Nutrient enrichment effects on biofilm metabolism in a Mediterranean stream

1. Biofilm biomass and metabolism were analysed in La Solana, a calcareous, undisturbed second-order stream. Measurements were carried out in two Mediterranean climatic extremes, summer and winter. Two on-site experimental channels were used to study changes following nutrient addition to one of them. 2. Algal biomass (chlorophyll a), NDPP (net daily primary production), GDPP (gross daily primary production) and R (respiration) increased in the enriched channel, and these increases were greater in summer. 3. Photosynthetic capacity (Pmax^chl) decreased during summer, possibly due to enhanced self-shading accompanying the increase in biomass. 4. In winter, Pmax^chl increased and reached values similar to those in the summer control channel. Because grazing was low, the higher values of Pmax^chl were attributed to partial substitution of the cyanobacterial assemblage by a green-algae dominated assemblage with higher photosynthetic capacity as well as the low increase in biomass preventing any significant self-shading.