Fish carcasses deliver toxic mercury pollution to the deepest parts of the ocean

A new study into deep-sea fish and crustaceans has revealed the presence of anthropogenic mercury pollution at the deepest points in the ocean, including the Marianna Trench. Researchers suggest that the most likely source of the toxic chemical is the sinking bodies of dead fish from the surface.

The sinking carcases of pelagic fish, such as tuna, are now thought to be the main source of mercury pollution in the deep ocean

Mercury pollution in the ocean is an issue that has been on the radar of marine scientists for quite some time now. Although the element does occur naturally on the ocean floor and dissolves in seawater, it is unsurprisingly human influence that has pushed its concentration to dangerous levels. We produce mercury pollution in a number of ways, including – mining operations, waste incinerators and cement factories, but most prominently through coal burning power stations. All of which create atmospheric mercury emissions totalling around 2,000 metric tonnes a year, which settle on land or the ocean’s surface. However, wind and run-off in rivers and estuaries means that most of it eventually ends up in the sea anyway. 

Once it makes its way into our oceans, man-made mercury is converted by microorganisms into a toxic compound known as methylmercury. As fish at the surface eat these microorganisms they accumulate the chemical in their tissue, which then further accumulates up the food chain reaching its highest concentrations in top predators. It can be very harmful to the marine creatures, but it is also a concern for seafood loving humans as well. Methylmercury has been shown to cause damage to the central nervous system, heart and immune system, as well as cause developmental problems in foetuses and young children. Unfortunately, as levels of mercury pollution increase, this is something that is becoming increasingly common.

Coal burning power stations, like these in China, are the biggest source of anthropogenic mercury pollution

Understanding the extent of mercury pollution and its impacts on human health has therefore become an important area of research in recent years. However, recent breakthroughs in this area are showing that the problem is much more widespread than previously thought. It has long been suspected that methylmercury is largely confined to the upper 1000m of the ocean, due to the fact it is passed up the food chain. But earlier this year two separate research teams (one from China and another from America) provided evidence that it can be found at the lowest points of the ocean, including the seven mile deep Marianna Trench – the deepest point on Earth.

Both studies found that crustaceans and small fish found in deep ocean trenches had high concentrations of methylmercury in their tissue, meaning they had been feeding on mercury contaminated food sources. They were able to determine that this was indeed man-made mercury rather than a natural source, because methylmercury is only formed at the surface by microorganisms. Both studies presented their findings at a scientific conference in June, but have since disagreed on exactly how the pollution has managed to reach the deepest parts of our oceans.

The first explanation came from the team of Chinese researchers, who in July released a paper in Nature Communications hypothesising that methylmercury was being transported to the deep via sinking detritus (aka marine snow). Their idea was that the compound was sticking to particles of decaying organic matter, which were later eaten at depths by the fish and crustaceans. This theory was initially accepted because it is a proven pathway for microplastic pollution to reach the deep ocean. However the second research team from the states disagreed.

Marine snow (made up of sinking detritus partciles) is a route for microplastic pollution into the deep ocean, but apparently not mercury as well

The US team, made up of researchers from the University of Michigan and the University of Hawaii, recently released a new paper in the journal PNAS, which suggests the mercury was coming from the sinking carcases of dead fish. They were able to determine this by analysing the isotopes ratios of the mercury in the deep sea specimens. After analysing them it became clear to the researchers that the mercury was actually coming from a range of fish species in the central Pacific that feed at depths of around 500 meters. Just to be sure they also double checked the isotope ratio against mercury found in marine snow and found they didn’t match.

Their new paper therefore disproves the original theory hypothesized by the Chinese and shows that the real cause for the mercury pollution reaching this deep is the pelagic fish. After they die, either from natural causes or maybe even mercury poisoning, their bodies sink to the seafloor and introduce the mercury to a whole new ecosystem.

You may be wondering why it really matters how the mercury is getting to the deepest points in our oceans? After all surely the real issue is that it is there at all. But according to the lead author of the study Joel Blum, speaking in a recent press release, “we need to understand the cycling of mercury through the entire ocean to be able to model future changes in the near-surface ocean”.

It is also another alarming example that even in some of the deepest and most remote parts of our oceans our influence is still being felt. Mercury joins a long list of anthropogenic contaminants found in deep sea trenches, including plastic, lead, PCB’s and even Carbon-14 from nuclear bomb tests. It just goes to show that nowhere in our oceans is really safe anymore.

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