Abstract
Widespread declines in mercury (Hg) in fish in pristine lakes in Fennoscandia since the 1970s are unexplained. Interactions between climate, atmospheric deposition, and elemental cycling of carbon (C), sulphur (S) and Hg are complex and affect Hg bioaccumulation. A parallel significant decline in methyl-Hg (MeHg) concentrations in aquatic macroinvertebrates (Chironomidae) was found between 1976–78 and 2004–15 in an intensely studied, pristine boreal lake (Langtjern, boreal Fennoscandia). Monitoring at Langtjern demonstrated a four-fold decrease in aqueous sulphate concentrations (SO4, 50-year record), significant lake browning (30-year records), increasing sediment Hg concentrations (50-year record), warming (45-year record) and increased runoff (40-year record). Contrasting Hg trends in biota (downward) and sediment (upward) indicated a disconnect between lake Hg loading and foodweb Hg bioaccumulation. We suggest that reduced SO4-deposition has 1) constrained substrate availability for SO4-reducing methylating bacteria (causing reduced foodweb MeHg exposure despite increased Hg loading to the lake), and 2), increased the binding affinity between aqueous organic matter and Hg species (leading to reduced MeHg bioavailability). The downward MeHg trend at the base of the foodweb at Langtjern is mirrored at higher trophic levels by strong declines in perch (Perca fluviatilis) and pike (Esox lucius) Hg concentrations in boreal Fennoscandia. A plausible explanation is that declining SO4-deposition, rather than climate change or reduced atmospheric Hg, is currently driving reduced MeHg contamination in northern freshwater foodwebs.
Highlights
International concerns about the toxic threat of mercury (Hg) have led to The Minamata Convention on Mercury, which aims to protect human health and the environment from adverse effects of Hg at a global scale (UNEP, 2014)
Aquatic macroinvertebrates have long been sampled from many European watercourses, and archived samples can potentially offer substantial information on the historical trends in bioaccumulation of Hg in lower trophic levels of the foodweb
Because of the strong correlation between Hg in top predators and Hg at lower trophic levels (Chasar et al, 2009), we argue that exploration of factors explaining MeHg at lower trophic levels from an intensively studied site that represents wide-spread environmental change, including lake browning, wetting and recovery from acidification, can be used to shed light on poorly-explained regional patterns in fish Hg from lakes with limited site-specific information
Summary
International concerns about the toxic threat of mercury (Hg) have led to The Minamata Convention on Mercury (hereafter The Minamata Convention), which aims to protect human health and the environment from adverse effects of Hg at a global scale (UNEP, 2014). Fulfilment of the Convention necessitates adequate methods for assessing time trends of Hg concentrations in biota. Concentrations of Hg in freshwater fish still often exceed limits for protection of human health (0.5 ppm) (FAO/WHO, 1995) and ecosystems (0.02 ppm) (WFD, 2014), recent studies show declines over the last half century, an indication of improved environmental status of some freshwater aquatic ecosystems (Åkerblom et al, 2014; Braaten et al, 2019). Archived biological material constitutes a potential data source for investigations of long-term change in environmental pollutants (Green and Scharlemann, 2003), including Hg (Poulopoulos, 2013), but there are few examples of such studies. Aquatic macroinvertebrates have long been sampled from many European watercourses, and archived samples can potentially offer substantial information on the historical trends in bioaccumulation of Hg in lower trophic levels of the foodweb
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