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CADDIS Volume 3: Examples & Applications

Worksheets: Little Scioto River, OH

List Candidate Causes

Table 2. List of candidate causes for the Little Scioto River

  1. Increased fine sediments. Channel modification (i.e., narrowing, deepening, and straightening of channel) leads to increased deposition of fine sediments.

  2. Altered pools and riffles. Channel modification (i.e., narrowing, deepening, and straightening of channel) leads to deeper pools and fewer riffle habitats.

  3. Low dissolved oxygen (DO) concentration. Several pathways may contribute to low DO levels: decreases in large woody debris and loss of riffle habitats can decrease aeration; increased organic carbon loading (e.g., from wastewater inputs) can increase biological oxygen demand (BOD); and/or increases in nutrients (nitrogen and/or phosphorus) can stimulate algal production and thus BOD (due to respiration of living plants and/or decomposition of algal detritus).

  4. Increased algal biomass. Nutrient inputs lead to moderate increases in algal biomass. These increases are insufficient to significantly reduce DO, but sufficient to stimulate secondary production in the system (i.e., lead to increased fish weights) and to alter invertebrate community structure.

  5. Ammonia toxicity. Increases in nitrogen loading lead to increases in total ammonia within the system; this ammonia dissociates into un-ionized ammonia, which is toxic to aquatic organisms. This candidate cause can be significantly affected by pH levels and DO concentrations, as the relative abundance of unionized ammonia increases with increasing pH and decreasing DO (due to reduction of nitrate to ammonium). Because increases in photosynthesis can raise pH, increased nutrients can both directly and indirectly increase unionized ammonia concentrations.

  6. Metal toxicity.

  7. PAH toxicity.

Map of the Little Scioto
                                      River with tributaries, roads
                                      and possible sources.
Figure 4. Map of the Little Scioto river showing locations of tributaries, roads, and possible sources.
A conceptual model of the six candidate causes for the Little Scioto stressor identification.
Figure 5. A conceptual model diagram of the six candidate causes for the Little Scioto stressor identification (revised August 2005).

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