Mercury trends in herring gull (Larus argentatus) eggs from Atlantic Canada, 1972e2008: Temporal change or dietary shift? Neil M. Burgess a, * , Alexander L. Bond b , Craig E. Hebert c , Ewa Neugebauer c , Louise Champoux d a Environment Canada, 6 Bruce Street, Mount Pearl, Newfoundland and Labrador A1N 4T3, Canada b Department of Biology, University of Saskatchewan and Environment Canada,11 Innovation Boulevard, Saskatoon, Saskatchewan S7N 3H5, Canada c Environment Canada, National Wildlife Research Centre, Carleton University, Raven Road, Ottawa, Ontario K1A 0H3, Canada d Environment Canada, 801-1550 avenue d’Estimauville, Québec City, Québec G1J 0C3, Canada article info Article history: Received 6 January 2012 Received in revised form 17 August 2012 Accepted 8 September 2012 Keywords: Herring gull Egg Mercury Stable isotope Atlantic abstract Mercury (Hg) is a pervasive contaminant that can adversely affect predatory wildlife. Bird eggs provide insights into breeding females’ Hg burdens, and are easily collected and archived. We present data on Hg trends in herring gull (Larus argentatus) eggs from five sites in Atlantic Canada from 1972 to 2008. We found a significant decrease in Hg at Manawagonish Island, New Brunswick and Île du Corossol, Quebec, but after correcting Hg for dietary shifts using stable isotopes (d 15 N), these trends disappeared. Decreasing temporal trends of stable isotopes in gull eggs were observed at four sites, suggesting shifts in gull diets. At Gull Island, Newfoundland, diet-adjusted Hg increased from 1977 to 1992, dropped sharply between 1992 and 1996, and rose again from 1996 to 2008. After adjusting Hg trends for dietary shifts of herring gulls, it appears that environmental Hg in coastal ecosystems has remained relatively constant at most sites in Atlantic Canada over the last 36 years. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. 1. Introduction Mercury (Hg) is an atmospherically transported contaminant, much of which is anthropogenically generated, and deposition is increasing worldwide (Nriagu, 1989; Nriagu and Pacyna, 1988; Streets et al., 2009). As top predators in the marine environment, seabirds are used frequently as sentinel species for monitoring ecosystem contamination (Burger and Gochfeld, 2004; Goodale et al., 2008; Hebert et al., 1999; Mineau et al., 1984; Pearce et al., 1979). Their high trophic position also means that seabirds can be exposed to high concentrations of biomagnified contaminants, including Hg (Braune et al., 2005). The biologically active and toxic form, methylmercury (MeHg), is the predominant type found in seabird feathers and eggs (Bond and Diamond, 2009; Thompson and Furness, 1989). Seabirds acquire MeHg through ingestion of contaminated prey. MeHg is then deposited in body tissues, demethylated in the liver or brain, or depurated into feathers or eggs (Braune and Gaskin, 1987; Monteiro and Furness, 2001; Spalding et al., 2000). In some marine ecosystems, Hg concentrations in seabirds have increased over time (e.g., Braune, 2007; Thompson et al., 1992). This increase could be due to two processes: first, the birds consumed a relatively constant diet over time and MeHg levels increased in that diet, or second, the birds shifted their diet over time to prey with higher Hg concentrations (see Hebert et al., 2009). The first possibility of changing Hg levels in prey could result from envi- ronmental changes, such as increased atmospheric deposition of Hg, increased bioavailability of MeHg, climatic or oceanographic changes (e.g., Aebischer et al., 1990; Drinkwater, 1996), or changes in food web structure (e.g., Carscadden et al., 2001; Montevecchi and Myers, 1996). Stable isotope analysis can be used to address the second possibility (a shift in diet) because isotope values ( 13 C/ 12 C, or d 13 C, and 15 N/ 14 N, or d 15 N) will reflect the consumer’s diet at the time of tissue synthesis (Hobson, 1995; Hobson and Clark, 1992). d 13 C can give an indication of the geographic foraging area, as inshore and more productive oceanic areas (including terrestrial sources, such as landfill sites) are enriched in 13 C(France, 1995; Goericke and Fry, 1994; Peterson and Fry, 1987; Popp et al., 1998), while d 15 N values increase 2e5& with each trophic level because 14 N is excreted preferentially in nitrogenous waste (Kelly, 2000; Minagawa and Wada, 1984; Steele and Daniel, 1978). However, analysing changes in carbon isotope values over time is potentially confounded by the Suess (1955) Effect, as the combustion of fossil fuels (naturally depleted in 13 C) have altered the baseline d 13 C value globally, including in the North Atlantic (Quay et al., 2007), but this can be taken into account * Corresponding author. E-mail address: neil.burgess@ec.gc.ca (N.M. Burgess). Contents lists available at SciVerse ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol 0269-7491/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envpol.2012.09.001 Environmental Pollution 172 (2013) 216e222