Relationships between the stream benthic macroinvertebrate assemblages and their stressors: Implications for benthic TMDLs

Relationships between benthic macroinvertebrates and their stressors were established so that stressor levels that would not adversely impact the benthic macroinvertebrates could be determined. Existing data on sediment, habitat, water quality. landuse, watershed characteristics, livestock numbers, and benthic macroinvertebrate assemblage conditions were compiled for 34 stations with 105 samples collected from the fall of 1996 through the fall of 1998 for watersheds dominated by agricultural, urban, and forested landuses. Since sediment is recognized as the Nation's leading pollutant and since the benthic macroinvertebrates live in the sediment on a stream bottom, this study placed emphasis upon the investigation of sediment as a primary stressor to the benthic macroinvertebrates. The stressor/benthos relationships were evaluated using statistical analyses procedures such as forward, backward, and stepwise multiple regression. The statistical results indicated that stressors such as dissolved oxygen, flow, % urban land, total suspended solids, temperature, stream velocity, substrate, hardness and alkalinity greatly impact the benthic macroinvertebrate assemblages. The study results also indicate that the individual metrics within the RBP procedure are just as critical as the final RBP values in describing the benthic macroinvertebrate assemblages. The implications of the relationships with regard to TMDLs were also studied using total suspended solids (TSS) loadings, turbidity levels, and embeddedness levels as the stressors of concern. The developed stressor/benthos relationships may be beneficial in developing benthic TMDLs.