During World War II, unused explosive munitions were frequently dumped at sea. Today, the waters off the coast of Germany contain 1.6 million tons of sunken weapons laden with chemicals that are highly toxic to marine life.
Surprisingly, researchers behind a new study in Nature Communications Earth & Environment observed more sea creatures clinging to debris than living on the ocean floor around it. The findings suggest some marine organisms can tolerate high levels of toxic compounds as long as there is a hard surface—such as metal weapons casings—for them to inhabit.
Exploring an unusual human-made ecosystem
The researchers, led by Andrey Vedenin of the Senckenberg Society for Nature Research in Frankfurt, Germany, investigated the composition of epifauna living on dumped munition in the Bay of Lübeck, located in the southwestern Baltic Sea. Epifauna—animals that live on or attached to the seafloor—can include fish, corals, mussels, barnacles, sponges, and other creatures.
The team used a remotely operated vehicle (ROV) to survey the study area, which is littered with warheads from V1 flying bombs. They found eight species of epifauna—five invertebrate species and three fish species—with an average density of more than 43,000 individuals per square meter.
Most of these animals were concentrated on the warheads, with the surrounding sediment showing a significantly lower average density of 8,200 individuals per square meter. While previous studies have observed similar levels of marine life abundance on natural hard surfaces in the bay, these findings were unexpected due to the toxic chemicals leaching from the warheads.
Marine life thriving in toxic waters
After sampling the water around several warheads, chemical analysis revealed high concentrations of explosive compounds such as trinitrotoluene (TNT), measured at 2.73 milligrams/liter.
These concentrations approach toxicity thresholds for aquatic organisms, according to the researchers. Despite this, epifauna have colonized the warheads in numbers comparable to natural hard surfaces, suggesting that some species of epifauna can tolerate high toxicity levels.
However, just because a species can tolerate potentially fatal conditions doesn’t mean it should. So why are these organisms choosing to live on the warheads instead of the seafloor? The researchers suggest that the hard surfaces provided by the munitions may be more attractive than softer sediments, outweighing the disadvantages of chemical exposure.
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