A diatom-based index for the biological assessment of eastern Canadian rivers: an application of correspondence analysis (CA)

We developed a diatom-based index that integrates the effects of multiple stresses on streams and provides information related to the distance from the nonimpacted state. The Eastern Canadian Diatom Index (IDEC) was based on a correspondence analysis (CA) to develop a chemistry-free index where the position of the sites along the gradient of maximum variance (first axis) is strictly determined by diatom community structure and is therefore independent of measured environmental variables. The index value indicates the distance of each diatom community from its specific reference community. A high index value represents a non- or less-impacted site, while a low index value represents a more heavily impacted site. Two sub-indices were developed based on two sets of reference communities. The IDEC-circumneutral includes the sites that have reference communities characteristic of slightly acidic or neutral environments. The IDEC-alkaline includes the sites that have reference communities characteristic of environments where pH values are naturally higher than 7.5. The distinction between the two sub-indices is fundamental to make sure that each stream has the potential to reach a high IDEC value following complete restoration of its ecosystem.