Macrozoobenthos Response to Sediment Contamination near the S/s Stuttgart Wreck: A Biological and Chemical Assessment in the Gulf of Gdańsk, Southern Baltic Sea

Abstract

This study provides an up-to-date assessment of the environmental status in the area of the S/s Stuttgart wreck in the southern Baltic Sea, focusing on macrozoobenthos, sediment chemistry, and contamination in Mytilus trossulus soft tissues. Comparative analyses from 2016 and 2023 revealed increased species richness and distinct benthic assemblages, shaped primarily by depth and distance from the wreck. Among macrozoobenthos, there dominated opportunistic species, characterized by a high degree of resistance to the unfavorable state of the environment, suggesting adaptation to local conditions. Elevated concentrations of heavy metals were detected in sediments, with maximum values of Cd—0.85 mg·kg⁻¹, Cu—34 mg·kg⁻¹, Zn—119 mg·kg⁻¹, and Ni—32.3 mg·kg⁻¹. However, no significant correlations between sediment contamination and macrozoobenthos composition were found. In Mytilus trossulus, contaminant levels were mostly within regulatory limits; however, mercury concentrations reached 0.069 mg·kg⁻¹ wet weight near the wreck and 0.493 mg·kg⁻¹ at the reference station, both exceeding the threshold defined in national legislation (0.02 mg·kg⁻¹) (Journal of Laws of 2021, item 568). Condition indices for Macoma balthica were lower in the wreck area, suggesting sublethal stress. Ecotoxicological tests showed no acute toxicity in most sediment samples, emphasizing the complexity of pollutant effects. The data presented here not only enrich the existing literature on marine pollution but also contribute to the development of more effective environmental protection strategies for marine ecosystems under international protection.

1. Introduction

The Baltic Sea, heavily impacted by human activity, faces additional threats that may further harm its biodiversity and stability, leading to ecosystem decline and increased mortality [1]. Over 415 shipwrecks remain deployed on the bottom of the Baltic Sea, within Polish marine areas, many of which may still contain fuel in their tanks. More than 100 of these vessels are located in the Gulf of Gdańsk, an area of intense military activity during World War II [2]. Among them, the S/s Stuttgart is considered one of the greatest environmental threats to the region. This German ship, intentionally sunk in 1943 after being bombed and set on fire, now lies in fragments scattered across the seabed. Considered a navigational hazard, it was targeted for removal starting in 1957 using pyrotechnic demolition [3]. This method involved breaking the wreck into smaller parts with explosives and retrieving the fragments with cranes. It is likely that during this operation, fuel leaked from the wreck’s tanks into the surrounding environment in unknown quantities. Previous studies have demonstrated a patchy distribution of oil contamination in the area [4,5], identifying the wreck as a significant source of pollution. Many samples collected near the wreck exhibited alarmingly high concentrations of polycyclic aromatic hydrocarbons (PAHs), total petroleum hydrocarbons (TPH), and total organic carbon (TOC) [6,7]. These substances, known for their toxic properties, can severely affect the local ecosystem. In light of such concerns, it is essential to assess whether these pollutants have a measurable impact on the local marine life.

Contemporary research highlights the growing threats associated with marine ecosystem pollution, including the bioaccumulation and biomagnification of toxic substances such as PAHs, PCBs, TPH, butyltins (BTs), and heavy metals. Additionally, it is important to note that hydrophobic pollutants can accumulate in bottom sediments, but sooner or later, sorption and desorption processes will occur, releasing toxic compounds into the water column. Consequently, this can negatively affect marine organisms, disrupting the ecosystem balance [8,9]. As a result, organisms living in sediments are constantly exposed to pollutants present in their environment, and toxic compounds can migrate, significantly impacting not only aquatic organisms but also human health [10,11,12]. As already mentioned, areas particularly vulnerable to contamination are the shipwreck surroundings, which can serve as reservoirs of harmful substances released into marine waters and seabed sediments [13]. Shipwrecks can pose a serious threat to marine ecosystems, accelerating the degradation of the aquatic environment. This risk stems from both sudden, large-scale releases of toxic substances and the gradual leaching of contaminants as the wrecks corrode and deteriorate over time. These processes contribute to the pollution of water and sediments with hazardous chemicals from fuel residues, protective coatings, and cargo once carried by the now sunken vessels [14].

One of the most effective ways to assess the ecological health of marine environments is through the study of macrozoobenthos—the community of invertebrate organisms inhabiting seabed sediments and playing a crucial role in marine ecosystems. Benthic organisms, such as bivalves, gastropods, oligochaetes, polychaetes, and crustaceans, contribute to sediment oxygenation, organic matter decomposition, near-bottom water filtration, and nutrient cycling, while also serving as a vital link in the marine food web. The taxonomic diversity, abundance, and sensitivity of benthic species provide valuable insights into the ecological quality of the seabed. Additionally, the long life cycles of certain organisms, particularly bivalves, make them reliable indicators of long-term environmental changes and pollution impacts. For these reasons, macrozoobenthos is widely used as a key bioindicator of marine ecosystem health [15,16]. The most abundant filter feeder in the Gulf of Gdańsk, i.e., the long-living mussel Mytilus trossulus, serves as a natural biofilter, especially in polluted and eutrophic waters, contributing to their purification. By absorbing suspended solids from the water column, through its ability to agglutinate and excrete in the form of pseudofeces, it contributes to the purification of the reservoir, enriching the bottom with detritus and organic matter. It also accumulates harmful and toxic substances such as metals, metalloids, PAHs, and PCBs [17,18,19]. Mussels are perfect for biomonitoring water quality and can be used to determine the levels of these contaminations in their tissues. Another common species of bivalve in the Gulf of Gdańsk is Macoma balthica, widely distributed in the Baltic Sea and playing a key role in the macrozoobenthos structure in this region. Moreover, this species inhabits heavily polluted areas, which indicates its high adaptive potential and wide range of tolerance, resulting from, among other things, genetic adaptation and hybridization [20,21,22]. As this bivalve is considered to be a pollution indicator in the Gulf of Gdańsk [23], the results of ecophysiological surveys reflect the state of the environment in which it lives.

The Baltic Sea is a unique ecosystem under international protection regulated by directives such as the Marine Strategy Framework Directive (MSFD, 2008/56/EC, with subsequent amendments) [24] and the Water Framework Directive (WFD, 2000/60/EC) [25]. These directives require EU member states to monitor and improve the quality of marine waters and seabed sediments. In Poland, their requirements are implemented through the Water Law Act and the regulations of the Council of Ministers, which defines environmental quality standards (EQS). A key role in the international assessment of sediment quality is that of the OSPAR Commission guidelines, including Background Assessment Concentrations (BAC), which define natural background levels of substances, and Environmental Assessment Criteria (EAC), which establish ecological thresholds. Exceeding these values may indicate anthropogenic contamination and potential threats to marine organisms.

This study aims to address the extent of ecological degradation caused by the wreck and its implications for the health of benthic ecosystems. It provides answers to questions such as the following: What changes in the structure, abundance, and biomass of macrozoobenthos occurred between 2016 and 2023? Is there a correlation between the condition of macrozoobenthos and environmental status, including chemical contamination, in the vicinity of the S/s Stuttgart wreck? What is the quality status of seabed sediments and the level of chemical contaminants in mussels? Do they meet the requirements for marine water protection? How can sediment contamination be assessed based on ecotoxicological tests?

