The purpose of this study was to evaluate the response of the Eastern Canadian Diatom Index (IDEC: Indice Diatomées de l’Est du Canada) with respect to temporal water chemistry variability. The results showed that along a large phosphorus gradient (8 to 1220 mg P L^-1), the IDEC was highly correlated with averaged water chemistry data. Along a small phosphorus gradient, specific to each stream, the IDEC integrated phosphorus over various periods of time, depending on the trophic status of the site studied and the variability in nutrient concentration. Where nutrient concentrations were low and generally stable, an input of phosphorus induced a rapid change in diatom community structure and IDEC value within the following week. In a mesotrophic stream, the observed integration period was approximately 2 weeks. Diatom communities in a eutrophic stream appeared to be adapted to frequent and significant fluctuations in nutrient concentrations. The IDEC therefore showed a slower response to short term fluctuations and integrated nutrient concentrations over a period of five weeks. These results indicate that (1) diatom communities have a better relationship with time-averaged water chemistry than punctual measurements and (2) underline the need for diatom-based monitoring that integrates temporal variations in stream nutrient concentrations.