752 BECKETT 



Environmental Protection Agency (1973) suggests a 6-week exposure 

 (colonization) period for artificial substrate samplers; my samplers 

 were exposed for 6 weeks for the June and September collections 

 and 7 weeks for the August collection. As colonized samplers were 

 retrieved, new samplers were attached to the sampling devices for 

 later collection. Mac roin vertebrates were collected from the washed 

 samples using a U. S. Standard 60 sieve. I used this sieve size rather 

 than the popular 30 sieve so that I could collect smaller or early 

 macroinvertebrate instars for analysis. 



Since species diversity and number of taxa are often dependent 

 on sampling effort (Sanders, 1968; Simberloff, 1972), I attempted to 

 collect a uniform number of samplers from each station. Also, since 

 samplers were occasionally lost as a result of vandalism or flooding, I 

 used four samplers per station as my uniform sample size. Pielou 

 (1966) found that diversity reached 95% of the asymptotic level by 

 the fourth sample for stream macroinvertebrates. For the 42 station 

 collections (3 collection dates x 14 stations), 27 collections yielded 

 at least four samplers, 14 yielded two or three samplers, and one 

 station on one date (June 7) yielded no samplers. From the four 

 sampler collections, I empirically determined that two samplers had a 

 mean of 75% of the total number of species for four samplers, and 

 three samplers had a mean of 90% of the total number of species for 

 four samplers. The reported total taxa levels (see Fig. 3) therefore 

 show an adjusted number of species if two or three samplers were 

 recovered instead of the usual four. 



In a similar manner two- and three-sampler Shannon— Wiener 

 diversity values (H') (see below) were compared to four-sampler H' 

 values. Although four -sampler collections generally had slightly 

 higher H' values than two- or three-sampler values, this difference 

 was not significant at the 0.05 level using a Wilcoxon signed rank 

 test. Reported H' values (see Fig. 2) are therefore pooled unadjusted 

 values (Pielou, 1966; Wilhm, 1970b). 



I used the Shannon— Wiener diversity index (Shannon, 1948; 

 Wilhm and Dorris, 1968; Wilhm, 1970a) to quantify the species 

 composition of the respective macroinvertebrate communities. To 

 obtain reliable estimates of diversity in this study I have (1) used 

 artificial substrates to reduce the confounding effects of substrate 

 difference (U. S. Environmental Protection Agency, 1973) and (2) 

 pooled samples at each site to approximate an asymptotic level of 

 diversity (Pielou, 1966; Wilhm, 1970b). 



Macroinvertebrates were identified in the laboratory to the genus 

 or species level for all taxa except oligochaetes, which were not 

 further identified (Table 1). Oligochaetes were not found on most 

 samplers and when present constituted only a small portion of the 



