A quantitative food web approach for estimating selenium flux in the Colorado River in grand canyon

Selenium (Se) biomagnifies in aquatic food webs and is toxic to fishes and terrestrial piscivores.  Se concentrations exceed fish and wildlife toxicity thresholds throughout the Colorado River Basin, yet Se date are lacking for the Colorado River within Grand Canyon.  We sampled the Colorado River food web (organic matter, invertebrates, fishes) for Se at six sites spanning ~360 river kilometers.  Our objectives were to quantify the magnitude of Se contamination in this reach and to estimate Se fluxes within the food web.  Mean Se concentrations for organic matter, macroinvertebrates, and fishes were 1966, 3752, and 2049 ng g^-1 wet weight, respectively, well above established risk thresholds.  At one site (Lees Ferry), we estimated total Se flux of 2071 µg m²y^-1 through the food web. Most (2061 µg m²y^-1) of this Se flowed from organic matter to invertebrates.  Dominant pathways were from diatoms and amorphous detritus to Gammarus and New Zealand mudsnail.  Flux to rainbow trout (the dominant fish species) was 9.60 µg m²y^-1, with the majority coming from amorphous detritus and algae.  The combined use of contaminant data and quantitative food webs is a novel approach for identifying critical exposure pathways in ecosystems.