MEIOFAUNAL ABUNDANCE 197 



(upper site) to 10 to 11 at Strouds Run and Margaret Creek, 

 showed a significant negative correlation with compensated non- 

 carbonate conductivity (P < 0.01). Geometric mean abundance per 

 station of total meiofauna and Diptera likewise showed significant 

 negative correlations (P < 0.01 and P < 0.05), but for no other taxon 

 was this the case. 



The fauna was dominated by Nematoda, Rotifera, and Diptera, 

 with occasionally high counts of Oligochaeta (Margaret Creek, south 

 site; Strouds Run, west site), Gastrotricha (Tick Ridge, lower site; 

 Strouds Run, west site), and Cyclopoida copepods (Strouds Run, 

 north site). There was a total of 16 taxa (listed in Table 3 in 

 decreasing order of Gx abundance per station, summed over all sites), 

 of which six (Turbellaria, Harpacticoida copepods, Pelecypoda, 

 Ostracoda, Collembola, and Ephemeroptera) were observed at but 

 three or four sites each and three (Cladocera, Gastropoda, and 

 Plecoptera) were found at but one site each. 



The two Long Run sites showed greatest taxon similarity, with 

 86% shared diversity (Fig. 1). The following sites joined at about the 

 same level (53 to 54%): Minkers upper with the two Long Run sites, 

 Margaret Creek west with south, Strouds west with Tick lower, and 

 Raccoon upper with Sandy lower. At a slightly lower level (49%), the 

 latter two pairs were joined, respectively, by Strouds north and 

 Sandy upper. Already, at 49% similarity, four of the seven stream 

 pairs have joined one another. At 46% similarity, Margaret Creek 

 joined the Minkers uppei^-Long Run group, and it in turn was joined 

 at 40% similarity by the Strouds Run— Tick lower group to form 

 one major complex of predominantly unpolluted streams. Also, at 

 46% similarity. Raccoon lower and Minkers lower joined. This group 

 joined the central complex at 35%, just 1% less than the point at 

 which Tick upper joined the Raccoon upper— Sandy Run group. 

 Finally, the latter group joined the enlarged complex at the 30% 

 similarity level to complete the series. 



Sites were arranged along the top of the figure, within the 

 confines of the axes provided by their sequential joining, in such a 

 manner as to maximize the Sh' values between adjacent members. A 

 pattern emerges v^dth two interesting features. One is that the H' 

 values for sites as arranged along the figure form a sine wave, with 

 but two reversals, increasing from a minimum on the left to a 

 maximum just right of center to a minimum again on the right. The 

 second feature is that this arrangement portrays a bipolarity of 

 faunal dominance, from rotifer-dominated faunae on the left to 

 nematode-dominated faunae on the right, with the high diversity- 

 low dominance faunae in the center. 



