Temporal change estimation of mercury concentrations in northern pike (Esox lucius L.) in Swedish lakes Staffan Åkerblom a,b,⇑ , Mats Nilsson b , Jun Yu c , Bo Ranneby c , Kjell Johansson a a Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden b Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden c Centre of Biostochastics, Swedish University of Agricultural Sciences, Umeå, Sweden article info Article history: Received 15 April 2011 Received in revised form 6 September 2011 Accepted 13 September 2011 Available online 19 October 2011 Keywords: Mercury Freshwater fish Temporal change Multiple linear regression abstract Adequate temporal trend analysis of mercury (Hg) in freshwater ecosystems is critical to evaluate if actions from the human society have affected Hg concentrations ([Hg]) in fresh water biota. This study examined temporal change in [Hg] in Northern pike (Esox lucius L.) in Swedish freshwater lakes between 1994 and 2006. To achieve this were lake-specific, multiple-linear-regression models used to estimate pike [Hg], including indicator variables representing time and fish weight and their interactions. This approach permitted estimation of the direction and magnitude of temporal changes in 25 lakes selected from the Swedish national database on Hg in freshwater biota. A significant increase was found in 36% of the studied lakes with an average increase in pike [Hg] of 3.7 ± 6.7% per year that was found to be pos- itively correlated with total organic carbon. For lakes with a significant temporal change the dataset was based on a mean of 30 fish, while for lakes with no temporal change it was based on a mean of 13 fish. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Mercury (Hg) exposure from consumption of freshwater pisciv- orous fish is considered to be an increasing health problem, as this constitutes the primary route of methyl-Hg exposure in humans and wildlife (Mergler et al., 2007). Environmental and health advi- sory limits (0.2–0.5 mg kg 1 )(UNEP, 2002) are exceeded in thou- sands or tens of thousands of lakes of many countries and regions, e.g. Scandinavia (Håkansson et al., 1988; Lindqvist et al., 1991; Johansson et al., 2001; Munthe et al., 2007), Canada (Evans et al., 2005b) and the USA (Bahnick et al., 1994; Hinck et al., 2009; Scudder et al., 2009). Using data from a Swedish environ- mental monitoring program, we examined temporal changes in [Hg] in freshwater northern pike (Esox lucius L.) from the late 1990s into the early 21st century. 1.1. Temporal variation in the past Temporal variation from the 1970s onwards in fish [Hg] from inland freshwaters have been examined in local (Hrabik and Watras, 2002; Suchanek et al., 2008; Gantner et al., 2009; Azim et al., 2011), regional (Kamman et al., 2005; Madsen and Stern, 2007; Levinton and Pochron, 2008; Hinck et al., 2009; Monson, 2009; Bhavsar et al., 2010) and national datasets from Scandinavia (Paasivirta and Linko, 1980; Håkansson et al., 1988; Johansson et al., 2001) and North America (Schmitt and Brumbaugh, 1990; Johnston et al., 2003; Muir et al., 2005; Chalmers et al., 2011). At the end of the 1970s, no significant temporal change in fish [Hg] was detected in Finnish (Paasivirta and Linko, 1980) and US (Schmitt and Brumbaugh, 1990) national monitoring programmes. However, data obtained from Swedish monitoring programmes for Northern pike revealed increasing [Hg] from 1967 to 1985 (Håkansson et al., 1988). The increase found in fish [Hg] was fol- lowed by an average decrease of about 20% from the early 1980s to the late 1990s (Johansson et al., 2001). A similar decreasing change as that found in Swedish lakes was also reported in other studies of long-term fish [Hg] changes in the USA (Hrabik and Watras, 2002; Madsen and Stern, 2007; Levinton and Pochron, 2008) and Canada (Johnston et al., 2003; Hinck et al., 2009) during the same period. These decreasing changes have mainly been as- cribed to reductions in anthropogenic Hg emissions (Johansson et al., 2001; French et al., 2006; Rasmussen et al., 2007; Monson, 2009). However, temporal changes in more recent data (2000– 2010) indicate a reversal, with increasing [Hg] reported in species in certain cases of North America (Muir et al., 2005; French et al., 2006; Monson, 2009), although decreases have also been noted be- tween 1992–1993 and 2005–2006 in lakes within the Adirondack region (Dittman and Driscoll, 2009). A bayesian modelling frame- work identified an increasing trend of [Hg] in lake Erie fish becom- ing particularly evident after the mid-1990s (Azim et al., 2011). 0045-6535/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2011.09.037 ⇑ Corresponding author at: Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden. Tel.: +46 18 67 30 42; fax: +46 18 67 31 56. E-mail address: staffan.akerblom@slu.se (S. Åkerblom). Chemosphere 86 (2012) 439–445 Contents lists available at SciVerse ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere