QUANTITY 67 



scavengers. The rooted vegetation is eaten only to a small extent. 

 Small floating or swimming plants and animals, called plankton (Figs. 

 12-18, pp. 75, 76) are the basis of the food supply of larger animals. 

 We could probably remove all the larger rooted plants and substitute 

 something else of the same form and texture without greatly affecting 

 the conditions of life in the water, that is, so far as the life habits of the 

 animals are concerned. The aquatic plants are commonly covered with 

 a coating of green algae, protozoa, and other small organisms, so that 

 animals such as small snails may rasp the surface of the plants and secure 

 food without eating the plant tissues themselves. Plants in water are 

 of particular use to animals as clinging- and nesting-places. 



V. QUANTITY (47) OF LIFE IN WATER 



The quantity of living matter in water, so far as it is plankton or 

 floating organisms, has been much studied. The quantity is usually 

 expressed in one of two ways: number of organisms per liter or cubic 

 meter of water, determined by counting a part of a collection; or in 

 cubic centimeters per cubic meter of water. In Lake Michigan (August) 

 Ward (75) found an average of 11.5 c.c. per cubic meter in water from 

 the surface to 2 m. ; from 2-25 m., 3 . 9 c.c. ; 25 m. to bottom, o . 4-1 . 5 c.c. 

 He found that Pine Lake (a small lake) contained relatively less plankton 

 than Lake Michigan, the surface stratum of Pine Lake containing more 

 and the deeper strata much less than the larger lake. Lake St. Clair 

 contains only one-half as much plankton as Lake Michigan. Lake 

 Michigan contains only about one-tenth as much plankton as some of the 

 small European lakes (Dobersdorfer See). Kofoid (77) found 71 .36 c.c. 

 per cubic meter the maximum record for the Illinois River. The 

 average for the year is 2.71 c.c. per cubic meter. The largest amount 

 recorded by Kofoid is 684.0 c.c. per cubic meter (Turkey Lake, Ind.) . 



Small streams and lakes with large inflow and outflow have but little 

 plankton. Large amount of plankton is commonly associated with 

 high CO 2 content, low oxygen content, and a large amount of carbonate 

 in solution. 



The amount fluctuates from season to season. Kofoid (77) found 

 the maximum for the Illinois River in April to June. The amount 

 gradually decreases until December and January, when the minimum 

 is reached. He also found evidence that the light of the moon increases 

 photosynthesis and the amount of plankton. The maximum of Crustacea 

 was found by Marsh (78) to fall in July, August, and September, differing 

 in different years. The maximum in Lake Michigan probably is usually 



