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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

Catchment vs. riparian urbanization

Where urbanization occurs in the watershed can affect its influence on stream ecosystems. Studies examining land use variables and stream characteristics typically consider land use at one (or more) of three general spatial scales:

Catchment: the entire catchment area above the site

Riparian: the entire riparian area in the catchment above the site

Reach: the riparian area for a relatively short distance above the site


King et al. (2005) examined whether macroinvertebrate assemblages in Coastal Plain, Maryland streams responded differently to development in the watershed versus development in areas closer to the focal site (Fig 3). They found that where development occurs can significantly influence its effects on benthic biota:

  • For % developed land in the watershed (Fig 3A), there was an apparent threshold between 21-32% where the probability of assemblage alterations increased rapidly; once >32% of the watershed was developed, all macroinvertebrate assemblages were affected.
  • When % developed land in the 250-m buffer was considered (Fig 3B), this threshold shifted left and all macroinvertebrate assemblages were affected once >22% of land in the 250-m buffer was developed.
  • A similar pattern was seen when developed land in the watershed was inverse-distance weighted (i.e., development closer to the focal site was weighted more than development farther away; Fig 3C), with the threshold for macroinvertebrate effects occurring between 18-23%.

The relative importance of development at different scales varies across studies (e.g., Sponseller et al. 2001, Wang et al. 2001, Morley & Karr 2002, Roy et al. 2003, Snyder et al. 2003, Schiff & Benoit 2007), and likely depends, at least in part, on the stressors considered (Allan 2004). For example, some stressors associated with urbanization (e.g., changes in flow) are highly dependent on catchment-scale processes, while other stressors (e.g., changes in basal energy sources) are more affected by reach-scale processes.


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Figure 3. Scatterplots of the threshold effect of developed land on macroinvertebrate assemblage composition (Bray-Curtis dissimilarity expressed as nonmetric multidimensional scale [nMDS] Axis 1 scores), for (A) % developed land in watershed, (B) % developed land within 250-m radius buffer of site, (C) % developed land in watershed weighted by its inverse distance (IDW) to site. Dotted lines indicate the cumulative probability of an ecological response to increasing % developed land. Sites within the watershed-scale threshold zone of 21-32% developed land in (A) are highlighted in black in all panels.
From King RS et al. 2005. Spatial considerations for linking watershed land cover to ecological indicators in streams. Ecological Applications 15(1):137-153. Reprinted with permission.

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