Fjords create mercury problems

HI 039582 — The mountains around the fjord funnel rainwater containing mercury into the fjord. Deep-sea fish in Sognefjorden contained almost as much mercury as fish from the industrial areas in Hardanger, even though there were no known sources of pollution.
Photo: Erlend A. Lorentzen / Havforskningsinstituttet

It has been a mystery why deep-sea fish contain so much mercury in fjords without any known sources of mercury pollution. In fact, fish in a clean fjord like Sognefjorden contain more mercury than fish around the polluted submarine wreck at Fedje. Can it be the fjord itself that is to blame?

Published: 28.01.2019 Updated: 14.02.2019 Author: Torhild Dahl og Atabak Azad

The scientists just couldn’t make sense of it. Deep-sea fish have high levels of mercury even in fjords where there haven’t been any industrial emissions or other known sources of mercury pollution. Meanwhile, the complete opposite is true of an open area such as off the Fedje archipelago in Hordaland. There the wreck of a World War II submarine has polluted the surrounding seabed with mercury, but nevertheless the fish in the area don’t contain much mercury.

Excessive levels of mercury, which is a highly toxic heavy metal, can affect brain function and development, particularly in young people. The Norwegian authorities therefore carefully monitor food to make sure that it doesn’t contain more than a safe level of mercury.

Mercury:

Mercury is a toxic heavy metal that is present in various parts of the environment. It is found in very low concentrations in the air, water and soil.
In the environment, mercury can be converted into its most toxic form, methylmercury. This is the form that is most easily absorbed by living organisms.
Animals retain more methylmercury from everything they eat than they excrete, so the concentration increases as you move up the food chain. Animals at the top of the food chain therefore contain relatively high mercury levels.
Mercury is one of the most important undesirable substances in seafood throughout the world, and it is the main cause of most health advisories relating to seafood.
Over time, methylmercury can be harmful to foetuses, infants and small children. It may inhibit the development of their nervous system and brain, as well as causing behavioural problems and learning difficulties.
In adults, potential long-term impacts of methylmercury include cardiovascular disease, disorders of the nervous system and mobility impairment.
In order to prevent the population from consuming too much methylmercury, the EU and Norway have established maximum levels for the concentration of mercury in fish sold as food. For most species of fish, the maximum level is 0.5 mg/kg wet weight.

Mercury in clean fjords

Sørfjorden, which is a branch of Hardangerfjorden, has been polluted by heavy industry for more than a century, and heavy metals like lead, zinc, mercury and arsenic create problems for fish and other seafood in the area. Sediment samples from the seabed near the industrial area contained a hundred times more mercury than samples from the outer part of the fjord, and the authorities advise against eating tusk and blue ling from all of Hardangerfjorden, since these deep-sea fish contain very high levels of this harmful heavy metal.

More surprisingly, deep-sea fish in Sognefjorden contained almost as much mercury as fish from the industrial areas in Hardanger, even though there are no known sources of mercury pollution. In Sognefjorden, the average values at seven of eight locations sampled exceeded the threshold set by the authorities, and the Norwegian Food Safety Authority therefore advises against eating tusk from most of Sognefjorden.

Similarly, mercury levels in deep-sea fish in Eidfjord, a less polluted branch of Hardangerfjorden, were even higher than they were closer to the heavy industry, in spite of Eidfjord only having a tenth of the amount of mercury in its bottom sediments.

So why are heavy metal levels so high in fish in Sognefjorden and other fjords without heavy industry? The answer may lie in the fjord itself.

Mercury levels in Hardangerfjorden, Sognefjorden and Fedje:

Mercury levels in Hardangerfjorden, Sognefjorden:

In Hardangerfjorden, levels in fish far exceeded the maximum level and high levels were also found in sediments near the heavy industry at Odda.
In Sognefjorden, levels in tusk far exceeded the maximum level and levels in sediments were low.
In both fjords, mercury levels gradually declined as you went further out towards the sea.
Based on these mercury levels, the Food Safety Authority of Norway advises against eating deep-sea fish from almost all of Hardangerfjorden and tusk from most of Sognefjorden.

Mercury levels around the submarine at Fedje:

The German submarine U-864 was sunk in 1945 with 67 tonnes of mercury on board. It split into two, and liquid metallic mercury ended up on the sea bottom.
The wreck was rediscovered in 2003, and since 2004 the Institute of Marine Research has made annual measurements of mercury levels in fish and crabs sampled near the wreck, acting on behalf of the Norwegian Coastal Administration.
The muscle tissue of fish and crab from this location don’t contain higher mercury levels than elsewhere along the coast, in spite of the high levels of mercury in the bottom sediments.
The brown meat of some crabs contains slightly more mercury than elsewhere along the coast, but the levels are not so high as to be a food safety concern.

Fjords funnel toxins

Mercury travels long distances in the atmosphere, and mercury released from a coal-fired power station in China may be deposited in Norway or the Arctic. In many places, long-range transported pollution is therefore the biggest source of mercury in the environment.

A fjord surrounded by high mountains acts like a funnel, with large rivers transporting rainwater into the fjord from their drainage basins in the surrounding mountains. As a result, fresh water that ends up in fjords contains mercury from rainwater that has been collected in a large area and then carried into the fjord by rivers. In addition, the water in fjords isn’t exchanged as often as in the open ocean, so the heavy metals accumulate. This is supported by the fact that you find the highest mercury concentrations in fish from the innermost part of the fjords where the rivers flow into them.

Converted into dangerous methylmercury

The river water also carries large quantities of biological material from the land, providing good conditions for micro-organisms to convert the mercury into its most toxic form, known as methylmercury. This is the form of mercury that is most easily absorbed by mammals, birds, fish and people. It is also the major form of mercury (70-100%) in fish fillets.

In the open ocean, on the other hand, the mercury is less accessible to micro-organisms, so it isn’t converted into methylmercury at the same rate as fjords. That explains why the deep-sea fish around the submarine wreck at Fedje don’t contain particularly high levels of this harmful form of mercury, since mercury from the submarine is not bioavailable and in the open ocean the water can freely circulate.

^{The blue dots on the map show locations in Western Norway where mercury levels were measured in tusk. The red dots show locations where mercury levels were measured in bottom sediments. The maps show that there isn’t always a link between the amounts of mercury in the sediments and in the fish.}