Impact of Metals on Macroinvertebrate Assemblages in the Forgotten Stretch of the Rio Grande Catalina Ordonez • Vanessa L. Lougheed • Jorge L. Gardea-Torresdey • Lisa J. Bain Received: 23 March 2010 / Accepted: 31 May 2010 / Published online: 20 June 2010 Ó Springer Science+Business Media, LLC 2010 Abstract The objective of this study was to examine how changes in the benthic macroinvertebrate community structure and a variety of abiotic variables, such as con- ductivity and sediment metal concentrations, are modified along the Forgotten River stretch of the Rio Grande. This stretch receives industrial effluent, raw sewage, and agri- cultural return flow from the El Paso (TX, USA)–Ciudad Jua ´rez (CHI, Mexico) metroplex and then flows relatively undisturbed for 320 km before its next significant input. The high degree of use, followed by the 320-km undis- turbed stretch, makes the Forgotten River a unique study site to examine downstream attenuation of contaminants and other abiotic variables to determine their potential effects on macroinvertebrates. Five different sites along the Forgotten Stretch were sampled over a 2-year period. Metal concentrations were low throughout the stretch and were predominantly correlated to percent sediment organic matter rather than explained spatially. Several sensitive invertebrate species, such as Leptophlebiidae, increased in relative abundance downstream, whereas the percentage of tolerant invertebrates decreased. Nonmetric multidimen- sional scaling separated the macroinvertebrate communi- ties upstream from those downstream, with the more sensitive species being found predominantly downstream and more tolerant taxa associated upstream. Additionally, there was a distinct seasonal gradient to the community. The most important drivers of the community assemblage appear to be distance downstream and seasonality, as well as water conductivity and concentrations of sediment cadmium, which was the only metal that exceeded pro- tective criteria. This study did not provide evidence of the downstream attenuation of heavy metals in the sediments in the Forgotten Stretch; however, downstream changes in macroinvertebrates toward more sensitive taxa suggests that other, unmeasured contaminants might be affecting biological communities in this isolated stretch of an international waterway. The Rio Grande forms the United States–Mexico interna- tional border from the El Paso (TX, USA)–Ciudad Jua ´rez (CHI, Mexico) metroplex to the Gulf of Mexico. It is heavily used as a source of irrigation and drinking water, as well as a receiving body for industrial effluent, raw sewage, agricultural return flow, and treated wastewater from a combined binational population of *2 million (EPA 2004). After leaving the metropolitan area, the Rio Grande then flows relatively undisturbed for *320 km. This stretch, termed the ‘‘Forgotten River,’’ has no significant towns and no perennial streams for 320 km, until its con- fluence with the Rio Conchos at Presidio (TX, USA)– Ojinaga (CHI, Mexico). The majority of the lands bordering this stretch are private ranches and rangeland (Brock et al. 2001) and the only inflows are springs. Because of heavy water usage patterns in the Rio Grande, the United States and Mexico issued the Integrated Environmental Plan for the Mexican-U.S. Border Area in 1992 (EPA 1997b). The monitoring was ultimately C. Ordonez Á V. L. Lougheed Department of Biological Sciences, University of Texas at El Paso, El Paso, TX, USA J. L. Gardea-Torresdey Department of Chemistry, University of Texas at El Paso, El Paso, TX, USA L. J. Bain (&) Department of Biological Sciences, Clemson University, Clemson, SC, USA e-mail: lbain@clemson.edu 123 Arch Environ Contam Toxicol (2011) 60:426–436 DOI 10.1007/s00244-010-9557-7