Water velocity attenuation by stream periphyton and macrophytes in relation to growth form and architecture

Periphyton and macrophytes alter water velocity in streams, influencing movement of solutes and providing microhabitat for other organisms. How assemblages with different growth form and architecture influence water velocity attenuation across mm to dm scales is not well described. A thermistor microprobe was used to measure water velocity through 4 morphologically distinct stream periphyton assemblages and 4 distinct stream macrophyte assemblages in flumes. All assemblages resulted in an exponential decay in velocity with depth. A dense assemblage of diatoms (primarily Cymbella) attenuated velocity more than filamentous green algae, filamentous green algae with interspersed diatoms, or a red alga (ANOVA, p , 0.05). External water velocity had no significant influence on the coefficient of attenuation in a filamentous green alga (ANOVA, p 5 0.76). Macrophytes also attenuated water velocity, but attenuation was more variable and, in all cases, attenuation coefficients were less for macrophytes than for periphyton. A model unifying attenuation by periphyton and macrophytes was developed using biomass density (g ash-free dry mass/m3) as the independent variable and it explained 80% of the variation in attenuation. The relative variance of attenuation coefficients increased sharply as Reynolds number increased above ;500 to 700, suggesting that variance in water velocity was dependent upon the spatial scale of the primary producer through which water is flowing, and that the distinction between periphyton and macrophytes may have real physical ramifications.