2. Materials and Methods

2.1. The Study Area

The shipwreck of S/s Stuttgart is located in the eastern part of the Gulf of Gdańsk, approximately 4 km from the shore (Figure 1). The wreck lies on the eastern slope of a sandy ridge extending in the SSW-NNE direction at a depth of approximately 21–22 m. The surrounding seabed has an uneven topography, with height differences of 1–3 m. The area consists mainly of fine- to medium-grained sands, with patches of silty sands and silts. Depressions in the seabed contain thicker layers of silty and clayey sediments, reaching up to 4 m in the central basin. These deposits rest on a substrate consisting of sands, gravels, and, in some places, glacial tills (Table S1). Chemical analyses were carried out at seven stations. Stations 1, 2, and 3 were selected to allow direct comparison with data from the 2016 survey, particularly for evaluating contaminant levels in mussel tissues. Additional stations were distributed to capture the potential spread of contaminants from the wreck site. A designated reference stations (REF_1, REF_2) was sampled to represent background conditions outside the assumed pollution plume. The number of chemical sampling points was optimized based on scientific priorities and limited by available financial resources.

2.2. Data Acquisition and Analysis

Macrozoobenthos samples were collected using a van Veen grab (0.1 m²) in the course of two survey campaigns: in April 2016 from aboard Imoros 1 and in June 2023 from Imoros 2. The 2023 campaign expanded the study area to include additional stations located at greater distances from the wreck, with two reference stations for comparative analysis. In 2016, one sample per station was taken, whereas in 2023, three replicates per station improved statistical robustness. Samples were sieved (using 1 mm mesh), preserved in 4% formaldehyde, and stored for further analysis. In 2023, water column parameters (depth, temperature, salinity, oxygenation) were recorded using a CTD 115M probe (Sea & Sun Technology GmbH, Trappenkamp, Germany), while sediment parameters (temperature, pH, redox potential) were measured in situ with a HANNA HI 98121 sensor (HANNA Instruments Sp.z o.o., Olsztyn, Poland).

The contamination assessment surveys of surface sediments from 2016 were repeated and extended to include chemical analyses of PAHs, PCBs, TPH, butyltins (BTs), and metals/metalloids. During both years sediment collection for ecotoxicological analysis, as well as chemical contaminants in M. trossulus, was performed. Additionally, in 2023, M. balthica was sampled to assess species conditions. M. trossulus and M. balthica specimens were collected with a 50 cm dredge (3 mm mesh). M. balthica samples were obtained at eight stations for condition assessment (≥40 individuals per station). M. trossulus was collected at five stations for chemical analysis. Between 144 and 286 M. trossulus specimens per station were conditioned in filtered seawater, then shelled, frozen, lyophilized, and homogenized for chemical analysis of metals, PAHs, and PCBs (Table S1).

2.3. Macrozoobenthos

The macrozoobenthos sample analysis followed the standards applicable to laboratory processing of biological samples used in international monitoring of the Baltic Sea [15,25,26]. The material for analysis was transferred onto Petri dishes, where organisms were selected and sorted using a stereoscopic microscope. Taxonomic identification was then performed to the level of species or a higher taxonomic unit. Polychaetes (Marenzelleria sp.) were identified to the genus, oligochaetes (Oligochaeta) to the subclass, chironomids (Chironomidae), and hydrobiid snails (Hydrobiidae) to the family. Epifaunal species (Amphibalanus improvisus and Einhornia crustulenta) were not included in the quantitative analyses; only their presence in the collected samples was recorded. The taxonomic classification and nomenclature of macrozoobenthos were adopted according to the WoRMS—World Register of Marine Species [27].

Organisms of the same taxon were counted, and after they were blotted on filter paper, their formalin wet weight was determined with an accuracy of 0.0001 g using an analytical balance. Bivalves were weighed with their shells after they had been crushed to remove water retained in the mantle cavity. Abundance was expressed in N∙m⁻² (N—number of individuals), and biomass in g wet weight∙m⁻².

Two biocenotic indices were selected for the analysis: the Constancy Index (C) and the Dominance Index (D) [28,29]. These indices provide complementary insights into the ecological role of taxa within the macrozoobenthic community, helping to assess species stability, distribution patterns, and dominance within the study area. To visualize the abundance and biomass of macrozoobenthos in the area surrounding the wreck of S/s Stuttgart in June 2023, Grapher 20 by Golden Software (Golden, CO, USA) was utilized.

2.4. Condition Indices

In the laboratory, the shell length of M. balthica specimens was measured to an accuracy of 0.01 mm using a caliper. The measurements were taken at 6 stations and 2 reference stations. After separating the shells from the soft tissue and removing water from them with filter paper, the prepared material was placed in a drying oven (55–60 °C) for 48 h. The dry weight of the soft tissue and the shells was then measured using an analytical balance with an accuracy of 0.0001 g.

Two morphometric condition indices, CI1 and CI2 [19,30,31,32], were calculated. The first condition index (CI1) was calculated using the following formula:

$$\mathrm{C}\mathrm{I}1={\displaystyle \frac{\mathrm{dry\; soft\; tissue\; weight} \left(\mathrm{g}\right)}{\mathrm{d}\mathrm{r}\mathrm{y} \mathrm{s}\mathrm{h}\mathrm{e}\mathrm{l}\mathrm{l} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \left(\mathrm{g}\right)}}\times 100$$

The second condition index (CI2) was calculated as

$$\mathrm{C}\mathrm{I}2={\displaystyle \frac{\mathrm{dry\; soft\; tissue\; weight} \left(\mathrm{m}\mathrm{g}\right)}{{\left(\mathrm{s}\mathrm{h}\mathrm{e}\mathrm{l}\mathrm{l} \mathrm{l}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{h}\mathrm{t} \left(\mathrm{c}\mathrm{m}\right)\right)}^3\left({\mathrm{c}\mathrm{m}}^3\right)}}$$

Graphical and statistical analyses of the condition indices were performed in Grapher 20.

2.5. Ecological Status Assessment

The assessment of the ecological status of the study area based on macrozoobenthos was conducted using the multimetric B index [15]. The classification of ecological status was determined according to the threshold values of the water quality index specified in the Regulation of the Minister of Infrastructure of 25 June 2021, on the classification of ecological status, ecological potential, and chemical status, as well as the methods for classifying the status of surface water bodies and environmental quality standards for priority substances [33]. Additionally, the classification follows the criteria of the Water Framework Directive [25], which identifies macrozoobenthos as one of the key elements for assessing the quality status of the seafloor.

2.6. Ecotoxicological Analysis

In 2016, seven sediment samples were analyzed for toxicity using the Microtox^® test with Vibrio fischeri bacteria (acute toxicity), the Ostracodtoxkit FTM with Heterocypris incongruens crustaceans (chronic toxicity), and the Phytotoxkit with Sorghum saccharatum plants (phytotoxicity). In 2023, five samples were tested for acute toxicity using Vibrio fischeri [34], Daphnia magna [35] Desmodesmus subspicatus algae [36] and Poecilia reticulata fish [37], while phytotoxicity was assessed with Sinapis alba seeds. Each sample was lyophilized before analysis to preserve its properties. Toxicity was evaluated based on bacterial luminescence inhibition, crustacean immobilization, algal growth inhibition, seed germination and root elongation effects, and fish mortality rates.

2.7. Chemical Analysis of Sediments and Mussels

Sediment and mussel samples collected in 2016 and 2023 were analyzed for contaminants, including PAHs, PCBs, TPH, butyltins (BTs), and metals/metalloids. All analyses were conducted in a PCA-accredited laboratory (accreditation no. AB 646) using standardized methods.

