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:

• ↑ fine sediment
  [Hogg & Norris 1991, Morley & Karr 2002, Roy et al. 2005, Taulbee et al. 2009, Walters et al. 2009]
• ↑ embeddedness and armoring
  [Borchardt & Statzner 1990, Blakely et al. 2006, Chin 2006, Walters et al. 2009]
• ↓ substrate stability
  [Pedersen & Perkins 1986]
• ↓ substrate complexity and heterogeneity
  [Morley & Karr 2002, Blakely et al. 2006]

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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