Long-range transboundary pollution and local inputs result in the deposition of heavy metals and persistent organic pollutants into lakes. High levels of mercury have been found in fish. Therefore, the Norwegian Food Safety Authority has issued general advice about the consumption of fresh water fish throughout the country.
Heavy metals and persistent organic pollutants in lakes
Norwegian lakes have elevated levels of lead, cadmium and mercury. Of these, mercury is the dominant metal found in fish.
Many lakes have been examined for well-known persistent organic pollutants such as PCBs, DDT and dioxins. In recent years the levels of brominated flame retardants, PFAS, chlorinated paraffins and Bisphenol A have been surveyed as well. Some lakes have high levels of PCBs and brominated flame retardants. These are mainly influenced by local point sources. However, most Norwegian lakes have low levels of persistent organic pollutants.
Mercury in fish results in consumption warnings
Due to high levels of mercury in fresh water fish, the Norwegian Food Safety Authority has introduced nationwide advice against consumption of pike, perch over 25 centimetres long and large trout and char (over one kilo). Pregnant and nursing mothers should not consume such kinds of fish at all. Other people should not eat them more than once a month. Heavy metals such as lead and cadmium are not easily transferred from sediments to the food chain, thus posing smaller risk for human consumption.
Environmental monitoring has shown that in some lakes, such as the mainland lake Mjøsa and Ellasjøen situated at Bjørnøya, concentrations of persistent organic pollutants increase in animals at the top of the food chain. Trout in both Mjøsa and Ellasjøen have high levels of PCBs. In Mjøsa, elevated levels of brominated flame retardants and chlorinated paraffins have also been detected.
Long-range transportation and local sources
Hazardous chemicals in Norwegian lakes are introduced through air currents or local sources. The advice against consumption caused by mercury is mostly due to long-range transported inputs. In contrast to most Norwegian lakes, Mjøsa has been influenced by local point sources which have discharged mercury, PCBs and brominated flame retardants into the lake.
The high levels of hazardous chemicals in Ellasjøen are mainly due to excrements from seabirds that have caught contaminated fish at sea. In addition, heavy precipitation over Ellasjøen is also considered a reason for the high levels of hazardous chemicals detected in the lake.
Lower inputs of lead, no reduction in mercury
The long-range transportation of lead has decreased. This is due to a reduction in the use of leaded petrol. There is however little evidence of a drop in inputs of mercury. Analyses of mercury content in perch fish show a 63 per cent increase between 1991 and 2008.
Inputs of PCBs, mercury and brominated flame retardants from local point sources into Mjøsa have stopped. But due to the nature of these substances, it will take considerable time before these hazardous chemicals are reduced in the food chain.