40 FRESH-WATER BIOLOGY 



animals. In large quantities it is rapidly fatal acting as a narcotic. 

 It is particularly injurious in the absence of oxygen which absence 

 is usually associated with it. Abundant oxygen decreases its 

 toxicity because blood has greater affinity for oxygen than for 

 carbon dioxide and the latter is crowded out of combination. On 

 account of the fact that it is usually accompanied by lack of 

 oxygen, putrescible muck bottom, etc., its presence in quantities 

 greater than 6 to 7 cc. per liter if accompanied by a bottom en- 

 tirely of such muck would indicate that the water was unsuitable 

 for trout, basses, sunfishes, and crappies. 



One of the most important characteristics of a water is its 

 acidity or alkalinity. Protoplasm must maintain essential neu- 

 trality or it will die. It possesses a very effective physico-chemical 

 mechanism based upon the presence in excess of very weak acids 

 (carbonic and phosphoric) and alkalies in the form of carbonates 

 and phosphates. Since protoplasm must remain nearly neutral 

 the acidity Or alkalinity of the surrounding medium cannot be 

 great. Thus Wells found that fishes do not live well in alkaline 

 water but become sluggish and inactive. Neutrality is likewise 

 toxic to some fresh- water fishes. They require a certain amount of 

 acid. The optimum acidity for the different species differs. The 

 optimum for the bluegill (Lepomis pallidus Mit.) is i to 3 cc. of 

 carbon dioxide per liter and for crappies (Pomoxis annularis Raf.) 

 4 to 6 cc. per liter. Wells showed by using various other acids 

 that the hydrogen ions are the important factor. In other words 

 fishes require a certain concentration of hydrogen ion. Neutrality 

 is avoided by fishes. In the absence of acidity they select alka- 

 line in preference to neutral water. Fishes and various crusta- 

 ceans will live in distilled water if it is slightly acid, while it is 

 rapidly fatal if neutral and more rapidly fatal if alkaline. The 

 toxicity of much ordinary distilled water is due to colloidal copper 

 or other metal from coolers, in suspension in it. 



Wells made a rearrangement of some of the data of Birge and 

 Juday which showed that various plankton organisms are distrib- 

 uted with reference to alkalinity, neutrality, and acidity, 1 a few 



1 In the determination of alkalinity and acidity great care should be exercised in 

 the making of collections so as to prevent the escape of CO 2 . The choice of indi- 



