Applicability of Severity-of-Ill-Effects (SEV) index for standards development in agriculturally-dominated watersheds

Agricultural activities are considered one of the most important sources of increased sediment loading to streams in North America. The Severity-of-Ill-Effects (SEV) index predicts biological effects on fish based on the magnitude and duration of exposure to total suspended sediments (TSS), and was generated largely from metadata produced in short-term laboratory experiments. We assessed the applicability of deriving a biotic standard for fish using the SEV index with TSS data from the outlet of Black Brook, New Brunswick; data that had been collected over the period 1992 to 2005. We used five options for SEV score calculation, with each approach having basic differences in how TSS events are defined. In comparison to other SEV calculation methods, the Sampling Frequency Method tended to produce the lowest SEV scores and always ranged in the sub-lethal range of effects on fish. Approximately 30% of the SEV values were ≥7, the level at which fish habitat degradation begins, while a mean of almost 3% of annual SEV values were in the lethal affects range. We conclude that the Sampling Frequency Method would be the most conservative approach for standards development and discuss the applicability of SEV to water quality monitoring data.