The V-1302 John Mahn was a German fishing trawler converted into a patrol boat during the Second World War. Her service ended on February 12, 1942 during the Operation Cerberuswhen an air attack from the Royal Air Force British ended up sinking it in the North Sea, off the Belgian coast. However, its presence can still be ‘felt’ eighty years later, as its remains continue to pour polluting substances, such as explosives and heavy metals, into the seabed, influencing the surrounding ecosystem. This is confirmed by a new study published in the journal ‘Frontiers in Marine Science’, which also warns about the danger of other forgotten sunken ships that may be negatively affecting the microbiology and geochemistry of their environment.
It all came about as part of the North Sea Wrecks project, whose objective is to locate and control the thousands of remains of ships, planes and ammunition that cover the seabed of the North Sea. “People are usually quite interested in shipwrecks from a historical point of view,” explains the author of the study, Josefien Van Landuyt, doctoral student at the University of Ghent. “However, the environmental impact of these remains is often forgotten. So we wanted to see if the old shipwrecks in our part of the sea (Belgium) were still shaping the local microbial communities and if they were still affecting the surrounding sediment,” she says.
The team took samples from both the steel hull and the surrounding soil. Analyzing them, they found various degrees of contaminationdepending on the distance from the remains: in the area where the coal was stored and in the sediment deposited after the shipwreck, a high content of heavy metalssuch as nickel or copper. They were also found polycyclic aromatic hydrocarbons (PAHs, chemicals naturally found in coal, crude oil, and gasoline), arsenic, and explosive compounds.
“Even though we don’t see these old shipwrecks, and many of us don’t know where they are, they may still be polluting our marine ecosystem,” says Van Landuyt. In fact, the passage of time could increase the environmental risk of these remains due to corrosion, which is breaking new ground. Therefore, its environmental impact is still evolving.”
They also found that the ship has influenced the microbiome – the set of bacteria, archaea, viruses, fungi and protists – that surrounded it. PAH-degrading microbes known as Rhodobacteraceae y Chromatiaceae in the samples with more contaminants; On the other hand, sulfate consuming bacteria (such as Desulfobulbaceae) were present in the hull samples, which probably led to the corrosion of the steel.
Up to 20.4 million tons of submerged debris
It is estimated that among all the shipwrecks of world wars have been dumped into the seas between 2.5 and 20.4 million tons of petroleum products. “Although shipwrecks can function as artificial reefs and serve to tell the story, we must not forget that they are objects created by people who have been forcibly introduced into a natural environment,” says Van Landuyt. “In fact, it is for this reason that new wrecks are currently removed.”
And the researcher warns that the John Mahn case is just the tip of the iceberg: “We only investigated one ship, at one depth, in one place. To get a better overview of the full impact of shipwrecks in the North Sea, it would be necessary to sample a large number of these wrecks in various locations.”