2.7.1. Organic Contaminants (PAHs, PCBs, TPH, BTs, TOC)

PAHs and PCBs were extracted using solvent-based methods, purified with SPE, and analyzed by gas chromatography-mass spectrometry (GC-MS; Hewlett Packard 6890 GC, 5973 MS, Palo Alto, CA, USA). The detection limits were 0.001 mg·kg⁻¹ DW for PAHs and 0.0001 mg·kg⁻¹ DW for PCBs. Total petroleum hydrocarbons (TPH, C10–C40) were determined by gas chromatography with flame ionization detection (GC-FID; Agilent 6890N, Santa Clara, CA, USA) following ultrasound-assisted extraction and SPE purification (LOQ: 5.0 mg·kg⁻¹ DW). Butyltins (MBT, DBT, TBT) were analyzed by GC-MS (Agilent 7890A GC, Agilent 5977A MS, Santa Clara, CA, USA) after alkaline extraction and in situ derivatization, with LOQs ranging from 0.001 to 0.01 mg·kg⁻¹ DW. Total organic carbon (TOC) was measured in freeze-dried and homogenized sediment samples using a TOC analyzer (TOC-L, Shimadzu Corporation, Kyoto, Japan). The limit of quantification (LOQ) for TOC was 1000 mg·kg⁻¹ DW.

2.7.2. Metals and Metalloids

Total metal content in sediments was determined by inductively coupled plasma optical emission spectrometry (ICP-OES; Agilent 5800, Santa Clara, CA, USA) following acid digestion, with detection limits of 0.25 mg·kg⁻¹ for most metals and 0.05 mg·kg⁻¹ for Cd. Labile metal forms were extracted using a diluted HCl solution prepared from hydrochloric acid, ACS reagent, 37% (Merck KGaA, Darmstadt, Germany). Mercury (Hg) was quantified by atomic absorption spectrometry (AAS) with gold amalgamation (MA-2, Nippon Instruments Corp., Kyoto, Japan; LOQ: 0.01 mg·kg⁻¹). Mussel tissue was analyzed for Pb, Cd, and As using inductively coupled plasma mass spectrometry (ICP-MS; NexION 350D, PerkinElmer, Waltham, MA, USA) after microwave-assisted digestion, with detection limits of 0.002 mg·kg⁻¹ for Cd and 0.005 mg·kg⁻¹ for Pb and As. Mercury (Hg) was determined by AAS (MA-2, Nippon Instruments Corp., Japan; LOQ: 0.005 mg·kg⁻¹). The results obtained were expressed per dry or wet weight.

2.8. Data Processing and Statistical Analysis

All univariate and multivariate analyses were performed using PRIMER v.7 and PERMANOVA+ (Primer-E Ltd., Plymouth, UK) [38].

A data matrix (taxa vs. stations) was realized with the average abundance values from replicates collected at each station and used to calculate the Bray–Curtis similarity after a square root transformation to normalize the data and downweigh the importance of the very abundant species. A shade plot was used to visualize global relationships among clusters of samples and species. Additionally, a two-dimensional representation of the comparison between the sampling stations was obtained by means of non-parametric multidimensional scaling (nMDS) with bubble plots representing five species most important for similarities between observed groups (as calculated by SIMPER routine).

The statistical significance of the spatial variations in the assemblages between stations, shown by nMDS, was tested using a 1-way analysis of similarities (ANOSIM routine). A 1-way similarity percentage procedure (SIMPER routine) was performed to further test the dissimilarities in species composition inside the designated groups and to identify the taxa most accountable for these similarities/dissimilarities.

Distance-based redundancy analysis (dbRDA), DistLM, and BEST routines were used to identify the abiotic factors that significantly co-varied with the macrozoobenthos community structure. Due to data limitations, the descriptors were divided into two subsets: environmental variables [depth, distance from wreckage (Dist), water temperature (T), sediment temperature (T sed.), salinity (S), oxygen concentration (O2), sediment pH (pH sed.), and sediment redox (Redox sed.)] for 18 stations from 2023 and chemical variables (TPH; metals: Cu, Zn, Ni, Cd, Pb, Hg; fluoranthene; indeno(1, 2, 3,-cd)pyrene; benzo(g, h, i)perylene; benzo(a)pyrene; benzo(b)fluoranthene; TBT; DBT; MBT) for 7 stations from 2023. Both data sets were normalized.

3. Results and Discussion

3.1. Macrozoobentos

3.1.1. Qualitative Composition

The macrozoobenthos samples collected at 10 stations in April 2016 contained a total of 13 taxa, representing eight classes, Polychaeta, Clitellata, Thecostraca, Malacostraca, Insecta, Gastropoda, Bivalvia, and Gymnolaemata, and one phylum (Priapulida). In contrast, the samples collected at 16 study stations and two reference stations in June 2023 revealed a total of 29 taxa, belonging to ten classes, Hydrozoa, Turbellaria, Polychaeta, Clitellata, Thecostraca, Malacostraca, Insecta, Gastropoda, Bivalvia, and Gymnolaemata, and one phylum (Priapulida).

In April 2016, the highest number of taxa (11) was recorded at station 17-120, while the lowest (5) was observed at station 10-140. The taxa classified as absolutely constant (76–100%) included Oligochaeta, Hydrobiidae, Macoma balthica, Mya arenaria, Hediste diversicolor, and Corophium volutator. In June 2023, the highest taxonomic richness (20 taxa) was recorded at station 1, while the lowest (8 taxa) was observed at station 37-20. The absolutely constant taxa in the study area were Macoma balthica, Bylgides sarsi, Hydrobiidae, Diastylis rathkei, Hediste diversicolor and Mya arenaria. At the reference stations, the number of taxa reached 11 at REF_1 and 9 at REF_2, with Halicryptus spinulosus, Macoma balthica, Mya arenaria, Bylgides sarsi, Diastylis rathkei, Peringia ulvae, and Potamopyrgus antipodarum classified as absolutely constant.

During the 2023 analyses, the presence of the North American mud crab Rhitropanopeus harrisii was recorded. However, due to its mobility and potential escape from the sampling device (van Veen grab), its presence in the samples was considered incidental, and therefore the species was excluded from the quantitative analyses.

The Gulf of Gdańsk, including Puck Bay (both Inner and Outer), is an area under strong anthropogenic influence, which can significantly affect the organisms inhabiting it. In the 1990s, a reduction in macrozoobenthos was observed in the eastern part of Outer Puck Bay. These changes were attributed to oxygen deficiency in the water and the presence of hydrogen sulfide in the sediment [39,40,41]. Later, in 2000, a decline in macrozoobenthos was noted across the entire muddy bottom of Outer Puck Bay. In the open waters of the Gulf of Gdańsk, changes in the macrozoobenthic structure have also been observed over the years, but these trends are hard to link to broader changes occurring in the open Baltic Sea [42].

The taxonomic composition of macrozoobenthos at the surveyed stations (mostly muddy seabed with sand admixture) in 2016 and 2023 was typical for seabed habitats in this region of the Gulf of Gdańsk. Species recognized as characteristic of this type of seabed included Macoma balthica, Hediste diversicolor, Mya arenaria, Diastylis rathkei, and Halicryptus spinulosus [43,44], all of which were consistently present in the studies presented in this article. These species are known for their high tolerance to organic matter pollution [44]. Macrozoobenthos surveys conducted in 1979 also confirmed a taxonomic composition typical for this seabed type [45]. Despite the observed reduction in macrozoobenthos in Outer Puck Bay, no complete disappearance of any species was recorded between 1979 and 2014. In 2000, the presence of a non-native polychaete, Marenzelleria sp., was noted in the samples, significantly contributing to the overall macrozoobenthos abundance [43].

