Decline of acid rain enhances mercury concentrations in fish.

M efforts have been made to reduce sulfur emissions in Europe. Acid rain pollution in Norway has diminished substantially, thanks to the 1979 Geneva Convention on Longrange Transboundary Air Pollution, which was implemented starting in 1983. Parallel with the reduction in acid rain, atmospheric concentrations and deposition of mercury (Hg) have also been reduced. Here, we show that reduced acid rain may have increased dissolved organic carbon in runoff, and enhanced transport of mercury from catchments, and increased in-lake Hg methylation. Together, these enhanced processes result in higher concentrations of methymercury in fish. Mercury pollution in Norway mainly originates from distant sources in other European countries. Similar to acid rain, the long-range atmospheric transport of Hg has declined significantly during the past decades. It was therefore unexpected when a recent nationwide survey showed strong increased Hg concentrations in brown trout (Salmo trutta) and European perch (Perca f luviatilis) in a representative set of lakes in southern Norway. We selected two of these lakes for further case studies based on the availability of historic lake chemistry data. The selection includes the lake with the highest Hgincrease in perch as well as one with a slight decline. The greatest health threat to human consumers comes from methyl-mercury (MeHg), a potent neurotoxin which is easily incorporated into living tissues. The methylation process is a key to the understanding of the potential danger posed by environmental Hg. Environmental Hg is methylated predominantly by naturally occurring bacteria known as sulfatereducing bacteria. The process occurs in saturated soils, anoxic lake waters, and in anoxic sediments, although the factors that control the methylation process are not fully understood. In October 2010 we sampled and analyzed modern sediments from two lakes in Norway (Figure 1).