parvulum typically develop large populations under nutrient-rich conditions. 

 The presence in abundance of Cyclotella meneghiniana in contrast to 

 Cyclotella stelligera and C. kutzingiana indicates an increase in the 

 nutrient levels of the water. There are a great many diatoms that indicate 

 these shifts. Studies by Patrick (1956) have shown that diatoms have a 

 toxicity threshold similar to that of fish and invertebrates to many 

 industrial wastes, and thus a very short-term test can tell a considerable 

 amount about the effects of a toxicant in a body of water on other members 

 of the food web. 



Various types of substrates have also been developed for monitoring 

 invertebrate communities. Whereas the diatometer substrates (Patrick et 

 al., 1954) have been devised to reliably represent the community in the 

 river under study, this has not been done so far as I know for inverte- 

 brates. However, various people have studied how many invertebrate 

 samplers one needs in an area in order to get reproducible results (Beak, 

 et al., 1973). 



One type of substrate that has been used are panels which collect 

 sessile organisms (Figure 6). This is a substrate sampler consisting of a 

 series of flat substrates placed in the water. This has been found to be 

 very good for the collection of certain invertebrates. Other people have 

 used various kinds of baskets from simple chicken-wire baskets filled with 

 rotted wood to baskets of very definite structure such as barbeque 

 baskets. These have been found very useful, but the important thing is to 

 calibrate them so that one can obtain reproducible results and know the 

 extent these organisms represent the area under study. In this way one can 

 compare over time changes in a given area and the shifts between areas. 



In the macroinvertebrate studies, as in periphyton or diatom studies, 

 one is concerned about shifts in the dominant forms or shifts in the sizes 

 of populations; shifts in kinds of species; and shifts in numbers of 

 species. For example, by insect traps placed by Dr. Roback of the Academy 

 in the Savannah River he was able to clearly define the effects of 

 dredging, because the filter-feeders such as caddisflies disappeared as 

 long as the water had a high suspended solids load. One can also determine 

 different degrees of nutrient loading in the water by shifts in the faunas 

 of insects such as shifts in a mayf ly-stonefly dominated fauna to one domi- 

 nated by damselflies and dragonflies, to one dominated by chironomids and 

 worms. Since mayflies and stoneflies are particularly sensitive to oxygen 

 concentrations, the loss of these species indicates an oxygen sag at times 

 in the river even though it is not noted chemically. In other cases shifts 

 of Hydropsyche caddisflies to dragonflies and Chematopsyche caddisflies 

 have indicated intermittent toxicity. 



A second type of monitoring is one in which given selected areas of a 

 river are studied over time. This type of monitoring is extremely valuable 

 because it tells a great deal more than monitoring by means of substrates. 

 In such monitoring a team of scientists is sent into a river. They deter- 

 mine not only changes in the chemical and physical characteristics but also 

 the characteristics of the aquatic ecosystems. They are able to determine 

 by increased growth of various types of organisms such as submerged or emer- 



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