Comparing diatom species, genera and size in biomonitoring: a case study from streams in the Laurentians (Quebec, Canada)

1. We studied the distribution of epilithic diatoms in streams subjected to different degrees of human impact in order to evaluate their potential as bioindicators for environmental changes such as nutrient enrichment and acidification. 2. Three descriptors of the diatom assemblages were tested with respect to their potential to predict environmental changes: species composition, genus composition and size distribution. 3. Water colour and pH explained the largest amount of variation in diatom assemblages. According to ordination analyses, water colour explained variations in size distribution (42%) better than those in generic (25%) or species composition (8%). On the other hand, pH was not correlated with size distribution while a significant fraction of variation was explained by species (11%) and especially generic (18%) composition. Only species composition responded to changes in phosphorus and grazer biomass, however. 4. Size distribution and coarse (genus level) taxonomic analyses sometimes outperformed fine taxonomy in describing the response of diatom assemblages to colour and acidity. In view of the simplicity of these alternative descriptions of diatom assemblages, their potential for routine stream monitoring should be further explored.