THE STREAM ENVIRONMENT 



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2000 



TC-1 TC-2 TC-3 TC-4 TC-5 TC-6 TC-7 

 SAMPLING STATIONS 



Fig. 2 Downstream changes in macroinvertebrate annual mean 

 density and biomass values on Trout Creek. 



Rather than decreasing, as expected, density and biomass 

 exhibited a general increase downstream (Fig. 2). Differences between 

 stations were significant (P < 0.01) for total numbers of organisms 

 collected during the study but were not significant on certain 

 collecting dates (June, July, and January). Significant differences 

 (P < 0.01) between dates reflect periods of emergence and recruit- 

 ment, effects of spring runoff, and onset of winter conditions. 



Although density increased downstream, similar numbers of taxa 

 occurred at all stations (Fig. 3). Shannon— Weaver index values also 

 indicated no longitudinal pattern. Median values at all stations were 

 between 3.0 and 4.0, the "normal" range defined by Wilhm (1970). 



Five orders of aquatic insects (Trichoptera, Diptera, Ephemer- 

 optera, Plecoptera, and Coleoptera) comprised well over 90% of the 

 mean annual density and biomass of macroinvertebrates at all 

 sampling stations. Eight taxa at each station represented 55 to 83% 

 of the density, and most of these taxa were among the most 

 abundant at many stations. The mayfly Baetis was among the top 

 eight at all stations. Others abundant at most stations included the 



