13. Sediment and aquatic habitat in rivers (a)Benthic organisms and bed sediments (b)Fish and bed sediments (c)Reach classification based on bed material properties

(a) Benthic organisms and bed sediments Benthos: organisms that live on or in the bottom of a body of water Bed materials inhabited by benthic organisms that provide food for fish Surficial bed material size primary influence on benthic community composition and density –Highest diversity found in cobble-gravel reaches –Sand-bed benthos characterized by low diversity and specialized species

Benthic organisms Two factors account for higher levels of species diversity in coarse sediments: –Poorly sorted sediments; offer wide range of void sizes used by different species –Bed stability; benthic community development requires bed stability periods longer than a life cycle

(b) Fish and bed sediment Fish use sediments directly for spawning and feeding (but use varies greatly with life stage and species) –Populations of fish (such as salmonids) that use bed sediments for spawning are clearly closely linked to sediment transport processes –Feeding also important; fish species prey on specific benthic types that specialize on certain bed sediments Some fish use large sediments (e.g. boulders) as velocity shelters and hiding cover from predators

Fish community… Habitat complexity increases community complexity Environmental stability appears to control community stability

Classification based on bed material properties (i)Boulder-cobble be streams -Physical heterogeneity and temporal stability provide habitat for a dense, diverse invertebrate community -Interstitial voids are important aquatic- habitat features; collectively called hyporheic zone

(ii) gravel reaches Benthic community generally dense and diverse (e.g. up to 40 species; densities between 2000 and organisms per square meter) Hyporheic zones often well aerated. Benthic density in a mixture of cobble, gravel, and sand inversely proportional to the % of sand.

Gravel reaches… Gravel deposit characteristics suitable for spawning:. Presence of interstitial voids for the eggs. Adequate permeability for transport of oxygen to eggs developing in the gravel. Stability during the period between spawning and emergence

(ii) gravel reaches… Salmonid egg survival inversely related to percentage of material finer than small gravel (  5 mm) and directly related to the ratio of bed sediment size to egg size. (Surficial deposition of sediments finer than gravel adversely impacts reproduction of all types of fish that spawn in gravel.)

Scour depth and spawning The depth of scour in gravel-bed rivers reflects the bedload transport rate Depth of scour can be estimated from bedload transport rate and bed sediment porosity d s = I b / [U b ρ s (1-p)] where ds: scour depth I b ; bedload transport rate U b : bedload velocity p: porosity ρ s : sediment density A slight increase in scour depth can jeopardize a significant proportion of egg pockets and affect salmon population

Hyporheic exchange in gravel-bed rivers with pools and riffles Salmon and trout bury their eggs in streambed gravels for incubation within the hyporheic zone Intensity and extent of hyporheic exchange is also a function of bed topography and consequent variations in near-bed pressure (i.e. in addition to bed material size, grain packing, and therefore, hydraulic conductivity).

Hyporheic exchange and riffle-pool sequences… Hyporheic exchange in riffle areas particularly important for salmonid ecology Exchange across the riffle characterized by strong downwelling on upstream side of riffle, and upwelling on downstream side. Salmonids preferentially spawn in this zone and may be attracted to the strong hyporheic circulation through the riffle sections

(iii) Fine bed sediments Sand Virtually constant movement of sand beds; harsh environment for most benthic species –High density of very small organisms –Most organisms found in the top 1-10 cm Silt-clay Depends on degree of consolidation – cohesiveness Often stable for most flows (high cohesion of clay banks and beds)

Effects of suspended sediment Fish apparently can physically withstand temporarily high levels of suspended sediments (e.g to 10 4 mg/L) In general, chronic exposure to elevated sediment levels is more critical than short- term exposure

Summary Benthic communitiesFish - Bed stability- Feeding (frequency of bed material- spawning movement)- voids spaces - Permeability- hyporheic. diversity sedimentation. size. stability. density. physical diversity