1. Nira Salant Department of Geography University of British Columbia Effects of Streambed Periphyton on Hydraulics and Sediment Deposition in Streams

2. What is periphyton?

3. What does periphyton do? Food and habitat Physical effects?  I. Hydraulics  II. Sediment deposition

4. Sediment deposition Trapping, Adhesion, Clogging Turbulence Algae: High profileDiatoms: ‘Sticky’

5. Sediment content of surface samples

6. Deposition from water column: Diatoms Diatoms: ‘Sticky’

7. Deposition from water column: Diatoms Highest deposition velocity when near-bed and upper flow shear stresses are low and biomass is moderate (moderate adhesion, low clogging) Biomass increases:  Near-bed shear stress increases (structural roughening)  Deposition velocity decreases (high upward stresses and infiltration decreases = ‘clogging’) Depositional velocity Max shear stress Near-bed shear stress

8. Deposition from water column: Diatoms Evidence for clogging?

9. Deposition from water column: Algae Algae: High profile

10. Deposition from water column: Algae Unclear relation between biomass, shear stress, and depositional velocity Deposition decrease with biomass? Clogging? But…

11. Deposition from water column: Algae Shear stress increases with growth stage Surface deposition decreases with growth stage Later growth stage  Increase in shear stress  Less surface deposition BUT  Higher advection and infiltration (subsurface deposition) Total deposition = balance of surface and subsurface deposition High biomass reduces infiltration Depositional velocity Max shear stress Near-bed shear stress Surface samples AM

12. Deposition from water column: Algae Turbulence  Less surface deposition, deeper infiltration (A8  A20) Biomass  Reduced infiltration despite high advection (A16)

13. Implications Streambed patchiness and complexity Flow conditions, sediment accumulation, interstitial infiltration  Habitat condition  Organism behavior …a function of periphyton structure and distribution

14. Decrease in concentration over time Exponential model C 0 = peak concentration at time t = 0 k = decay (or deposition) rate (T -1 ) w s = settling velocity (D/T) = depositional velocity w d when fit to exponential model h = flow depth (D)

15. I. Hydraulics ‘Closed’ ‘Open’ Filamentous periphyton ‘patches’

17. Velocity distribution u0u0 UxUx u max 0.010.020.030.040.050.0 u (cm/s) 50.0 0.6 u (cm/s)

18. Shear stress distribution Two-layered flow Closed Open Logarithmic layer Peak shear = top of Roughness layer Periphyton No periphyton Shift in height of roughness layer top Same thickness

19. 0 0.05 0.1 0.15 0.2 0.25 0.3 00.0050.010.0150.020.025 Re/ρU x 2 ( z/H Near-bed turbulence reduction Periphyton None 2) Hydrodynamic smoothing (Closed mats) 1) Shift in location of peak shear (Open mats) Higher upper flow stress Reduced turbulent transfer

20. Diatoms 24 WeeksDiatoms 4 Weeks None