Macrozoobenthos studies conducted in the open waters of the Baltic Sea, near another wreck—the Sleipner—revealed the presence of not only species more tolerant to environmental and anthropogenic factors but also sensitive species, suggesting favorable environmental conditions in the sediment [46]. In contrast, studies conducted in 2016 and 2023 near the S/s Stuttgart wreck showed a dominance of opportunistic species, characterized by a high degree of resistance to unfavorable environmental conditions and typically inhabiting sediments with a high organic matter content.

3.1.2. Quantitative Composition

In 2016, the highest macrozoobenthos abundance was recorded at station 09-140 (5780 ind. m⁻²), where gastropods from the Hydrobiidae family, Oligochaeta, and the bivalve Macoma balthica were the dominant taxa. The lowest abundance was noted at station 37-340 (2590 ind. m⁻²). In the study area in 2023, the highest average macrozoobenthos abundance was again recorded at station 09-140 (7350 ind. m⁻²), with Hydrobiidae gastropods, bivalves (mainly M. trossulus), and Oligochaeta being the dominant groups. The lowest average abundance was observed at station 10-140 (677 ind. m⁻²). At the reference stations, the average abundance was 1260 ind. m⁻² at REF_2 and 753 ind. m⁻² at REF_1 (Figure 2).

In 2023, a decrease in macrozoobenthos abundance was observed compared to 2016 at most stations, except for station 09-140, located just north of the S/s Stuttgart wreck. This station had the highest abundance among all surveyed sites in both years, with an even higher density recorded in 2023 than in 2016. This increase was primarily driven by the high occurrence of Hydrobiidae snails and M. trossulus mussels at this station in 2023 (Figure 2).

The composition of the macrozoobenthos is presented in pie charts below (Figure 3). In both 2016 and 2023, the community was dominated by Hydrobiidae snails and bivalves, including M. balthica, a species tolerant of polluted waters [23], and M. trossulus, which typically forms aggregations attached to the substrate. In 2023, the polychaete Hediste diversicolor also had a significant share in the community structure. Taxa with an abundance below 1% were classified as “varia”, and they accounted for 1.9% (5 taxa) in 2016 and 3.4% (13 taxa) in 2023.

Across the 10 stations sampled in both 2016 and 2023, macrozoobenthos abundance ranged from 2710 ind. m⁻² (station 37-20) to 5780 ind. m⁻² (station 09-140) in 2016 and from 677 ind. m⁻² (station 10-140) to 7350 ind. m⁻² (station 09-140) in 2023. In 2016, Hydrobiidae gastropods (1520–2890 ind. m⁻²) and M. balthica bivalves (1010–1870 ind. m⁻²) dominated the stations. Similarly, in 2023, Hydrobiidae gastropods (187–2955 ind. m⁻²), M. balthica (117–720 ind. m⁻²), and H. diversicolor polychaetes (187–383 ind. m⁻²) were the most abundant taxa (Figure 4).

Over previous decades, studies conducted northeast of the S/s Stuttgart wreck (1978–1979, 2000–2001, and 2014) reported M. balthica abundance ranging from 40 ind. m⁻² to 2542 ind. m⁻² [43]. However, those studies did not report Hydrobiidae gastropod dominance. In contrast, our study found Hydrobiidae gastropods and M. balthica to be dominant in 2016, with M. balthica abundance ranging from 1010 ind. m⁻² to 1870 ind. m⁻², suggesting similar values to those recorded in 2014. In 2016, station 29-540 was primarily dominated by the amphipod Corophium volutator (1400 ind. m⁻²), which prefers sediment rich in detritus, its main food source [47]. In 2023, Hydrobiidae snails and M. balthica remained dominant but at lower abundances, while H. diversicolor, a common Baltic polychaete known for its high tolerance to variations in oxygenation, temperature, and salinity, became a significant component of the community. This omnivorous polychaete exhibits diverse feeding strategies depending on the trophic conditions of its habitat [48].

In the 2023 study, the reference stations were dominated by the bivalve M. balthica (41.2%) and Hydrobiidae gastropods (13.0%). Taxa with an abundance not exceeding 1% (15 taxa) accounted for 1.7% of the total abundance (Figure 5). Another significant presence was the amphipod Pontoporeia femorata (710 ind. m⁻² at REF_2). This soft-bottom species, which feeds on organic matter and tolerates low oxygen conditions [49], was previously identified as dominant in studies from 1978 to 1979, 2000 to 2001, and 2014. Its abundance in 2023 was comparable to historical data (not exceeding 450 ind. m⁻²) [43].

A comparison of macrozoobenthos biomass at individual stations in 2016 and 2023 is presented in Figure 6. In 2016, the highest biomass was recorded at station 17-120 (282.610 g WW m⁻²), primarily due to the dominance of the bivalves M. trossulus and M. balthica. In contrast, in 2023, the highest biomass values were observed at stations 09-140 (808.151 g WW m⁻²), 1 (789.395 g WW m⁻²), and 2 (645.375 g WW m⁻²), where M. trossulus was the predominant species. The lowest biomass values were recorded at station 19-140 (34.640 g WW m⁻²) in 2016 and at station 10-140 (42.048 g WW m⁻²) in 2023. The reference stations exhibited lower biomass compared to the highest values in the study area, with station REF_2 showing a moderate biomass level (133.659 g WW m⁻²), while station REF_1 had the lowest recorded value (29.802 g WW m⁻²). The observed variations in biomass distribution between the years may indicate shifts in community composition and environmental conditions affecting the study area.

The pie charts in Figure 7 illustrate the dominance structure of macrozoobenthos biomass in 2016 and 2023. In both years, M. balthica and M. trossulus were the key contributors, with M. balthica dominating in 2016 and M. trossulus becoming the predominant species in 2023. The polychaete H. diversicolor also had a significant share in 2023, likely due to the presence of larger individuals, as this species reaches its maximum size in organic matter-rich environments [44]. Taxa whose biomass contribution did not exceed 1% were classified as “varia”, accounting for 1.5% (5 taxa) in 2016 and 2.5% (22 taxa) in 2023.

The biomass dominance structure of macrozoobenthos at 10 common stations surveyed in 2016 and 2023 is presented in Figure 8. Across these stations, macrozoobenthos biomass ranged from 34.640 g WW m⁻² (19-140) to 282.610 g WW m⁻² (17-120) in 2016 and from 42.048 g WW m⁻² (10-140) to 808.151 g WW m⁻² (09-140) in 2023. In 2016, bivalves, primarily M. balthica, dominated most stations, with biomass values ranging from 27.280 to 92.310 g WW m⁻². M. trossulus was the dominant species at two stations: 17-120 (150.280 g WW m⁻²) and 09-140 (134.796 g WW m⁻²). In 2023, M. balthica continued to be the dominant species at most stations, with a biomass ranging from 23.125 to 62.330 g WW m⁻². However, Mytilus trossulus exhibited significantly a higher biomass at two stations: 09-140 (716.393 g WW m⁻²) and 19-140 (201.478 g WW m⁻²) (Figure 8).

