Bacterial growth on stream insects: potential for use in bioassessment

Growth of filamentous bacteria (Sphaerotilus sp., Leptothrix sp.) on aquatic insects was evaluated for its usefulness as a bioindicator of detrimental nutrient levels in streams. Field measurements of insect abundance, nutrient concentrations, and incidence/degree of bacterial growth on insects upstream and downstream of livestock pastures were made in 2 Virginia, USA streams. Laboratory studies were conducted to determine the effect of bacterial growth on insect survival. Elevated concentrations of dissolved nutrients (0.13-0.35 mg/L orthophosphate, 1.29-2.13 mg/L nitrate) downstream of pastures were associated with growth of filamentous bacteria, which colonized the gills and body surface of aquatic insects. Significantly lower densities of insects (up to 66% less) occurred at downstream sites. In laboratory studies, 100% mortality of heavily infested mayflies (>25% of body covered, including gills) occurred within 30 d, whereas >85% of individuals without bacterial growth survived and grew normally. The pattern of mortality in the laboratory closely paralleled the differences in density observed in the field. Bacterial growth on aquatic insects appears to be a reliable bioindicator of nutrient enrichment, and the degree of infestation associated with reduced insect survival can be quickly detected in the field or laboratory using a hand lens (10-15x magnification). This bioindicator shows promise as a significant addition to EPA Rapid Bioassessment Protocols because simple visual assessment of benthic samples may be sufficient to identify a cause for impaired macroinvertebrate communities. Bacterial growth should be useful for detecting nutrient impacts in streams as well as evaluating the success of management practices to control nutrients from point or non-point sources.