'oducts of the summer's 

 yi'owtli. and aive them l)ack for use. only late in the autumn 

 wlieu the season for active life s i)assin,ii- away. 



Some lakes may Hiul aid from another source in the task of 

 secui'ino- carbon dioxide. Most n;itural waters contain a ceilain 

 amount of calcium and nu^unesium s;ilts in solution, and. for 

 the greater part, these exist in thi' foi'ui of hit-arhonate-'. Lakes 

 whose water is hard contain a considerahk' amount of these 

 hicai'lionates and soft-water lakes have little or nont'. Li hard- 

 watci' lakes it is found that durin.ii' the <ir()win_o- season, when 

 al^ae are active, the u])|)er water contains no free carhon diox- 

 ide, hut is. on the contrary, alkaline to indicators like |)henolph— 

 thalein. This alkalinity comes from the fact that one molecule 

 of carhon dio\id(> has l)een withdrawn from part of the l)icarhon- 

 atcs con\crtin,ii- them into carbonates. It appears that the alpie 

 are ahle lo elfect this reduction and that they can obtain their 

 suppiv of carhon from the carbon dioxide of the bicarbonates dis- 

 solveil in the water. This fact introduces a wholly new feature 

 into the story of the food su])i)ly of the plants. It provides a 

 chemical carrier for the carhon dioxide, which may carry this uas 

 somewhat as the haemoglobin carries oxygen in the l)lood. All 

 carbon dioxide set free in this alkaline Avater as the result of de- 

 composition or other processes, will 1)e taken u|) immediately hy 

 the cai'honates. Thus if jjlants are not at hand to utilize the car- 

 hon dioxide at once, it :s not lost hut kept until it is nt'i'ded. So 

 ill th<' night, the lake is able to I'etain all tlie carbon dioxide set 

 fi'ee and which the plants do not use at that time. 



Such alkaline water has also a great advantage in absorbing 

 cai'hon dioxide from the air. It presents for absorption, not 

 merel\- the relati\ely weak and slow powers of tlie water for dis- 

 solving the gas. but the eager and vigorous powers of cliemical 

 aflinitv. And until these alkaline cai-honates are sat\irated. no 

 free carbon dioxide will appear in the water to diminVh the 

 rai)iditv of absorption from the air. Thus hard-water lakes 

 have an advantage over soft-water lakes in the matter of securing 

 l)lant food, and in fact the population of soft-water lakes seems 

 to he smaller than that of lakes of the other tvpe. 



