8 FRESH-WATER BIOLOGY 



form resistance in contrast with related organisms. The buoyancy 

 is achieved by oil droplets and gas bubbles formed in the cells 

 whereas heavy cell walls and skeletal structures are wanting. Flo- 

 tation-apparatus in the shape of lateral wings, bristles, spines, 

 or a body form like a parachute, a spiral thread, or a gelatinous 

 cover provides against rapid sinking. Ostwald has determined 

 that the rate of sinking is equal to the excess weight of the organism 

 above that of an equal water volume divided by the product of the 

 form resistance and the viscosity of the fluid. 



Generally speaking great depth in a water body and large inflow 

 in proportion to volume are unfavorable to the abundant develop- 

 ment of the plankton organisms whereas minimal depth and scanty 

 inflow favor the production of plankton. 



When water is first deposited on the earth it is almost absolutely 

 pure, containing only the minute amount of materials which it 

 has leached out of the atmosphere. From the ground over which 

 it flows or the soil through which it percolates come substances 

 organic or inorganic, in solution and suspension, here of one type 

 and there of another, that serve to enrich it and make of it an 

 environment capable of supporting life. "The aquatic popula- 

 tion of a lake or stream is thus sustained by the wastes of the land, 

 materials which would otherwise be carried down practically un- 

 altered to the sea; and rivers and lakes may be looked upon as a 

 huge apparatus for the arrest, appropriation, digestion, and assimi- 

 lation of certain raw materials about to pass from our control" 

 (Forbes). 



For the determination of physical data on the character of bodies 

 of water, methods and apparatus of considerable complexity have 

 been devised, largely by students of oceanography, and adapted 

 later to fresh-water conditions. By such means the investigator 

 is enabled to measure in a comparative way, and sometimes in 

 absolute fashion, and to record environmental conditions such as 

 the depth, temperature, turbidity, and other physical features of 

 the water body. Some of these determinations are simple and 

 require only limited apparatus; others are complex and beyond 

 the powers of the ordinary student of aquatic biology. The appli- 

 cation of such data to biologic problems is discussed in part in the 