Comparing these findings to previous macrozoobenthos studies conducted in Outer Puck Bay, northeast of the S/s Stuttgart wreck, in 1978–1979, 2000–2001, and 2014, a similar dominance of the tolerant and widespread bivalve M. balthica was observed. In those studies, its biomass did not exceed 480 g WW m⁻² [43], which is higher than the values recorded in the S/s Stuttgart wreck area in 2016 and 2023. However, M. trossulus generally reaches larger sizes than M. balthica, which likely explains the substantial differences in biomass structure at stations where M. trossulus was recorded in higher numbers.

The dominance structure of macrozoobenthos biomass at the reference stations in June 2023 is presented in Figure 9. At these stations, M. balthica was the dominant species in terms of biomass. The polychaete H. diversicolor also had a significant share. Taxa whose biomass contribution did not exceed 1% were grouped into the “varia” category, which accounted for a total of 2.1% (21 taxa).

In Outer Puck Bay, during the studies conducted in 1978–1979, 2000–2001, and 2014, priapulids (H. spinulosus), known for their tolerance to oxygen depletion in near-seabed waters [50], dominated the biomass structure. The highest recorded biomass values for priapulids in those studies reached 12.520 g WW m⁻² [43]. In comparison, in the 2023 survey, at the reference stations, the H. spinulosus biomass ranged from 0.111 g WW m⁻² to 7.357 g WW m⁻², which is slightly lower than historical records.

When comparing the reference stations to the other study sites sampled in both 2016 and 2023, a distinct difference in biomass composition and overall biomass values is evident. The reference stations were characterized by the dominance of M. balthica and had lower biomass values (133.659 g WW m⁻² at REF_2 and 29.802 g WW m⁻² at REF_1) compared to most other stations. In contrast, at sites near the S/s Stuttgart wreck, particularly station 09-140, M. trossulus exhibited significantly higher biomass values, reaching 808.151 g WW m⁻² in 2023—over six times higher than at REF_2 and more than 27 times higher than at REF_1.

3.1.3. Ecological Quality Assessment Based on B Index

Figure 10 presents the distribution of the B index at various stations in the S/s Stuttgart wreck area in 2016 and 2023. In April 2016, the B index values ranged from 1.91 (37-340) to 2.56 (09-140), with an average of 2.15 for the 10 common stations, indicating a poor ecological status. By June 2023, the lowest B index value in the study area was recorded at station 17-120 (2.24), while the highest was at station 4 (3.07), with an overall average of 2.72, corresponding to a moderate ecological status. At the common stations in 2023, the B index ranged from 2.24 (17-120) to 3.02 (05-20), averaging 2.63, still indicating a poor ecological status. At the reference stations, the average B index was 2.79, with the highest value at REF_2 (2.94) and the lowest at REF_1 (2.64), reflecting a moderate ecological status. In 2016, all surveyed stations exhibited poor ecological conditions. In contrast, in 2023, ecological status ranged from poor to moderate, with an improvement observed at three stations (05-20, 09-140, and 37-20) near the S/s Stuttgart wreck. Despite a general increase in the B index value in 2023, the average index for the study area still indicates poor ecological quality (Figure 10).

The observed improvement at certain stations suggests a localized recovery of benthic communities, potentially influenced by changes in environmental conditions or organic matter availability. However, the overall ecological status remains poor, emphasizing the need for continued monitoring to assess long-term trends and potential anthropogenic or natural drivers of change in the wreck area.

3.1.4. Macoma balthica Condition Index

The following charts (Figure 11 and Figure 12) present the results of condition indices CI1 and CI2 at six stations in the study area and two reference stations. The CI1 index values at stations 1–6 ranged from 12.89 ± 4.89 at station 6 to 18.80 ± 6.60 at station 5, with the highest mean value of CI1 observed at the reference station REF_2 (20.24 ± 6.78). The CI2 index showed a similar trend for the lowest mean value, recorded for bivalves from station 6 (CI2—7.06 ± 2.73). The highest CI2 values were recorded at stations 2 and 3 (10.34 ± 2.48 and 10.34 ± 4.41, respectively), as well as at reference station REF_2 (10.47 ± 2.77).

Morphometric condition indices can be used to describe the physical state of a population regardless of location, season, or population structure [51]. The CI1 index is based on the relationship between dry body mass and shell mass, allowing for the assessment of physiological activity levels under different environmental conditions. The use of dry tissue mass eliminates errors caused by fluctuations in water content. The shell represents the cumulative growth of the bivalves as a product of its secretory capacity, whereas soft tissue reflects metabolic and reproductive activity. The CI2 index is based on the ratio of dry body mass to shell length cubed, with very low values indicating the cessation of gamete production and the redirection of metabolic effort toward survival [52]. Condition indices also monitor the deleterious effects of various contaminants and diseases [53]. The condition index, which is at its lowest when gonads are depleted, increases during the reproductive development phase [54].

In the study area, sediments contained substances particularly harmful to benthic organisms, including petroleum hydrocarbons, metals (cadmium, lead, mercury), TBT, DBT, MBT, and polycyclic aromatic hydrocarbons (PAHs) such as fluoranthene, indeno(1,2,3-cd)pyrene, benzo(g,h,i)perylene, and benzo(a)pyrene. The spatial and temporal distribution of organisms may be influenced by various natural factors that interfere with establishing clear connections between contaminant exposure and biological effects [55]. Environmental instability, combined with excessive levels of the above-mentioned toxic compounds, may create stress conditions for bivalves, which is reflected in their physiological condition, indirectly inhibiting gametogenesis and reproduction while increasing mortality. Copper inhibits bivalves’ oocyte development and vitellogenesis. Zinc, though less toxic than copper, suppresses oocyte development and causes gamete lysis. Cadmium, in turn, inhibits gametogenesis only in its early stages [56,57]. Butyltin compounds are strong endocrine disruptors in benthic organisms. PAHs are also of concern due to their persistence and potential to accumulate in bivalves’ soft tissues [58].

In 2023, the concentrations of bioavailable metals such as copper, zinc, and cadmium in the sediments near the S/s Stuttgart wreck were slightly higher compared to the open waters of the Baltic Sea unaffected by fuel spills. Condition index values of bivalves generally fluctuate seasonally, increasing from March to May and decreasing during summer and early autumn after the spawning period [59]. The condition index values for Macoma balthica in June 2023 could have been influenced both by the biological effort associated with gamete production and release and by the impact of the aforementioned pollutants, leading to reduced values. Comparing the M. balthica condition indices obtained in this study with literature data, it is noteworthy that they were lower than those recorded in 2003 for the same month and depth range at station SOPOT-30 (30 m) in an oil spill-free area of the Gulf of Gdańsk, where CI1 was approximately 20 and CI2 around 15 [60]. In contrast, Sokołowski (1999) [21] reported CI2 values for M. balthica in the Vistula estuary ranging from 7.4 to 17.3. Honkoop and van der Meer 1997 [52] identified a critical CI2 value of 5.6 for M. balthica in the Wadden Sea, below which gamete production ceases, and all metabolic effort is directed toward survival.

