Introduction A large body of evidence indicates that dissolved con- centrations of many potentially toxic trace metals ex- hibit substantial diel (24-h) cycles during low-flow conditions in streams with neutral to alkaline pH (Fuller and Davis 1989; Bourg and Bertin 1996; Brick and Moore 1996; Nimick and others 1998; Scott and others 2002; Nimick and others 2003; Jones and others 2004). For example, concentrations of dissolved Cd, Mn, Ni, and Zn in northern Rocky Mountain streams increased as much as 119–500% from minimum values during mid to late afternoon to maximum values shortly after sunrise the next morning (Nimick and others 2003). Dissolved As concentrations exhibited the opposite timing, but with smaller increases of as much as 54% (Fuller and Davis 1989; Nimick and others 1998, 2003). These diel cycles of dissolved metal con- centrations are widespread and persistent, occur over a wide range of metal concentrations, and are found in small streams as well as larger rivers (Nimick and others 1998, 2003). Diel metal cycles have an important bearing on the understanding of the mobility of trace metals in watersheds and on the spatial and temporal complexity of biogeochemical processes in stream ecosystems. Scientists have only recently begun to document and appreciate the implications of short-term variations of stream chemistry and to reevaluate David A. Nimick Thomas E. Cleasby R. Blaine McCleskey Seasonality of diel cycles of dissolved trace-metal concentrations in a Rocky Mountain stream Received: 15 July 2004 Accepted: 13 September 2004 Published online: 13 January 2005 Ó Springer-Verlag 2005 Abstract Substantial diel (24-h) cycles in dissolved (0.1-lm filtration) metal concentrations were observed during summer low flow, winter low flow, and snowmelt runoff in Prickly Pear Creek, Montana. During seven diel sampling episodes lasting 34–61.5 h, dissolved Mn and Zn concentrations increased from afternoon minimum values to maxi- mum values shortly after sunrise. Dissolved As concentrations exhib- ited the inverse timing. The magni- tude of diel concentration increases varied in the range 17–152% for Mn and 70–500% for Zn. Diel increases of As concentrations (17–55%) were less variable. The timing of mini- mum and maximum values of diel streamflow cycles was inconsistent among sampling episodes and had little relation to the timing of metal concentration cycles, suggesting that geochemical rather than hydrologi- cal processes are the primary control of diel metal cycles. Diel cycles of dissolved metal concentrations should be assumed to occur at any time of year in any stream with dis- solved metals and neutral to alkaline pH. Keywords Diel, diurnal Æ Metal, geochemistry, adsorption Æ Arsenic, manganese, zinc Æ Prickly Pear Creek, Montana, USA Environ Geol (2005) 47: 603–614 DOI 10.1007/s00254-004-1178-x ORIGINAL ARTICLE Electronic Supplementary Material Supple- mentary material is available for this article if you access the article at http://dx.doi.org/ 10.1007/s00254-004-1178-x. A link in the frame on the left on that page takes you directly to the supplementary material. D. A. Nimick (&) Æ T. E. Cleasby U.S. Geological Survey, 3162 Bozeman Avenue, Helena, MT 59601, USA E-mail: dnimick@usgs.gov Tel.: +1-406-4575918 Fax: +1-406-4575990 R. B. McCleskey U.S. Geological Survey, Boulder, CO, USA