Long‐term moderate nutrient inputs enhance autotrophy in a forested Mediterranean stream

1. The effects of long‐term nutrient addition at moderate levels were examined in the food web of a forested Mediterranean stream. Basal concentrations of N and P were increased twofold (to c. 750 μg N) and threefold (to c. 30 μg P) from ambient concentrations in an experimental reach. Variations in the abundance of microbes (bacteria and algae), meiofauna and macrofauna, microbial processing of organic matter (extracellular enzyme activities) and stoichiometry of biofilms and invertebrates were compared to an upstream control reach during 4 years of artificial nutrient enhancement. 2. Effects were faster in the bacterial compartment but more substantial in the algal compartment. Epilithic algal biomass doubled in the enriched section jointly triggered by nutrients and increased light irradiance in winter and early spring. Only a few animal groups reacted to the enrichment, including the meiofaunal Copepoda, linked to their high use of enriched FPOM, and macrofaunal grazers (Ancylus), which followed the large algal biomass increase. 3. The enrichment caused biofilm phosphatase activity to decrease, while activities related to the use of algal‐related materials (peptidase, β‐glucosidase) increased. Enzymatic activities related to the use of allochthonous organic matter showed only minor and episodic increases. 4. Changes in stoichiometric ratios were apparent in the epilithic compartment, but not in the sand sediment or in the FPOM. Increases in P content were delayed for 9 months in epilithic biofilms and for nearly 2 years in the case of N. 5. After 2 years of enrichment, the flatworm Schmidtea polychroa (predator), oligochaetes (detritivore) and tadpoles of Bufo bufo (grazer) showed higher per cent N. 6. Enrichment effects were produced in spite of flow cessations that occurred commonly in summer. The results show that forested streams subjected to sustained (though minor) nutrient enrichment changed aspects of their biological structure and metabolism and that changes were especially favoured by periods when light was not limiting.