CADDIS Volume 2: Sources, Stressors & Responses
Urbanization & basal energy sources
There are two main sources of fixed energy that drive stream food webs:
- Organic carbon produced by photosynthesis outside the stream, or allochthonous production
- Organic carbon produced by photosynthesis within the stream, or autochthonous production
Most streams rely on both allochthonous and autochthonous energy, although the relative importance of each varies with elevation, stream size and other factors. For example, terrestrial carbon is more important in forested headwater streams, whereas autochthonous carbon is more important in open-canopied, mid-sized rivers.
Urbanization alters the energy sources available to stream food webs, as well as the in-stream retention and storage of those basal resources. Key changes associated with urbanization are summarized at right; examples include:
↑ riparian deforestation, resulting in:
- ↑ light and algal production
- ↓ terrestrial litter and wood inputs
↑ nutrient enrichment, resulting in increased algal production and microbial respiration
↑ input of sewage-derived particulate organic matter
↓ algal biomass, due to scouring flows
Δ in the relative importance of physical vs. biological factors in determining leaf decay rates
Changes in resources can result in changes in the consumer community. For example invertebrate functional feeding groups may change: reduced leaf litter may lead to few shredder invertebrates; increased algal production may lead to increased scrapers; and increased input of particulate organic matter may lead to increased filterers. However, these changes often are mitigated by concurrent changes in habitat and water quality.
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