Periphyton distribution and abundance on substrata of different size along a gradient of stream trophy

In stream, substrata of different size present different degree of stability, current, erosion, and deposition to colonizing organisms. In this study, we tested the importance of substratum size ranging from sand to small boulders for periphyton distribution and abundance. Because trophy strongly affect stream organisms, we sampled at nine sites chosen to represent the range of nutrients typical of Eastern Ontario and Western Quebec. A large part of the variability in algal biomass (as chlorophyll) among sites was explained by trophy (as seston or total phosphorus). However, there was also an effect of substratum size. Cobbles had the highest biomass, and gravel the lowest; sand and boulders were intermediate. Assemblages on different substrata were different in taxonomy and life forms. Cyanobacterial colonies and motile diatoms dominated the finer substrata while adnate and filamentous algae were more developed on the larger ones. Consequently periphyton on fine sediments was more loosely attached than on rocks. Average algal size was not related to substratum size but increased significantly with trophy confirming observations in benthic and planktonic assemblages in lakes.