3.2. Chemical Contaminants in Sediments and Mytilus trossulus Tissues: Insights into Marine Pollution

3.2.1. PAHs

The studies conducted in 2023 at the M. trossulus sampling sites (stations 1–6) revealed that the total PAH concentrations in the analyzed sediments ranged from 1.28 to 4.4 mg·kg⁻¹ DW, with the highest value recorded at station 3. In the reference point REF_1, the concentration was lower (0.89 mg·kg⁻¹ DW) (Figure 13a,b). A comparison of these results with data from 2016 indicates a decrease in PAH concentrations in the immediate vicinity of the wreck, where previous studies reported significantly higher PAH and TPH levels [6]. This decreasing trend is consistent with reports from other regions of the southern Baltic Sea, where systematic reductions in PAHs have been observed over the past decades, likely as a result of environmental regulations, improvements in maritime transport standards, and reduced industrial discharges [61].

In the context of bottom sediments in the southern Baltic Sea, where PAH concentrations ranged from 0.01 to 7.0 mg·kg⁻¹ DW [63], the obtained results are comparable, although in some cases, higher values were observed, particularly at station 3. Compared to sediments from the Gulf of Gdańsk, where PAH concentrations ranged from 0.235 to 2.205 mg·kg⁻¹ DW [64] and up to 2.24 mg·kg⁻¹ DW [65], the studied sediments in some locations exhibited higher values, especially around station 3 (

Table 1. The range of PAH concentrations in bottom sediments, given in mg·kg⁻¹ DW.

Compound	Concentration Range (Station 1–6)	REF_1 Station
Naphthalene	0.007–0.038	0.014
Acenaphthene	0.008–0.022	0.016
Acenaphthylene	0.004–0.012	0.002
Fluorene	0.010–0.042	0.009
Phenanthrene	0.068–0.200	0.050
Anthracene	0.016–0.221	0.012
Fluoranthene	0.152–0.660	0.117
Pyrene	0.105–0.440	0.076
Benzo(a)anthracene	0.091–0.340	0.069
Chrysene	0.074–0.320	0.053
Benzo(b)fluoranthene	0.007–0.038	0.122
Benzo(k)fluoranthene	0.087–0.440	0.088
Benzo(a)pyrene	0.110–0.390	0.075
Indeno(1,2,3-c,d)pyrene	0.100–0.560	0.073
Dibenzo(a,h)anthracene	0.016–0.218	0.024
Benzo(g,h,i)perylene	0.071–0.285	0.093

).

Notably, the concentrations of PAHs in sediments near station 3 exceeded typical levels reported for the Gulf of Gdańsk by Ruczyńska et al. (2011) [66], where ∑15PAHs ranged from 29.3 to 103 µg·kg⁻¹ DW (i.e., 0.029 to 0.103 mg·kg⁻¹ DW). In the present study, concentrations reached up to 4.4 mg·kg⁻¹ DW, indicating that sediments in the vicinity of the wreck are heavily contaminated compared to background levels in the region. This suggests the presence of localized historical contamination or ongoing inputs, which may be linked to the degradation of wreck structures or residual fuels. Similarly elevated PAH concentrations have been reported in other coastal regions worldwide. In Brunei Bay, Σ16PAH levels in sediments ranged from 0.83 to 2.96 mg·kg⁻¹ DW, indicating significant anthropogenic inputs [67]. Along the west coast of Peninsular Malaysia, the mangrove oyster Crassostrea belcheri accumulated PAHs at concentrations between 0.31 and 2.23 mg·kg⁻¹ DW, confirming its suitability as a biomonitor species for assessing PAH bioavailability in the aquatic environment [68,69].

The analysis of persistent organic pollutants in M. trossulus tissues revealed a significant decrease in PAH concentrations in 2023 compared to 2016 (Figure 14). In 2016, the highest levels of fluoranthene, benzo(a)pyrene, and anthracene were recorded at stations 1 and 3. By 2023, the concentrations of these compounds had declined, although fluoranthene remained highest at station 1 and lowest at station 3. Despite the overall reduction in PAH concentrations, fluoranthene, anthracene, and benzo(a)pyrene levels in the soft tissues of Mytilus edulis in the studied area remained higher than those reported for Pomeranian Bay [70]. Interestingly, similar bioaccumulation patterns in mussels have been described by Karlson and Faxneld (2021) [71], who demonstrated that bivalves exposed to chronic pollution can accumulate significant levels of polycyclic aromatic hydrocarbons (PAHs), reflecting environmental contamination gradients. The present results confirm that, although PAH levels in mussel tissues have decreased since 2016, localized contamination persists, especially at station 1.

A comparison of PAH concentrations in sediments and mussel tissues at different stations showed that the highest PAH levels in sediments did not always correspond to elevated concentrations in organisms. For example, in 2023, station 3 had the highest total PAH concentration in sediments, yet the highest fluoranthene levels in M. trossulus tissues were recorded at station 1. Such discrepancies between sediment and biota contamination are well documented in the literature and can be explained by variations in PAH bioavailability, sediment characteristics (e.g., grain size, organic carbon content), and the metabolic capacity of organisms [66]. Moreover, it is known that Mytilus species selectively accumulate certain low-molecular-weight PAHs, while high-molecular-weight compounds tend to remain sediment-bound and are less bioavailable to filter feeders. This discrepancy may be attributed to differences in PAH bioconcentration mechanisms, metabolism in organisms, and the bioavailability of these compounds in sediments [70]. Additionally, the reduction in PAH levels in mussel tissues compared to previous years may suggest ongoing biodegradation processes or a decline in new emission sources. A decline in PAH concentrations in M. trossulus tissues in 2023 compared to 2016 suggests a reduced exposure of organisms to these compounds.

3.2.2. PCBs

The obtained PCB concentration results in bottom sediments from the wreck site fall within the range of values reported for the Gulf of Gdańsk and the Gdańsk Deep and are comparable to literature data [64,72]. PCB concentrations in the analyzed samples (0.0023–0.0143 mg·kg⁻¹ DW) are slightly higher than those reported for the western Baltic Sea (0.0019–0.0025 mg·kg⁻¹ DW) but similar to concentrations observed in the Gulf of Gdańsk (0.0003–0.0122 mg·kg⁻¹ DW) and the Gdańsk Deep (0.005 mg·kg⁻¹ DW). Significantly higher values were recorded near the port of Gdynia (14 mg·kg⁻¹ DW) and the Vistula River mouth (7 mg·kg⁻¹ DW) [73]. The results do not indicate the wreck as a significant source of PCB contamination in the studied area.

PCB contamination levels in the soft tissues of M. trossulus collected near the S/s Stuttgart wreck in 2023 are presented in Figure 15. Concentrations ranged from 3.3 to 8.5 ng·g⁻¹ WW (0.0033–0.0085 mg·kg⁻¹ WW), with the highest values recorded at station 4. Compared to the Gulf of Gdańsk, where the total PCB concentration in Mytilus spp. was 10.0–11.7 ng·g⁻¹ WW (0.0100–0.0117 mg·kg⁻¹ WW) [74,75], levels near the S/s Stuttgart wreck were lower. Literature data indicate that PCB concentrations in mussels are highest in the Gulf of Gdańsk near the Vistula River mouth and significantly lower at sites located near the fishing grounds of Władysławowo [76]. Thus, PCB levels in 2023 near the Stuttgart wreck are relatively low.

