42 FRESH-WATER BIOLOGY 



are necessary. This is believed to be due to the neutralization of 

 the toxicity of the NaCl by the other salts; this is known as antag- 

 onism. The effects are due to the cations, one anion being suffi- 

 cient though some are more favorable than others. 



Salts present in excess, or without the proper antagonistic salts 

 or ions, and salts not commonly present in quantity in fresh water 

 are toxic to fresh-water animals. The toxicity varies for different 

 salts and according to the concentration of hydrogen or hydroxyl 

 ions which accompany it. Ammonia salts are poisonous to fishes 

 if present in company with carbonates. Carbonates are not essen- 

 tial to the life of fishes as sulphates may be substituted entirely, 

 at least for short periods. Carbonates alone are fatal to fishes 

 because of their alkalinity. In the presence of COz, however, 

 carbonates are converted into bicarbonates which are normally 

 present in all natural fish waters. Bicarbonates accompanied by 

 a small excess of CO 2 are not harmful. Of the salts of potassium, 

 the sulphate is most poisonous; sodium salts are less injurious than 

 those of potassium. The presence of an excess of calcium causes 

 the tail fins of the rock bass to degenerate and this fact was prob- 

 ably responsible for the tailless trout found in certain waters of 

 the British Isles where the water was contaminated with waste 

 from paper mills. There is much evidence that calcium tends 

 to lower the metabolic activity of organisms. 



As shown by Wells fishes react to salts in solution. They are 

 usually negative to nitrates, more or less positive to chlorides 

 (markedly so to NaCl) but are decidedly negative to CaCl 2 and 

 MgCt They are positive to ammonium chloride and are usually 

 very negative to sulphates. The reaction of the fishes to the salts , 

 was shown to have a distinct relation to the acidity of the water, as 

 fishes that were decidedly negative to Na2SO 4 for instance in slightly 

 acid water were made positive to this salt by running the experiment 

 in strongly acid water (i.e., 20 cc. CO 2 per liter). A part of the 

 effect of ions lies in their effect on permeability. Alkalies increase 

 permeability of protoplasm. Acids first decrease and later increase it. 



In animals and plants there are various rhythms of activity con- 

 stituting parts of their physiological life histories or recurring 

 functions lying within them. These often coincide with rhythms 



