Improving the Diagnosis of Nutrient Impairments with Visual Assessments and Habitat Data

Nutrient impairments are more difficult to correctly assess than those associated with toxic substances. Virginia’s Academic Advisory Committee has shown that visual assessment bears promise for identifying nutrient impairments of wadeable streams. However, the regulated wastewater community has commented on the need to make visual assessments more objective and reproducible, and the need to include a mechanism for categorizing stream segments as nonimpaired. The Virginia Association of Municipal Wastewater Agencies performed an independent evaluation of Virginia’s stream database to address these needs. A contingency table method was used to determine whether individual visual assessment metrics were useful predictors of impairment or non-impairment. Classification and regression tree analysis was applied to determine if the accuracy of assessment could be improved by the combined use of multiple metrics. Results demonstrated that algal growth was secondary stressor to benthic macroinvertebrates, compared with siltation and other habitat impacts. Visual algal metrics were shown to be effective for screening sites as non-impaired by nutrients, especially if used in conjunction with habitat metrics. Similarly, the use of specific individual algal metrics (e.g., measures of filamentous algae and total algal coverage) was shown to hold promise for making visual assessments more objective and reproducible. In contrast, vascular plant coverage was of little use in correctly identifying nutrient impairments.