Chronic, non-lethal stressors occurring gradually (in space or time) can result in cumulative impacts, more dramatic than higher intensity or occasional critical level of one of these contaminants singly. The negative effects of these stressors trigger enduring impacts that may grow in intensity and become problematic over time and/or to higher trophic levels. In a river, for instance, aquatic organisms experience this type of cumulative stress along the up- to downstream gradient in natural and anthropogenic contaminants generally observed in inhabited watersheds. Diatoms are a major component of the periphyton in rivers; their richness and diversity in natural communities are directly related to their varied ecological preferences. In this study, we monitored from 2003 to 2008 the changes in structural features and traits along a small river (Riou-Mort, South West France), at three sites: one site upstream considered as a reference, one intermediate site with high nutrient load, and one downstream site exposed to both nutrient and metal pollution. The cumulative impacts of nutrients plus metals led to a gradual decrease in species richness and diversity, and potential capacity to cope with additional stresses, e.g. climate change-related ones. This is reflected by, for example, the decrease in species richness downstream, more dramatic in Summer 2003 (heat wave) than in cooler summers. With the increasingly protective environmental regulations (e.g. Water Framework Directive in Europe), cumulated stresses on aquatic resources are recommended to receive increasing attention, in particular considering the expected changes in global climate.