The analysis of PCB concentrations in bottom sediments and M. trossulus soft tissues does not indicate a clear correlation between their presence in both matrices. PCB values in sediments exhibit some spatial variability, but this does not directly translate into contamination levels in filter-feeding organisms. For example, the highest PCB concentrations in sediments do not always correspond to the highest values in mussel tissues, suggesting that the primary source of these compounds for M. trossulus is likely the water column, rather than direct contact with sediments. The lack of a clear correlation may result from differences in the bioavailability of individual PCB congeners, their solubility, and their ability to adsorb onto organic and mineral particles. As filter-feeding organisms, mussels accumulate PCBs mainly from the water column, with bioaccumulation levels depending on the hydrophobic properties of the compounds and their composition in dissolved and suspended fractions. Additionally, PCB levels in tissues are influenced by metabolic processes and differences in the elimination rates of individual congeners. Similar patterns have been observed in other regions of the Baltic Sea, where PCB concentrations in mussels were higher in areas with intense riverine inflows and anthropogenic influences than in locations with low contamination sedimentation dynamics. In the context of the S/s Stuttgart wreck study, the obtained results suggest that its presence is not a dominant source of PCBs in the area, and contamination levels in mussels remain relatively low compared to other parts of the Baltic Sea.

3.2.3. TPH

The TPH concentrations at the studied sites (stations 1–6) ranged from 38 to 278 mg·kg⁻¹ DW, while at the reference site (REF_1), the concentration was 104 mg·kg⁻¹ DW. According to previous studies conducted in this region [78], TPH content in bottom sediments from the Gdynia disposal site (Gulf of Gdańsk), adjacent to the approach channel to the Port of Gdynia, ranged from 5 to 60 mg·kg⁻¹ DW.

In general, TPH concentrations in sediments from the Polish sector of the southern Baltic Sea vary between <5 mg·kg⁻¹ DW and 25 mg·kg⁻¹ DW, while in bottom sediments from Polish ports, oil content fluctuates between 300 and 625 mg·kg⁻¹ DW [79]. In sediment samples collected in 2016 from the immediate vicinity of the S/s Stuttgart wreck, TPH concentrations were significantly higher, ranging from 94 to 11.529 mg·kg⁻¹ DW [6].

The current results indicate a considerable reduction in TPH concentrations compared to 2016, which is consistent with the expected natural attenuation processes. It is well documented that petroleum hydrocarbons, particularly low- to mid-range fractions, undergo degradation in marine sediments through microbial activity, oxidation, and hydrodynamic dispersion [80]. Biodegradation is facilitated by the availability of oxygen and the presence of hydrocarbon-degrading microbial communities, which have been previously identified in sediments impacted by oil contamination [81]. Additionally, natural water movements, such as currents, waves, and sediment mixing, can contribute to the removal or relocation of TPH-contaminated sediments. These physical processes, combined with microbial degradation, likely explain the significant reduction in TPH concentrations observed near the wreck over the past several years.

3.2.4. Butyltins (BTs)

In 2023, seven sediment samples were analyzed for the presence of butyltin compounds. TBT (<0.01 mg·kg⁻¹ DW) and DBT (<0.001 mg·kg⁻¹ DW) concentrations were below the limit of quantification (LOQ) at all stations, whereas MBT was detected at levels ranging from 0.0001 to 0.0110 mg·kg⁻¹ DW, with the highest values recorded at station 1.

Despite the EU ban on TBT-based antifouling paints since 2003 (Regulation No. 782/2003 OJ.EU.L.2003.115.1), these compounds persist in the marine environment and degrade into less-toxic but still harmful forms [82]. Previous studies have reported higher concentrations of these substances in port sediments and near shipping lanes. In the Arkona Basin, TBT concentrations ranged from 0.003 to 0.006 mg·kg⁻¹ DW, while in the Gulf of Gdańsk, its levels reached 0.02 mg·kg⁻¹ DW [83,84]. Significantly higher values have been observed in port sediments, e.g., in Gdynia (2.7 mg·kg⁻¹ DW) and near the shipyard in Gdańsk (up to 8.5 mg·kg⁻¹ DW) [14,85].

Some Baltic countries have established sediment quality standards for TBT. In Denmark, the first threshold value is 0.007 mg·kg⁻¹ DW, while in Finland, concentrations below 0.005 mg·kg⁻¹ DW are classified as the cleanest (first class), whereas levels exceeding 0.1 mg·kg⁻¹ DW belong to the fourth sediment quality class [86]. Sweden, in collaboration with HELCOM, initially set the environmental quality standard (EQS) at 1.6 µg·kg⁻¹ DW, later lowering it to 1.3 µg·kg⁻¹ DW (0.0013 mg·kg⁻¹ DW) [58,87].

The 2023 results indicate that the studied sediments fall within the two cleanest classes in Finland and remain below the first threshold in Denmark. Due to methodological limitations, it was not possible to determine TBT at the levels required by HELCOM. However, considering the detected MBT values, it can be assumed that overall butyltin contamination in the study area is low.

3.2.5. Metals, Metalloids, and Labile Forms of Metals

The concentrations of metals and metalloids in sediment samples collected in 2023 at stations 1–6 and at the reference station REF_1 are presented in Figure 16.

The spatial distribution of heavy metals corresponds, to some extent, with the variation in organic matter content, expressed as total organic carbon (TOC), which is known to influence the retention of metals in sediments [88]. The highest concentrations of metals were recorded at stations 4, 5, and 6, where TOC levels were also among the highest: 41,118 mg/kg, 28,721 mg/kg, and 30,019 mg/kg, respectively. In particular, station 4 showed both the highest TOC and the highest concentrations of As, Zn, Cd, Cu, and Pb.

Conversely, the lowest heavy metal content was observed at station 1, which also had the lowest TOC value (5935 mg/kg). This pattern supports the hypothesis that organic matter in sediments may play a significant role in metal accumulation, as many heavy metals are known to bind with organic compounds. The relatively high TOC level at the reference station (27,341 mg/kg) accompanied by moderate metal content may further confirm the influence of localized anthropogenic inputs near the wreck.

The analysis of the graphs indicates that the highest concentrations of heavy metals were recorded at stations 4, 5, and 6, where Zn, Cr, Cu, Ni, and Pb levels are particularly high, in some cases significantly exceeding those from other locations. In particular, station 4 exhibited the highest concentrations of As, Zn, Cd, Cu, and Pb. The lowest metal content was observed at station 1, suggesting minimal contamination impact in this area.

From an environmental perspective, the labile form of metals is crucial, as they can desorb into the water and accumulate in benthic organisms [89]. Near the S/s Stuttgart wreck, Pb (78%), Zn (61%), Cu (61%), and Cd (69%) exhibit significant bioavailability, while Cr (21%), As (21%), Hg (47%), and Ni (40%) are mostly bound in stable forms.

Comparing these results with literature data, the metal concentrations in the studied area fall within the typical range for sandy sediments in the Gulf of Gdańsk [65,78]. However, given that samples were taken from a potentially contaminated site, localized elevated values may be expected. Previous studies have shown that Zn, Cu, and Cd concentrations in the Gulf of Gdańsk are higher than in open Baltic waters, reaching Zn 5–60 mg·kg⁻¹ DW, Ni 2.5–16 mg·kg⁻¹ DW, Cu 1.5–9 mg·kg⁻¹ DW, and Cd 0.05–0.9 mg·kg⁻¹ DW [90]. For comparison, studies near other wrecks in the outer part of Puck Bay did not reveal significant heavy metal contamination [91].

The concentrations of metals in the soft tissues of Mytilus trossulus mussels collected near the wreck in 2023 and 2016 are shown in Figure 17.

