BACTERIAL-GROWTH ON STREAM INSECTS - POTENTIAL FOR USE IN BIOASSESSMENT

Growth of filamentous bacteria (Sphaerotilus sp., Leptothrix sp.) on a quatic insects was evaluated for its usefulness as a bioindicator of d etrimental nutrient levels in streams. Field measurements of insect ab undance, nutrient concentrations, and incidence/degree of bacterial gr owth on insects upstream and downstream of livestock pastures were mad e in 2 Virginia, USA streams. Laboratory studies were conducted to det ermine the effect of bacterial growth on insect survival. Elevated con centrations of dissolved nutrients (0.13-0.35 mg/L orthophosphate, 1.2 9-2.13 mg/L nitrate) downstream of pastures were associated with growt h 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% mort ality of heavily infested mayflies (>25% of body covered, including gi lls) occurred within 30 d, whereas >85% of individuals without bacteri al growth survived and grew normally. The pattern of mortality in the laboratory closely paralleled the differences in density observed in t he field. Bacterial growth on aquatic insects appears to be a reliable bioindicator of nutrient enrichment, and the degree of infestation as sociated with reduced insect survival can be quickly detected in the f ield or laboratory using a hand lens (10-15x magnification). This bioi ndicator shows promise as a significant addition to EPA Rapid Bioasses sment Protocols because simple visual assessment of benthic samples ma y be sufficient to identify a cause for impaired macroinvertebrate com munities. Bacterial growth should be useful for detecting nutrient imp acts in streams as well as evaluating the success of management practi ces to control nutrients from point or non-point sources.