Photosynthetically active radiation (PAR, μmol m−2 s−1) was recorded every 10 min during the sampling period using a spherical quantum sensor (Li-Cor Li-193SA, Lincoln, NB, USA) connected to a data logger (Li-Cor, Li-1000). The sensor was placed 20 cm above the stream water surface at the middle of the reach. Temperature and PAR for each addition were estimated as the average of all data recorded during the experiment. On each date and for each reach, we also
measured stream width at 6 transects evenly distributed along the reach. At each transect, water depth, substratum type and coverage by biofilm or filamentous green algae were measured every 20 cm across the channel width. Stream width and water depth for each reach were computed as the average from transect measurements.