Velocity gradients and turbulence around macrophyte stands in streams

1. Submerged macrophytes strongly modify water flow in small lowland stream s. The present study investigated turbulence and vertical velocity gradient s using small hot-wire anemometers in the vicinity and within the canopies of four macrophyte species with the objective of evaluating: (a) how plant canopies influence velocity gradients and shear force on the surfaces of th e plants and the stream bed; and (b) how the presence and morphology of pla nts influence the intensity of turbulence. 2. Water velocity was often relatively constant with water depth both outsi de and inside the giant canopies, but the velocity declined steeply immedia tely above the unvegetated stream bed. Steep vertical velocity profiles wer e also observed in the transition to the surface of the macrophyte canopy o f three of the plant species forming a dense shielding structure of high bi omass. Less steep vertical profiles were observed at the open canopy surfac e of the fourth plant species, growing from a basal meristem and having the biomass more homogeneously distributed with depth. The complex distributio n of hydraulic roughness between the stream bed, the banks and the plants r esulted in velocity profiles which often fitted better to a linear than to a logarithmic function of distance above the sediment and canopy surfaces. 3. Turbulence increased in proportion to the mean flow velocity, but the sl ope of the relationships differed in a predictable manner among positions o utside and inside the canopies of the different species, suggesting that th eir morphology and movements influenced the intensity of turbulence. Turbul ence was maintained in the attenuated flow inside the plant canopies, despi te estimates of low Reynolds numbers, demonstrating that reliable evaluatio n of flow patterns requires direct measurements. The mean velocity inside p lant canopies mostly exceeded 2 cm s(-1) and turbulence intensity remained above 0.2 cm s(-1), which should be sufficient to prevent carbon limitation of photosynthesis in CO2-rich streams, while plant growth may benefit from the reduced physical disturbance and the retention of nutrient-rich sedime nt particles. 4. Flow patterns were highly reproducible within canopies of the individual species despite differences in stand size and location among streams. We p ropose that individual plant stands are suitable functional units for analy sing the influence of submerged macrophytes on flow patterns, retention of particles and biological communities in lowland streams.