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 lager geographic area, allowing for the inclusion of environmental conditions that were not well represented in the first version. Moreover, the approach used to define the biological interpretation of the index values was revised. Based on the first IDEC, the alteration gradient was divided into 5 equal classes reflecting qualitative levels of biological integrity. However, the limits between the classes were arbitrarily defined and lacked a meaningful biological consideration. The new 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. This approach, therefore, suggests more ecologically significant boundaries between qualitative classes, and reduces the subjectivity associated with arbitrarily and equally defined classes.