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CADDIS Volume 3: Examples & Applications

Worksheets: Little Scioto River, OH

Identify Probable Cause

The following probable causes were identifed for each specific effect:

Biological effect: Increased relative weight
Probable Cause
Degraded riffles/pools
Evidence for: Stressor co-occurs with effect
Cause is mechanistically plausible

Biological effect: Increased % DELT
Probable Causes
Decreased DO Increased Metals
Evidence for: Stressor co-occurs with effect Stressor co-occurs with effect
Cause is mechanistically plausible Cause is mechanistically plausible
Causal pathway exists

Biological Effect: Decreased % mayflies and increased % tolerant macroinvertebrates
Probable Causes
Increased sediment Degraded riffle/pools Decreased DO
Evidence for: Stressor co-occurs with effect. Stressor co-occurs with effect. Stressor co-occurs with effect.
Cause is mechanistically plausible. Cause is mechanistically plausible. Cause is mechanistically plausible.
Causal pathway established
Effect consistent with field stressor-response relationship
DO is less than minimum value from criterion.

Conclusions

Different candidate causes seem to be responsible for each of the biological effects that were considered in this analysis. However, closer examination of the candidate causes suggests a common thread. First, it is likely that the three causes associated with decreased mayflies (increased sediment, degraded pools and riffles, and decreased DO) all derive from the same human activity, namely channelization of the stream. When the streamcourse is channelized, fine sediment can increase and riffle habitats can be degraded. Furthermore, the deepening of the channel and lost riffles can reduce dissolved oxygen. The deepened channel is also identified as a likely cause for the increased fish weight. Finally, reduced DO from the deepened channel may also be the cause of the increased proportion of DELT. Therefore, we identify the channelization of the stream as the most likely source of the observed impairment at this site.

Discussion

Increased relative weight: Only one candidate cause was found to be probable from the strength of evidence analysis. The available evidence suggested that the change in channel structure, and the resulting deepening of the stream was the most likely factor causing the increase in fish weight. Most other candidate causes were unlikely because no plausible mechanisms existed by which they could increase fish weights (sediment, dissolved oxygen, ammonia, metals). The argument for increased algae was weakened by a negative relationship between increased nutrients (used as a surrogate for algae) and relative fish weight. That is, the largest fish weight was observed at the site with one of the lowest nutrient concentrations. Although only one cause was identified, the evidence in support of this cause is weak, and further data collection is suggested. In particular, the effects of different sampling methodologies between the upstream site and the site of interest should be investigated. Increased relative weight is not a metric in either IBI or ICI, and so this effect is difficult to relate back to the original definition of the impairment.

Increased proportion of DELT anomalies: The cause of the change in DELT was either decreased DO or increased metal concentrations, but the available evidence does not distinguish between these two possibilities. The available lines of evidence for these two candidate causes were virtually identical. Decreased DO included the extra line of evidence that a causal pathway existed. Increased metals lacks this line of evidence only because the causal pathway is simple: increased metals can directly cause increased DELT. Other candidate causes were unlikely for a variety of reasons: Sediment and riffle/pool quality were unlikely causes because they lacked plausible mechanisms. Increased ammonia concentrations were not found at the site, and therefore an unlikely cause. Nutrient concentrations (used as a surrogate for increased autotrophs) were lower than the levels associated with increased DELT anomalies from other field studies in Ohio. Collecting additional evidence would be useful, particularly data that establishes relationships between DO or metal concentrations and the proportion of DELT anomalies. Information on the specific types of anomolies found would also be useful.

Decreased proportion of mayflies and increased proportion of tolerant macroinvertebrate taxa: Three candidate causes possibly explain the changes observed in the macroinvertebrate assemblage. Stressor-response relationships between sediment levels, habitat quality, and the observed biological responses were difficult to establish using the available data. However, changes in substrate composition and the quality of riffle habitat are known to have profound effects on macroinvertebrate assemblages. Several lines of evidence suggest that decreased DO is the cause of the observed changes. Collecting additional evidence linking sediment levels and habitat quality with the macroinvertebrate assemblage response would strengthen the argument for these causes.

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