This pilot study of the Bay of Luebeck demonstrates that routine water sampling and blue mussel monitoring at specific dumping sites are principally suitable for the surveillance of energetic compounds in munition contaminated waters. However, an issue was the low oxygen levels in the Bay of Luebeck, particularly during the summer months (Lutterbeck 2019). When recovering the mussels deployed at the Pelzerhaken A position in August 2020, only empty shells were retrieved, indicating that the mussels had died. This disadvantage with living species can principally be compensated for by using passive sampling systems. Apart from natural obstacles such as weather, water currents and poor visibility underwater, mooring systems could be subject to being dragged by fishing nets. This could be the reason, why, in our case, two moorings could not be recovered on separate occasions: An original mooring deployed in the Haffkrug-Area in August 2020 and a further one in Pelzerhaken B in March 2021. An additional limiting factor in our special case was that some of the originally planned cruises in the years 2020 and 2021 had to be canceled due to restrictions from the Covid-19 pandemic.

EC were detected in all water samples of the monthly sampling campaign. This is consistent with previous publications from the German Baltic Sea, where EC were found at every investigated location (Beck et al. 2025; UDEMM et al. 2019). No seasonal trend regarding the concentration of EC could be identified in the monthly water samples. Similarly, there was no general trend between the samples taken at the seafloor and those collected 1 m below the water surface, which indicates strong vertical mixing within the water column. Only for 1,3-DNB and RDX measured values near the seafloor appear to be slightly elevated on average, but was only determined to be statistically significant for RDX at the sampling location Trave at Schlutup.

Particularly striking in the Bay of Luebeck is the increased occurrence of 1,3-DNB in relation to TNT. Only in four out of around one hundred water samples was the 1,3-DNB concentration lower than the TNT concentration. In all other samples, 1,3-DNB was measured at levels 2–34 times higher than TNT (1,3-DNB: 0.3–9.4 ng/L; TNT: < LOD-5.2 ng/L). In the passive samplers, 1,3-DNB was measured at levels 3.9–15.8 times higher than TNT (1,3-DNB: 20.4–105.5 ng/PS; TNT: 1.5–11.8 ng/PS). The nitroaromatic 1,3-DNB is an intermediate product in the production of the explosives TNT and trinitrobenzene (TNB) and was used as a readily available substitute for TNT in varying percentages in the filling of bombs during World War II. While 1,3-DNB almost always appears together with TNT in the environment, it is usually present in much lower concentrations than TNT. For example, in the munition dumping area of the Oosterschelde estuary—a North Sea region in the Netherlands, the ratio of 1,3-DNB to TNT in the analyzed water ranged from 3.9 to 26.8% (1,3-DNB: 0.2–7.8 ng/L; TNT: < LOD–56.7 ng/L) (Den Otter et al. 2023). In the area with hexanite (German: “Schiesswolle” of the Kolberger Heide dumping ground with uncovered explosive chunks, the proportions were even lower, ranging between 0–7.9% (1,3-DNB: 0–2.0 ng/L; TNT: 6.8–1346.1 ng/L) in water and 0.18% and 5.7% in passive samplers (1,3-DNB: 101.1–1499.0 ng/PS; TNT: 2183.5–119,754.4 ng/PS). Other studies reported 1,3-DNB to TNT ratios of 5.9/14.2 µg/L (Halifax Harbor, Canada (Rodacy et al. 2001)), 0.008/7.5 µg/L (Bahia Salina del Sur, Puerto Rico (Rosen et al. 2018)), 23.4/105 µg/L (Isla de Vieques Bombing Range, Puerto Rico (Porter et al. 2011)), 0.18/0.17 µg/L (Lake Mjøsa, Norway (Rossland et al. 2011)), and 10/50 µg/L (Kolberger Heide, Germany (Beck et al. 2019)).This atypical ratio in favor of 1,3-DNB compared to TNT has already been observed in previous investigations along the German Baltic Sea coast, with 30-fold higher 1,3-DNB concentrations in the water of the Bay of Luebeck compared to the Kolberger Heide (UDEMM et al. 2019). The reason for this, however, remains unclear. It is possible that multiple exposed explosive objects, presumably warheads from the Fieseler Fi 103 flying bomb, are the source. A significant dissolution of their uncovered, sponge-like explosive material was observed between March 2023 and 2024 (Greinert et al. 2024).

