ducing convection currents from top to bottom, (4) 

 circulation of the total water volume by wind action, 

 and (5) vertical temperature equality, 4 C. The over- 

 turn results in a uniformity of the physical and chemi- 

 cal properties of the water. 



Standing Crop — The biota present in an environment on 

 a selected date. 



Stenotopic Organisms — Organisms with a narrow range 

 of tolerance for a particular environmental factor. Ex- 

 amples are trout, stonefly nymphs, etc. 



SUBLITTORAL ZoNE — The part of the shore from the low- 

 est water level to the lower boundary of plant growth. 



Submerged Aquatic Plant — A plant that is continu- 

 ously submerged beneath the surface of the water. 

 Examples are the pondweed and coontail. 



SwiMBLADDER — An internal, membranous, gas-filled or- 

 gan of many fishes. It may function as a hydrostatic 

 or sense organ, or as part of the respiratory system. 



Swimmers' Itch — A rash produced on bathers by a para- 

 sitic flatworm in the cercarial stage of its life cycle. 

 The organism is killed by the human body as soon 

 as it penetrates the skin; however, the rash may per- 

 sist for a period of about 2 weeks. 



Symbiosis — Two organisms of different species living 

 together, one or both of which may benefit and neither 

 is harmed. 



Systematics — The science of organism classification. 



Thermocline — That layer in a body of water where the 

 temperature difference is greatest per unit depth. It 

 is the layer in which the drop in temperature equals 

 or exceeds 1 C (1.8 F) per meter (39.37 inches). 



TLm (see Median Tolerance Limit). 



Tolerant Association — An association of organisms 

 capable of withstanding adverse conditions within the 

 habitat. It is usually characterized by a reduction in 

 species (from a clean water association) and an in- 

 crease in individuals representing a particular species. 



Trophogenic Region — The superficial layer of a lake 

 in which organic production from mineral substances 

 takes place on the basis of light energy. 



Tropholytic Region — The deep layer of a lake, where 

 organic dissimilation predominates because of light 

 deficiency. 



Vertebrate — ^Animals with backbones. 



Warm and Cold-Water Fish — Warm-water fish include 

 black bass, sunfish, catfish, gar, and others; whereas 

 cold-water fish include salmon and trout, whitefish, 

 miller's thumb, and blackfish. The temperature factor 

 determining distribution is set by adaptation of the eggs 

 to warm or cold water. 



Waterfleas (Daphnia) — Mostly microscopic swimming 

 crustaceans, often forming a major portion of the zoo- 

 plankton population. The second antennae are very 

 large and are used for swimming. 



Zooglea — Bacteria embedded in a jellylike matrix formed 

 as the result of metabolic activities. 



Zooplankton — Protozoa and other animal micoorga- 

 nisms living unattached in water. These include small 

 Crustacea, such as daphnia and Cyclops. 



110 



