NUTRIENT ENRICHMENT EFFECTS ON BIOFILM METABOLISM IN A MEDITERRANEAN STREAM

1. Biofilm biomass and metabolism were analysed in La Solana, a calcar eous, undisturbed second-order stream. Measurements were carried out i n two Mediterranean climatic extremes, summer and winter. Two on-site experimental channels were used to study changes following nutrient ad dition to one of them. 2. Algal biomass (chlorophyll a), NDPP (net dai ly primary production), GDPP (gross daily primary production) and A (r espiration) increased in the enriched channel, and these increases wer e 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 val ues similar to those in the summer control channel. Because grazing wa s low, the higher values of Pmax(chl) were attributed to partial subst itution of the cyanobacterial assemblage by a green-algae dominated as semblage with higher photosynthetic capacity as well as the low increa se in biomass preventing any significant self-shading.