The German submarine U-864 was sunk in World War II off the Norwegian island of Fedje, loaded with 67 tons of metallic mercury. When the wreck was discovered in 2003, some of the mercury was found leaking from broken containers. Now, researchers show that this material has contaminated sediments surrounding the wreck. But surprisingly, the scientists think the marine food web may not be substantially affected by the pollution, based on their analysis of crabs sampled near the sub (Environ. Sci. Technol. 2016, DOI: 10.1021/acs.est.6b02128).
Crabs and other seafood are often contaminated with neurotoxic mercury from industrial pollution and fossil fuel burning that is deposited from the air onto the ocean surface, making its way into the food chain. So when the mercury-laden vessel was discovered at a relatively shallow depth of 150 m, scientists, fishermen, and government officials were concerned. To determine the wreck’s impact, Norway’s Coastal Administration sampled sediments, and the National Institute of Nutrition & Seafood Research collected crabs near the site. Frank Vanhaecke of Ghent University and his colleagues then analyzed the samples with a variety of methods, including measuring mercury isotope ratios using multicollector inductively coupled plasma-mass spectrometry.
“In some sediment samples we could see small droplets of mercury,” Vanhaecke says. The wet sediments had 60 to 24,000 mg/kg of mercury—in comparison, background levels in ocean sediments are 0.02 to 0.1 mg/kg. The isotopic composition of mercury in the submarine was similar to that in the sediments. Notably, crabs collected within a four-nautical-mile radius of the wreck did not have significantly different mercury concentrations than those collected from other areas along the Norwegian coast. However, the isotopic results indicate that crabs in the immediate vicinity of the submarine did take on some mercury from the wreck in their brown meat.
The researchers hypothesize that mercury levels in crabs are not elevated because the sandy seafloor has relatively little organic matter; this may have limited methylation of mercury from the wreck by microbes. Methylmercury is the most bioavailable and toxic form of the element. Instead of methylmercury, what little mercury the crabs absorbed from the wreck was likely ingested directly from the sediments. The results show no evidence that the metal transforms into the more toxic methylmercury, Vanhaecke says. That “would make the problem much more risky,” he says, and needs to be further assessed.