Nira L. Salant, Marwan A. Hassan Department of Geography, University of British Columbia Physical effects of streambed periphyton on particle deposition and flow hydraulics Paper Number H----- Introduction What is periphyton? Periphyton is a complex matrix of algae, bacteria and associated polysaccharide secretions found in the hyporheic and benthic zones of streambeds. Periphyton assemblages dominated by green algae are typically thick and filamentous, while diatom and bacteria are usually low-profile and mucilaginous. Hydraulics: Velocity and shear stress profiles Conclusions Deposition: Inorganic content of surface samples Hypothesis Streambed periphyton influences the deposition of fine particles directly by adhesion or trapping and indirectly by changes to nearbed hydraulics; the magnitude and nature of change depends on periphyton density and structure Predictions Hydraulics 1) Low density, low-profile forms will not significantly alter near- bed flow patterns, but low-density, rigid filamentous algae will increase surface roughness, reducing near-bed flow velocity and turbulence intensity. 2) High density and/or flexible filaments shielded from the flow will cause flow constriction and accelerated velocities throughout the flow depth. Deposition 1) ‘Sticky’ exopolysaccharides of mucilaginous biofilm (e.g. diatoms and bacteria) will enhance particle adhesion and deposition relative to filamentous forms (e.g. green algae). 2) Particle deposition will increase with biofilm density, but will plateau as mats become saturated with material. What does periphyton do? Periphyton provides food and habitat for many aquatic organisms, but some evidence suggests that periphyton assemblages may indirectly influence stream organisms by altering the geomorphic and hydraulic conditions of their local environment. Recent research suggests that periphyton can enhance particle deposition via adhesion and may alter hydraulic conditions above and within streambeds. Like large- scale macrophytic canopies, periphyton mats may significantly alter flow and sedimentation patterns according to their thickness, structure and density U x (cm/s) z/H Periphyton-Open None BE p ‘Closed’ BE eff = BE p BE p BE eff ‘Open’ BE eff < BE p U x (cm/s) z/H Periphyton-Closed None BE p Reynolds shear stress (N m/s) z/H Reynolds shear stress (N m/s) Table: Values are averages (SE) of all trials in one of six categories based on flow level (HF=high, LF=low) and type of periphyton mat (C=‘closed’, O=‘open’, or N=none). Bed shear stresses (τ Re0 ) and turbulence intensities (TKE 0 ) were determined from 3-D velocity fluctuations of the near-bed measurement. U x, U max, and U 0 are the mean, maximum, and nearbed horizontal velocity. Volumetric density (AFDM/h) of each mat was determined from the areal density (measured as AFDM) and the measured height of the filaments (h). Flume experiments Field studies and flume experiments BE p Acceleration Reduced near-bed velocities Slight acceleration Reduced shear stresses