Benthic macroinvertebrate community structure and metal concentration along the Forgotten River stretch of the Rio Grande

The purpose of this study is to evaluate heavy metal concentration in the water and sediments, as well as macroinvertebrate communities along this undisturbed stretch to assess the current status of the river ecosystem and determine if a pollution gradient is developed with increased distance from El Paso/Juarez area. Mean Conductivity (4282uS) and salinity (2.34ppt) were higher at site 3, which is just downstream from El Paso/Juarez. Levels of metals in the sediments were variable at different sampling times. Concentrations of iron and zinc were highest among the metals present reaching concentrations of 25,000 mg/kg and 22 mg/kg respectively. Arsenic was not detected during the course of this study. Despite the great deal of variation, taxa richness and percent tolerant families were observed to increase in the downstream sites. In addition a more balanced trophic structure was observed at these sites as well. Overall patterns in the distribution of the invertebrates seemed to follow the irrigation season. Total number of chironomids were positively correlated with salinity and conductivity, and negatively correlated with dissolved oxygen. In addition, Family Biotic Index values were found to be negatively correlated with dissolved oxygen and positively correlated with temperature. Concentrations in the sediments of both Cr and Cd were found to be negatively correlated to total number of invertebrates collected at each site. Further studies are needed to determine other potential stressors that are acting on the system. (Abstract shortened by UMI.