Mathematical Analysis of Temporal and Spatial Trends in the Benthic Macroinvertebrate Communities of a Small Stream

Abstract

Macroinvertebrates were collected at four sites in Padden Creek, a small second-order stream in Whatcom County, Washington, USA. Two upstream sites were characterized by high densities of sensitive taxa, predominantly mayflies, stoneflies, and caddisflies, and two downstream sites showed high densities of tolerant taxa, especially true flies, annelids, Baetis mayflies, and gastropods. Despite the small sample size, some statistical techniques proved useful. The first two components of correspondence analysis were used to confirm the existence of both seasonal and spatial trends in the benthic macroinvertebrate populations of the stream. Neither component alone, however, ordinated the samples with respect to these trends. Combinations of the first two components were required. A standard clustering technique, k-means clustering with squared Euclidean distance, further confirmed the seasonal trend. Nonmetric clustering, not widely used in the analysis of ecological data, was necessary to confirm the spatial trend. Nonmetric clustering was also able to identify a small number of "significant" taxa, i.e. taxa that reliably served as indicators of spatial position on the stream.