Arsenic transfer and biotransformation in a fully characterized freshwater food web

•We have shown for the first time how arsenic is distributed in >90% of the biomass of a freshwater food web.•Our data vividly show the biodiminution of arsenic between three levels of a freshwater food web.•We report a living organism, Motobdella montezuma, with an exceptionally high arsenic content of 2810mgkg−1.•Our data illustrate that the transformations of arsenic in the Montezuma Well ecosystem appear to be species specific. X-ray absorption near edge spectroscopy (XANES) was combined with ICP-MS to understand arsenic transfer and transformation within the freshwater Montezuma Well (central Arizona, USA) food web. Montezuma Well water contains 110μgL−1 arsenic (100% H3AsO4), which was shown previously to originate from ore deposits approximately 30km to the west, and transported underground to enter Montezuma Well through vents in the bottom of this collapsed travertine spring. The Montezuma Well food web contains three trophic levels with only five organisms in the top two levels, making it possible to account for the arsenic in >90% of the biomass of the food web. Arsenic diminution generally occurs between trophic levels, with 702mgkg−1 As in the primary producers, 3.4mgkg−1 As in the second trophic level, and