CHAPTER I. INTRODUCTION 



It is necessary to examine all aspects of water quality to gain 

 a clear understanding of past and present characteristics of a stream. 

 Chemical and physical measurements provide data on water conditions at 

 a particular moment. On the other hand, biological observations reflect 

 conditions that have existed for some time previously. A study of bio- 

 logical characteristics and the ecological relationships of plants and 

 animals in a stream may therefore provide the best overall picture of 

 past and present water quality conditions in that stream. 



Various assembled^es of organisms are known to be associated 

 with particular types of water quality. For example, heavy concentrations 

 of midge larvae or tubificid worms are often associated with organic en- 

 richment. These organisms are quite tolerant of low dissolved oxygen 

 concentrations and, in the absence of competition from other tolerant 

 forms, may increase their numbers enormously. On the other hand, waters 

 with little pollution may be characterized by smaller total numbers, but 

 a much greater variety of animals. Thus, in clean-water streams, we may 

 expect to find organisms such as mayflies, stoneflies, caddisflies, midge 

 larvEie, damselflies, and a host of other types. It must be noted, however, 

 that aquatic forms often associated with poor water quality are also found 

 in clean water areas but are far less numerous . 



In euidition to effects brought about by the chemicfiLL quality 

 of water, the physical characteristics of the habitat are extremely impor- 

 tant in modifyii^ plant and animal life. Certain types of habitat are 

 not conducive to large bottom fauna popiilations . A shifting saiid bottom, 

 for example, supports a much smaller biomass than does a silt or boulder 

 habitat . 



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