Biogeochemistry 40: 293–310, 1998. c 1998 Kluwer Academic Publishers. Printed in the Netherlands. System controls on the aqueous distribution of mercury in the northern Florida Everglades JAMES P. HURLEY 12 , DAVIDP. KRABBENHOFT 3 , LISA B. CLECKNER 2 , MARK L. OLSON 3 , GEORGE R. AIKEN 4 & PETER S. RAWLIK JR 5 1 Bureau of Integrated Science Services, Wisconsin Department of Natural Resources, 1350 Femrite Dr., Monona, WI 53176, U.S.A.; 2 Water Chemistry Program, University of Wisconsin, 660 North Park St., Madison, WI 53706, U.S.A.; 3 U.S. Geological Survey, 6417 Normandy Lane, Madison, WI 53719, U.S.A.; 4 U.S. Geological Survey, 3215 Marine St., Boulder, CO 80303, U.S.A.; 5 South Florida Water Management District, 3301 Gun Club Rd., West Palm Beach, FL 33406, U.S.A. ( corresponding author: Phone: 608-262-3979; Fax: 608-262-0454; Email: hurley@engr.wisc.edu) Key words: aqueous mercury species, bioaccumulation, methyl mercury, marshes, wetland systems Abstract. The forms and partitioning of aqueous mercury species in the canals and marshes of the Northern Florida Everglades exhibit strong spatial and temporal variability. In canals feeding Water Conservation Area (WCA) 2A, unfiltered total Hg (HgTU) is less than 3 ng L 1 and relatively constant. In contrast, methyl mercury (MeHg) exhibited a strong seasonal pattern, with highest levels entering WCA-2A marshes during July. Stagnation and reduced flows also lead to particle enrichment of MeHg. In the marshes of WCA-2A, 2B and 3A, HgTU is usually 5 ng L 1 with no consistent north–south patterns. However, for individual dates, aqueous unfiltered MeHg (MeHgU) levels increase from north to south with generally lowest levels in the eutrophied regions of northern WCA-2A. A strong relationship between filtered Hg species and dissolved organic carbon (DOC), evident for rivers draining wetlands in Wisconsin, was not apparent in the Everglades, suggesting either differences in the binding sites of DOC between the two regions, or non-organic Hg complexation in the Everglades. Introduction Mercury cycling in wetland systems has recently been the focus of several studies due to observations that production of bioaccumulative monomethyl mercury (MeHg) appears to be favored in wetlands relative to other land use/land cover patterns (St. Louis et al. 1994, 1996; Hurley et al. 1995; Krabbenhoft et al. 1995; Rudd 1995; Branfireun et al. 1996). The processes which lead to the formation of MeHg in wetlands are not fully understood (Zillioux et al. 1993), but similar formation processes, observed in lacustrine and oceanic systems (Winfrey & Rudd 1990; Gilmour et al. 1991), may be influencing MeHg formation in wetland systems.