SECTION 7 



MONITORING THE CONDITION OF FLOWING WATERS BY 

 BIOLOGICAL ORGANISMS 



Ruth Patrick 



Whether one is concerned with the assimilative capacity of the river or 

 its sport and commercial fisheries, one eventually has to be concerned with 

 the whole ecosystem. In monitoring we may study one or a few selected 

 species or we may assay the condition of the whole aquatic ecosystem in 

 certain areas. It is important that natural streams be set apart and kept 

 in their natural state, so we can have base lines against which we can 

 measure natural change and changes due to man's effects. 



In monitoring one may wish to learn about immediate change or more long- 

 term, subtle changes. In such studies one must remember that time is a 

 relative parameter. Organisms that reproduce once a day would show effects 

 in two days that might take years to show in an organism that reproduces 

 much more slowly. One type of monitoring the condition of organisms is by 

 bioassay tests. The time and duration of the test to show acute or 

 subacute effects depend considerably on the organism under study. However, 

 generally we use bioassay tests of a few hours or a few days to determine 

 acute effects—that is, those effects that show up immediately. In the 

 United States these short-term tests are designed to determine the concen- 

 tration at which 50 percent of the organisms die in a given length of 

 time. They are, if batch tests, often considered as more-or-less "rough 

 and ready" tests to get an idea of the effects of a given substance on 

 aquatic organisms in an ecosystem. In carrying out such tests it is ad- 

 visable to use organisms representing various stages of the food web, be- 

 cause the food web may be altered if any stage of nutrient and energy trans- 

 fer is impaired. For this reason an alga that is a good source of food, an 

 invertebrate, and a fish are often tested. 



Long-term monitoring tests are aimed at showing sublethal effects and 

 often follow acute tests, because by the acute tests one has found that con- 

 centration or range of concentrations that probably will not kill the or- 

 ganism being studied. Whereas acute tests are concerned with 

 concentrations that cause death; sudden morphological changes such as the 

 sloughing of mucus by fish; or avoidance reactions to low oxygen; long- 

 term tests are more concerned with physiological changes and changes re- 

 lating to the fecundity of the organism. 



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