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.

The identification of biological reference conditions specific to each type of water body is essential for the development of sound biological indicators and criteria. The purpose of the present study was to establish the reference conditions of each stream type sampled in southern Québec (Canada) using benthic diatoms and environmental variables characterizing streams and watersheds. First, stream reaches were classified as a function of their natural watershed and habitat characteristics. Second, diatom communities were classified based solely on taxa abundance data. Resulting groups were graphically presented on ordinations to interpret, a posteriori, the environmental gradients associated with diatom groups and to identify the diatom communities representing the reference conditions of each of the stream reach groups. A final classification based solely on diatom reference communities found pH and conductivity to be the main discriminating factors, regardless of ecoregion and stream type. Although a specific diatom reference community may be identified for each stream group, our results suggest that many of these communities exhibit strong similarities. Only two reference communities may therefore be used, one for circumneutral conditions and one for alkaline conditions. These reference communities represent the baseline for biocriteria development.

No abstract

In 2006, the Eastern Canadian Diatom Index (IDEC: Indice Diatomées de l'Est du Canada) was developed to monitor the biological integrity of streams and rivers in Eastern Canada. The first version of the IDEC has been successfully used to evaluate the biological status of numerous sites. A new version of the index (IDEC 2.0) was recently developed to cover a larger geographic area that encompasses additional geological characteristics. IDEC 2.0 is now applicable for the biological assessment of streams in Quebec, Ontario, New Brunswick, Nova Scotia, and Prince Edward Island. Moreover, the approach used to define the biological interpretation of the index values was revised. IDEC 2.0 presents ecologically meaningful differences among biological communities (ecological thresholds or class boundaries) based on diatom biotypes, providing a more relevant interpretation of the diatom community changes along the alteration gradient.

The Eastern Canadian Diatom Index (IDEC) was developed to evaluate the ecological integrity of streams along a pollution gradient, as a function of the dissimilarity between current diatom communities and suitable reference communities. Distinguishing natural variations in community structure from those induced by human activities is essential for proper assessment of dissimilarity. To account for the effect of the natural variation in pH on this assessment, two IDEC subindices were used: one for sites with diatom reference communities typical of naturally alkaline water pH, and another for sites with communities typical of naturally circumneutral water pH. This study used three statistical models, namely classification trees (CT), random forests (RF), and artificial neural networks (ANN) to: (i) identify the environmental variables discriminating between alkaline and neutral reference communities (“biotypes”), and (ii) compare their predictive capacities. Models identified clay rocks, gneiss/paragneiss rocks, siliceous rocks, and carbonated rocks as the main geological features discriminating reference biotypes. For the reference streams, clay, siliceous, and carbonated rocks were associated with high water pH while gneiss/paragneiss rocks were associated with low water pH. Both ANN and RF models behaved similarly across all performance criteria and yielded general models useful for identifying the appropriate IDEC sub-index.