1. Leaf breakdown rates of Alnus glutinosa were determined and the structure of decomposer assemblages associated with leaves were analysed to assess the effect of pollution on the ecological condition of the Ave River (North-west Portugal).
2. Increase in organic and inorganic nutrients was associated with an increase in density and a decrease in richness of macroinvertebrates, a dramatic decline in the conidial production of aquatic hyphomycetes, but no major change in the richness of aquatic hyphomycetes.
3. Downstream nutrient enrichment was correlated with accelerated leaf breakdown rates.
4. The degree of functional impairment assessed by the ratio of leaf breakdown rates in coarse-mesh and fine-mesh bags was in accordance with the gradient of pollution defined by two biotic indices.
5. This study supports the contention that leaf breakdown experiments are a valuable tool to assess the effect of pollution on the ecological condition of rivers.