An Investigation of the Relationships Between Stream Benthic Macroinvertebrate Assemblage Conditions and Their Stressors

Agriculture, urbanization, and human activities, if not managed carefully, can expose a water body to environmental degradation, decreased water quality, and ultimately impaired benthic macroinvertebrate assemblage conditions. In streams where the benthic macroinvertebrates are impaired, the stream itself will not be meeting the water quality standards set forth in the Clean Water Act. As a result, the goal of this study was to establish relationships between benthic macroinvertebrates and their stressors so that stressor levels that would not adversely impact the benthic macroinvertebrates could be determined. Stressors such as sediment, habitat, water quality, landuse, watershed characteristics, and livestock numbers impact the benthic macroinvertebrate assemblage conditions. 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 specific objectives of this study were to develop relationships between the benthic macroinvertebrates and sediment and other stressors for Virginia streams, to evaluate the accuracy of the stressor/benthos relationships, and to discuss the implications of the study results for development of benthic TMDLs. A procedure to determine the relationships between stressors and benthic macroinvertebrate assemblage conditions was developed. 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 to the fall of 1998. The 34 stations were located within 13 counties in Virginia (Rockbridge, Rockingham, Augusta, Frederick, Shenandoah, Page, Loudoun, Fairfax, Prince William, Fauquier, Culpeper, Rappahannock, and Madison) and in watersheds dominated by agricultural, urban, and forested landuses. Virginia currently uses the Rapid Bioassessment Protocol (RBP) method in its Biological Assessment Program. The RBP compares habitat and biological measures of the benthic macroinvertebrates to reference conditions using individual metrics. VADEQ's Biomonitoring Database, together with Ambient Water Quality Monitoring reports, GIS data layers, and VADCR's Hydrologic Unit Animal Census Database provided all of the necessary information for the stressor variables and benthic macroinvertebrate conditions. Accordingly, the stressor/benthos relationships were evaluated using statistical analyses procedures such as forward, backward, and stepwise multiple regression techniques; correlation analysis; principal component analysis; and r-square analysis. The statistical results indicated that sediment alone cannot be used to assess the benthic macroinvertebrate assemblage conditions. Other stressors such as dissolved oxygen, flow, % urban land, total suspended solids, temperature, stream velocity, substrate, hardness and alkalinity greatly impact the benthic macroinvertebrate assemblage conditions. 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 assemblage conditions. Upon completing the steps needed to develop stressor/benthos relationships, the validity of the relationships were verified for their application to other streams in Virginia. Validation was completed using 10 stations with 29 samples from the fall of 1996 to the fall of 1998. The 10 stations were located within 8 counties in Virginia (Bedford, Montgomery, Pulaski, Giles, Botetourt, Albemarle, Orange, and Culpeper) and in watersheds dominated by agricultural, urban, and forested landuses to correspond with the stations used to develop the stressor/benthos relationships. 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 results for all benthic stations within Virginia showed that moderately impaired streams generally need to reduce the amount of embeddedness by 11, reduce turbidity levels by 5 FTU (57%), and reduce TSS values by 7 mg/L (68%) to meet a threshold value that would no longer adversely impact the benthic macroinvertebrates. Similarly, for the severely impaired stations throughout Virginia to meet threshold values, embeddedness amounts need to be reduced by 22, turbidity reduced by 57 FTU (93%), and TSS reduced by 74 mg/L (96%). This study was important since the proposed stressor/benthos relationships can provide policymakers with a useful tool to determine stressor thresholds that will not adversely impact the benthic macroinvertebrate assemblage conditions for use in developing benthic TMDLs in Virginia. The stressor/benthos relationships could also be used to determine the impact of certain activities or stressors on the benthic macroinvertebrates assemblage conditions in a given stream.