As could be expected, higher EC concentrations were measured in the water at the four moorings compared to the routine monthly water samples, albeit these differences were only marginal. Overall, EC water concentrations in the Bay of Luebeck from our study correspond to findings from other munition dumping sites with concentrations in the single- or double-digit ng/L, and with higher concentrations reported in close proximity to solid munitions material: Eastern Scheldt, Netherlands (Den Otter et al. 2023), Sejerø Bay, Denmark (Maser et al. 2023a), Kolberger Heide, Germany (Beck et al. 2019; Esposito et al. 2023; Gledhill et al. 2019), and at a shipwreck in the North Sea of Belgium (Maser et al. 2023b).

Blue mussels in our study showed very low levels of contamination. Only 1,3-DNB and RDX were regularly detected in very low concentrations, while TNT and its metabolites 2- and 4-ADNT were found only in individual mussels. In previous studies, much higher tissue concentrations were found at other dumping sites close to munition objects. In the Kolberger Heide, up to 10 ng/g wet weight (approximately 100 ng/g dry weight) of the TNT metabolite 4-ADNT were detected in the tissue of mussels, deployed directly at corroded moored mines (Appel et al. 2018), and up to 150 ng/g wet weight (~ 1500 ng/g dry weight) of 4-ADNT were measured near free-lying chunks of hexanite (German: “Schiesswolle”) (Strehse et al. 2017). In the area around the ship wreck of the V1302 (“John Mahn”), which sank in the North Sea during World War II,TNT concentrations in mussels reached up to 3 ng/g dry weight (Maser et al. 2023b). Interestingly, concentrations of 1,3-DNB, 4-ADNT, and 2-ADNT above the detection limit were found onwards from the year 2017 until today in wild blue mussels collected since 1992 by the Environmental Specimen Bank from the Darssßer Ort area, located further east of the Bay of Luebeck. However, these concentrations were below the quantification limit (Strehse et al. 2023). Similar findings were reported in the same study for two other locations at the coastline of the North Sea region—Eckwarderhoerne located in the Lower Saxony Wadden Sea and the island of Sylt (Koenigshafen) located in the Schleswig–Holstein Wadden Sea (Strehse et al. 2023).

The question now is what the results of our study mean for the marine environmental ecology and the safety of human seafood consumers. Literature reviews of the effects of EC to aquatic organisms were first provided by Talmage et al. (1999), and later by Juhasz and Naidu (2007), Nipper and Lotufo (2009) and Lotufo (2013). Lotufo et al. (2013, 2017, 2021), Voie and Mariussen (2017), Beck et al. (2018), and Barbosa et al. (2023) provided comprehensive summaries and concise overviews on the toxicity of EC to aquatic biota. As a conclusion from all of these studies it appears that EC toxicity benchmarks values for aquatic biota are in the μg/L to mg/L ranges when considering acute toxicity studies that were performed under short-term laboratory conditions.

However, the situation with submerged munitions is much more complex in the oceans and involves the following important aspects. Firstly, a distinction must be made between acute (lethal) and sublethal toxicity. Acute toxicity is considered a somewhat different issue with mortality as an endpoint, while sublethal toxicity has an impact on the general health of marine organisms. A deteriorating state of health not only reduces the longevity of individuals, but can also negatively influence the population dynamics of marine organisms. Together with other anthropogenic influences (microplastics, pharmaceuticals, endocrine disruptors, pesticides), EC from world war relics can significantly affect sea food stocks. For example, Schuster et al. (2021) observed significant responses of the antioxidant defense system in blue mussels (Mytilus spp.) at the biochemical and cellular biomarker level, while Koske et al. (2019) found lethal and sublethal effects of TNT and its transformation products 2-ADNT and 4-ADNT to zebrafish embryos (Danio rerio) in a 120-h exposure scenario. Lethal concentrations (LC50) were 4.5 mg/l for TNT, 13.4 mg/l for 2-ADNT, and 14.4 mg/l for 4-ADNT, while sublethal effects such chorda deformation and genotoxicity were induced by all three compounds already at 0.1 mg/l as lowest tested concentration. In field studies, low levels of EC in the 1–10 ng/L range have been occasionally detected directly adjacent to submerged munitions (Gledhill et al. 2019; Maser et al. 2023a), whereas higher TNT concentrations of up to the two-digit µg/L range have been observed near exposed explosive solids (Beck et al. 2019), thereby reaching concentration that might impact the marine life.

