Ecological Applications, 18(8) Supplement, 2008, pp. A227–A238 Ó 2008 by the Ecological Society of America MERCURY RESIDUES AND PRODUCTIVITY IN OSPREY AND GREBES FROM A MINE-DOMINATED ECOSYSTEM DANIEL W. ANDERSON, 1,3 THOMAS H. SUCHANEK, 1,4 COLLIN A. EAGLES-SMITH, 1,5 AND THOMAS M. CAHILL,JR. 1,2 1 Department of Wildlife, Fish, and Conservation Biology, University of California, Davis, California 95616 USA 2 Department of Chemistry, University of California, Davis, California 95616 USA Abstract. Mercury (Hg) and reproduction and status of Western and Clark’s Grebes (Aechmophorus sp.) and Osprey (Pandion haliaetus) were studied from 1992 through 2001 and then less intensely through 2006 at Clear Lake, California, USA. Remediation to reduce Hg loading from the Sulphur Bank Mercury Mine was initiated in 1992. Mercury in grebe feathers declined monotonically from ;23 mg/kg dry mass (DM) in 1967–1969 to 1 mg/kg in 2003, but then increased to 7 mg/kg in 2004–2006. Mercury in Osprey feathers varied similarly, with mean values of 20 mg/kg DM in 1992, declining to a low of 2 mg/kg in 1998, but increasing to 23 mg/kg in 2003, and 12 mg/kg in 2006. Mercury in Osprey feathers at our reference site (Eagle Lake, California) remained low (1–8 ppm) throughout the entire period, 1992–2003. Grebe productivity at Clear Lake improved from ;0.1 to 0.5 fledged young per adult during the latter part of the study when human disturbance was prevented. At that period in time, improved productivity did not differ from our reference site at Eagle Lake. Human disturbance, however, as a co-factor made it impossible to evaluate statistically subtle Hg effects on grebe productivity at Clear Lake. Osprey reproduced sufficiently to maintain increasing breeding numbers from 1992 to 2006. Mercury in Clear Lake water, sediments, invertebrates, and fish did not decline from 1992 to 2003, but a shift in trophic structure induced by an introduced planktivorous fish species may have caused significant alterations in Hg concentrations in several species of prey fishes that may have produced concomitant changes in Osprey and grebe Hg exposure. The temporary declines observed in grebe and Osprey feather residues in the late 1990s, with coincidental improvements in reproductive performance, however, could not be attributed to remediation at the mine site. Key words: Aechmophorus sp.; Clark’s Grebe; Clear Lake, California, USA; mercury; mine site remediation; Osprey; Pandion haliaetus; populations; productivity; residues; Sulphur Bank Mercury Mine; Western Grebe. INTRODUCTION This study is one component of an ecosystem-level project that attempted to trace the origin and pathways of mercury (Hg) from the ore body at an abandoned Hg mine through the abiotic (sediment and water) com- partments to lower trophic level species (benthic invertebrates and plankton) and ultimately to higher trophic level species (e.g., fish and birds). We utilize those and other data to evaluate the fate and potential effects of mine-derived Hg on two avian groups: Western and Clark’s Grebes (Aechmophorus sp.; hereaf- ter referred to as ‘‘grebes’’) and Osprey (Pandion haliaetus). Our objectives were: (1) to evaluate potential changes in Hg exposure and reproductive rates in these two high-trophic-level avian piscivores from 1992 to 2001 (with less intense monitoring in subsequent years through 2006) and (2) to evaluate whether changes observed in these avian piscivores might relate to reported changes in other trophic levels and to remediation activities initiated at the mine in 1992. Mercury affects various reproductive parameters in birds (e.g., Heinz 1979, Eisler 1987, Wren et al. 1995, Thompson 1996, Poole et al. 2002, Heinz et al. 2006). In field studies, Furness et al. (1989) and Newton and Galbraith (1991) related trends of decreasing reproduc- tive success to elevated Hg levels in feathers of adult Golden Eagles (Aquila chrysaetos), but also urged caution due to other confounding variables. Mercury residues from ingested fish are transferred to bird feathers as a depuration mechanism, and most feather residues are in the organic form, methylmercury (MeHg) (Thompson and Furness 1989a). The proximate cause of elevated Hg residues in biota in Clear Lake, California, USA, has been mostly point source erosion or acid mine drainage input from the Sulphur Bank Mercury Mine Superfund Site (Suchanek et al. 1998, 2000a, b, 2008b, d; Fig. 1). In 1992, the U.S. Manuscript received 2 November 2006; revised 9 July 2007; accepted 11 July 2007. Corresponding Editor (ad hoc): A. Fairbrother. For reprints of this Special Issue, see footnote 1, p. A1. 3 E-mail: dwanderson@ucdavis.edu 4 Present address: U.S. Geological Survey, Western Eco- logical Research Center, 3020 State University Drive East, Sacramento, California 95819 USA. 5 Present address: U.S. Geological Survey, Western Eco- logical Research Center, Davis Field Station, One Shields Avenue, Davis, California 95616 USA. A227