Montezuma Well is an unusual fishless desert wetland in central Arizona. Water in the Well is naturally enriched with >100 g/L dissolved geogenic arsenic (As) and supports a relatively simple aquatic desert food web with several endemic invertebrate species. Previous studies of As among various environmental compartments and organisms in Montezuma Well have not included turtles despite their potential importance in the As cycle by virtue of their substantial biomass and role as top predators. We measured As concentrations in water, sediment, and organisms (macrophytes, amphipods, insects, leeches, and turtles) representing a range of food web trophic levels, in order to document the importance of turtles in the As cycle at the apex of the Montezuma Well food web.