Examining the Relationship between Aquatic Insect Assemblages and Water Variables by Ordination Techniques

Aquatic insects were sampled quarterly at five sites along the upper Keelung River, which runs through the Taipei metropolitan area and receives various forms of pollution and anthropogenic disturbances, between August 1996 and April 1997 to examine the distribution patterns of aquatic insects and their relationships with the water variables using univariate and multivariate analyses. Upstream sites 1 to 3 significantly differed from the downstream sites 4 and 5 in the number of taxa, density, Shannon diversity index, and the proportion of dominant taxon in the univariate analyses. Baetis spp. was the dominant taxon (14.22%) and Chironomidae was the dominant family (21.78%) in the entire study reach. The results of canonical correspondence analysis (CCA), a eigenanalysis-based ordination, were compared with the results of non-metric multidimensional scaling (MDS), a distance-based ordination. Differences in aquatic insect assemblages at the first three sites were only found by the multivariate approaches, not by th