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CADDIS Volume 2: Sources, Stressors & Responses

energy sources physical habitat hydrology temperature water and sediment quality stormwater runoff wastewater inputs riparian and channel alteration urbanization

Bed substrates & biotic condition

Urbanization typically affects both sediment supply and transport capacity in streams, resulting in altered substrate composition and stability—both of which are key factors influencing stream biotic communities (see the Sediments module for further discussion of sediment as a stressor).

Many streambed substrate changes associated with urban development have been linked to changes in biotic condition, including:

For example, Morley & Karr (2002) found that invertebrate biotic integrity (B-IBI) scores and taxa richness metrics increased with substrate size and roughness, but that these substrate parameters decreased with urbanization (Table 7).

However, fine sediments are not always higher in urban streams. Fines may be scoured from these systems as stream discharge increases with impervious cover, resulting in coarser, more armored streambeds (Chin 2006).

Sediment increases related to urbanization also can have indirect effects on stream biota, via sediment-associated contaminants. Urban sediments can contain high concentrations of metals, organics, and other toxics, and these compounds can adversely affect biotic condition (see Water / Sediment Quality).

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Increased erosion and sediment runoff from disturbed soils.
Courtesy of U.S. EPA.
Table 7. Spearmen rank correlation coefficients for associations of urbanization and macroinvertebrate biotic condition parameters with substrate measures. D16 and D50 refer to the substrate diameter below which 16% & 50% of particles are smaller, respectively; roughness was calculated as the 84% particle diameter divided by bankfull depth. Coefficients in italics had p < 0.10; coefficients in bold had p < 0.05.
Substrate Measure
Urbanization, n 17 17 17
    % sub-basin -0.20 -0.35 -0.60
    % local -0.12 -0.49 -0.70
Biotic condition, n 18 18 18
    B-IBI +0.27 +0.12 +0.51
    Total taxa richness +0.34 +0.17 +0.43
    EPT richness +0.59 +0.41 +0.50
    Clingers richness +0.60 +0.39 +0.52
Modified from Morley SA & Karr JR. 2002. Assessing and restoring the health of urban streams in the Puget Sound basin. Conservation Biology 16(6):1498-1509.

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