ScCrtUHO/ity 



The invertebrate fauna of the Yellowstone River is rich in numbers and 

 species. The number of species and the population are greatest in the upper 

 river (stations 1-5), and both decrease downstream. 



The invertebrate fauna is dominated by mayflies (Ephemeroptera) , 

 caddisflies (Trichoptera) , and true flies (Diptera). The stonefly 

 (Plecoptera) fauna is diverse but not abundant, and there is a steady 

 decrease in number of species downstream. The mayfly fauna is composed of 

 a mountain fauna and a prairie fauna, although several species are found 

 throughout the river. In the lower five sampling stations, mayflies are the 

 most diverse order. Caddisflies are abundant and diverse throughout the 

 Yellowstone River. The caddisfly family Hydropsychidae dominates the 

 invertebrate fauna in the lower half of the river. True flies, in 

 particular the midge family, Chironomidae, are abundant and diverse 

 throughout the river. 



The invertebrate fauna of the Tongue River is similar to but distinct 

 from the fauna of the lower Yellowstone River. 



Baseline species diversity calculations showed that the Shannon-Weaver 

 index was near or below 3.0 for most stations. Generally an index above 

 3.0 illustrates a healthy unstressed community, while an index below 1.0 is 

 indicative of a monospecific community under stress. The index range of 1.0- 

 3.0 seems to illustrate a community under some stress. 



The current preferences of many species and genera were examined. For 

 most species, increasing current (up to 3 ft/sec) means a larger population. 



At present, dissolved oxygen concentrations in the Yellowstone River 

 are high enough to sustain invertebrates and fish. Lack of dissolved oxygen 

 could influence invertebrate communities if reduced flows are so low that 

 domestic sewage or decaying organisms tax the capacity of the river. With 

 reduced flows, increased concentrations of nutrients could result in an 

 increase in periphyton (alga) growth which probably would result in a larger 

 standing crop of benthic organisms and a shift in benthic species composition. 



Increased water temperatures as a result of reduced flows would 

 affect invertebrate growth, emergence, egg hatching, and metabolism. The net 

 effect would probably be a reduction of the fauna. 



A reduction in flow which results in a reduction of current velocity will 

 result in a faunal reduction because most species prefer swift currents. 

 Flow reduction also decreases the river stage, exposing large amounts of 



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