A second aspect is the time of exposure to contaminants. While laboratory studies are generally performed with relatively high EC concentrations and short exposure periods, the biota in the marine dumping sites are usually exposed to lower concentrations but lifelong. In the southwestern Baltic Sea, Beck et al. (2025) detected at least one EC compound in nearly every water sample in concentrations ranging from sub-pmol/L up to several thousand pmol/L. In this area, Blue mussels (Mytilus spp.) had been deployed as a biomonitoring system for the presence of EC near corroding mines (Strehse et al. 2017, 2020; Appel et al. 2018; Maser and Strehse 2020). After a deployment period of 93 days, mussels bioaccumulated TNT and its transformation products and a decrease in shell growth in combination with weight loss were observed. Moreover, a statistically significant induction of the carbonyl reductase gene in the tissues of Mytilus spp. placed in cages adjacent to a chunk of explosive material was shown after a 58-day exposure (Strehse et al. 2020). The enzyme carbonyl reductase plays an important role in the detoxification of TNT-caused carbonyl stress, as has been proven in lab studies with Mytilus spp. (Strehse et al. 2020) and Daphnia magna (Jacobsen et al. 2022, reviewed in Adomako-Bonsu et al. 2024). Whether such chronic toxic effects can be detected in the biota living in the Bay of Luebeck remains a question for future studies.

For humans, as consumers of potentially contaminated seafood, such as mussels, health impacts cannot be entirely ruled out. On the one hand, it is important to carefully infer the actual concentrations of energetic compounds present in seafood. On the other hand, the average and actual consumption behavior of different population groups with regard to this EC contaminated seafood is crucial. Calculations based on the mussels collected near the corroded anchor buoys in Kolberger Heide, where an average of 5 ng/g fresh weight (equivalent to about 50 ng/g dry weight) of TNT metabolites were detected (Appel et al. 2018), indicate that a health risk would only arise from a daily and lifetime consumption of several kilograms of these mussels. Only mussels from areas with exposed explosives, containing up to 350 ng/g fresh weight (equivalent to 3500 ng/g dry weight), would pose a health risk, based on the assumption that 39 g of mussels were consumed daily over a lifetime (Maser and Strehse 2021). This latter calculation is based on the average daily intake of fish and seafood of 39 g per person in Germany (FIZ 2017). Since the lifelong daily consumption of 39 g of mussels from the immediate vicinity of chunks of explosives lying without covers on the sea bed is very unlikely, the consumption of mussels from the Baltic Sea can be generally considered as being safe from today’s point of view (Maser and Strehse 2021). Therefore, based on the concentrations measured in the mussels of the Bay of Luebeck (< 1 ng/g dry weight), there is currently no health risk for humans even with daily and lifelong consumption of these mussels (Beck et al. 2022; Maser and Strehse 2021, 2020).

However, the aspect of ongoing corrosion of the metal casings of the munition bodies comes into play. After 80 years of resting on the seabed, there are clear signs that corrosion is progressing continuously and that increasing amounts of EC are entering the marine environment. Hence, as time goes on, not only the marine biota is becoming increasingly endangered, but also the human seafood consumer. As already mentioned above, this temporal aspect was recently shown in a study on mussels from the environmental specimen bank. Here, mussels from years 1985 to 2021 were examined for their EC content. While mussels from the Darßer Ort from years 1992 to 2016 showed no evidence of EC, these were clearly detected in mussels from years 2017 to 2021, and with increasing concentrations (Strehse et al. 2023).