Life of the Waters 383 



Such temporary pools may be restocked, perhaps chiefly from 

 the outside by the action of wind, birds or other agencies. 



But not only do inland waters show an annual cycle in 

 their life, dependent on the changing seasons; there may be 

 several cycles, some greater and some less, in the life of lake 

 or pond or river. These cycles depend in large measure 

 upon changes in amount of food-stuffs in the water. They 

 are initiated by the plants upon which the animals are pri- 

 marily ^dependent for their food. When the melting snows 

 and rains of spring are washing the surface of the land and 

 draining through the soil into lakes and rivers, they carry 

 with them large quantities of mineral and organic matters 

 in solution ; which substances, especially the latter, with their 

 high nitrogen content, furnish abundant food supply for 

 plants, which in turn serve as provender for animals. Rainy 

 periods later in the season may be followed by temporary in- 

 crease of life in inland waters. Even the phases of the moon 

 are seemingly reflected in the abundance of the plankton, for 

 Professor Kofoid, in his extensive studies on the plankton of 

 the Illinois Eiver, found a series of monthly increases or 

 "pulses" of the plankton, corresponding roughly at least 

 with the periods of full moon, and possibly due to the in- 

 crease of light at these periods resulting in increased activity 

 of chlorophyl-bearing plants in the water. 



The movements of fresh water animals, to which reference 

 has already been made, furnish a fascinating field for study. 

 They display vertical and horizontal migrations similar to 

 those of marine animals. The surface of a lake which may 

 be swarming with animals by night, may be almost depopu- 

 lated by day. In the cool days of spring and fall the shal- 

 low shore waters, readily heated by the sun, may be an at- 

 tractive haven for animals of many sorts, which in the hot 

 days of midsummer seek the cooler waters farther out from 

 shore. But not only are the movements of animals controlled 

 by evident physical and chemical factors, there appear to be 

 biological factors also which determine them, which is prob- 

 ably only another way of saying physical and chemical fac- 

 tors in a less evident form. But however that may be, fresh 

 water animals often congregate in great swarms, just as do 

 insects, fishes, birds or mammals. One region of a pond may 

 be almost free from some species of animal, while another 

 region only a few feet distant may be so thickly populated 

 as to appear ''milky" or "soupy" to the observer. In one 

 such swarm of the crustacean Moina the writer has estimated 

 more than 80,000 individuals in a quart of water. The cause 

 of such swarms is as yet unknown. 