Pb and Cd concentrations in 2023 were lower than in 2016. The mean Pb values decreased from 0.2652–0.2856 mg·kg⁻¹ DW (2016) to 0.183–0.258 mg·kg⁻¹ DW (2023), while Cd decreased from 0.278–0.331 mg·kg⁻¹ DW to 0.133–0.142 mg·kg⁻¹ DW, remaining below environmental thresholds. In contrast, Hg concentrations increased in 2023, reaching 0.026–0.069 mg·kg⁻¹ DW, compared to 0.010–0.0156 mg·kg⁻¹ DW in 2016. An exceptionally high Hg concentration of 0.493 mg·kg⁻¹ DW was recorded at the reference station (REF_1) in 2023, exceeding the HELCOM threshold (0.020 mg·kg⁻¹ DW). Arsenic concentrations in 2023 were comparable across all stations.

This trend aligns with broader regional observations in the southern Baltic Sea. Jędruch et al. (2022) [92] reported that despite a consistent decrease in atmospheric Hg deposition since the 1990s, concentrations of mercury in fish such as herring and cod began to rise in the early 2000s. This divergence was attributed to remobilization of historical mercury deposits, shifts in food web structure, and other emission-independent anthropogenic pressures. These factors may also be relevant in the studied area and could explain the persistent and increasing Hg levels in mussel tissues despite improved emission controls. Compared with other studies in the Gulf of Gdańsk, Cd and Pb concentrations in 2023 were lower than the average values recorded between 2003 and 2014, where Cd ranged from 0.218 to 0.227 mg·kg⁻¹ WW, and Hg was 0.011 mg·kg⁻¹ WW [72,89]. However, Pb concentrations in 2023 (0.218 mg·kg⁻¹ WW) were higher compared to literature data (0.150–0.157 mg·kg⁻¹ WW) [93,94]. Relative to other Baltic Sea regions, the metal concentrations in 2023 were comparable to results from the Gulf of Gdańsk, where Cerastoderma glaucum bivalves had Cd levels of 3.04–9.90 mg·kg⁻¹ DW and Pb levels of 7.6–8.0 mg·kg⁻¹ DW [95], while Macoma baltica had Cd concentrations ranging from 0.25 to 3.5 mg·kg⁻¹ DW and Pb levels of 2.2–7.6 mg·kg⁻¹ DW [90]. In 2023, Hg concentrations in soft mussel tissues ranged from 0.22 to 0.51 mg·kg⁻¹ DW, aligning with previous studies from Puck Bay [96,97]. The 2023 study also included arsenic (As), the concentrations of which in M. trossulus soft tissues ranged from 1.048 to 1.184 mg·kg⁻¹ DW at all stations, comparable to other Baltic regions and below reference values indicating poor environmental conditions [98,99].

The analysis of heavy metal concentrations in sediments and the soft tissues of M. trossulus indicates no clear correlation between metal content in both matrices. For Pb and As, mussel tissue concentrations remain relatively stable despite significant variations in sediment concentrations. In contrast, Hg and Cd show inverse relationships—high sediment concentrations do not correspond to elevated values in mussel tissues. This discrepancy may be because M. trossulus does not inhabit sediments but is an epibenthic organism attached to the substrate surface. As a result, its primary metal source is water rather than direct contact with sediments. Metal uptake in mussels is therefore more dependent on their bioavailability in the water column than on sediment concentrations. Other factors, such as metal chemical speciation or hydrodynamic conditions, may also influence bioaccumulation. These findings highlight the importance of analyzing both sediments and water to gain a comprehensive understanding of metal accumulation mechanisms in marine organisms. The exceedances of Hg in mussel tissues indicate potential long-term contamination risks in the area. Similar patterns of Hg accumulation in mussels have been reported for the Tagus Estuary, where sediment resuspension and hydrodynamic processes contribute to metal bioavailability in bivalves [100].

3.3. Correlations and Redundancy Analysis

The shade plot below (Figure 18) indicates clear distinctions among five groups of samples (I–V). All designated groups are strongly distinct from one another, and the differences are statistically significant (ANOSIM global R = 0.972, p = 0.001) (

Table 2. Results of ANOSIM test performed for groups of macrozoobenthos samples. Significant results (p < 0.05) are shown in red.

Groups	R	p
2016 A, 2016 B	0.829	0.0080
2016 A, 2023 D	1	0.0010
2016 A, 2023 E	1	0.0030
2016 A, 2023 C	1	0.0002
2016 B, 2023 D	1	0.0180
2016 B, 2023 E	1	0.0290
2016 B, 2023 C	1	0.0050
2023 D, 2023 E	1	0.0080
2023 D, 2023 C	0.956	0.0005
2023 E, 2023 C	0.958	0.0010
Global	0.972	0.0010

). An especially clear distinction (23.4% similarity) can be seen between groups of samples from 2016 and 2023, the latter characterized by a higher number of species and higher abundance.

Based on the SIMPER routine, two groups of samples from 2016 (I and II) are distinguished mainly by the presence of M. trossulus (43.97% dissimilarity). Sample groups from 2023 are characterized by the presence of two species responsible for at least 40% similarity. In group III, those were H. diversicolor (21.1% similarity) and Hydrobidae (20.2%) (

Table 3. Results of the SIMPER test with the average similarity for each group and contributing taxa.

Group	2016 I	2016 II	2023 III	2023 IV	2023 V
Av. Simil.	71.29	77.60	75.68	72.83	73.79
Taxa	%	%	%	%	%
Macoma balthica	62.42	41.73	19.51	8.75	36.08
Mytilus trossulus	-	21.84	-	15.24	-
Hydrobiidae	18.99	-	20.24	28.65	-
Hediste diversicolor	-	-	21.09	14.72	-
Bylgides sarsi	-	-	10.03	-	-
Oligochaeta	-	-	-	7.77	9.77
Halicryptus spinulosus	-	-	-	-	13.88
Pontoporeia temorata	-	-	-	-	13.60

); this group included mostly samples from stations localized in close vicinity of the wreckage (<400 m) at a relatively low depth (<25 m). Group IV, which included samples from stations located in close proximity but to the south and north of the wreckage, was characterized by the presence of Hydrobidae (28.7%) and M. trossulus (15.2%). Finally, group V was characterized by the presence of M. balthica (36.1%) and H. spinulosus (13.9%), and included samples from stations located to the north and far away from the wreckage (>2 km) (Figure 19).

The DistLM marginal test performed for biological and physical variables was significant for depth, distance from the wreckage (Dist.), water temperature (T), salinity (S), oxygen concentration (O2), and substrate temperature (

Table 4. Distance-based linear model (DistLM) marginal and sequential tests describing the association between environmental variables and macrozoobenthos taxonomic composition. Significant results (p < 0.05) in the marginal test are marked in red. Prop. var. = proportion of variation; Prop. cum. var. = cumulative proportion of variation.

	Marginal Test			Sequential Test
Variable	Pseudo-F	p	Prop. var.	Pseudo-F	p	Prop. var.	Prop. Cum. var.
Depth	14.2	0.0001	0.47	14.2	0.0001	0.47	0.47
Dist.	10.6	0.0001	0.40	0.9	0.4083	0.03	0.50
T	3.5	0.0239	0.18	3.5	0.0295	0.10	0.60
S	4.9	0.0068	0.23	2.6	0.0584	0.07	0.67
O2	8.2	0.0005	0.34	2.7	0.0526	0.06	0.73
T sed.	3.4	0.0244	0.18	0.3	0.9256	0.01	0.73
pH sed.	1.3	0.2672	0.07	1.4	0.2341	0.03	0.77
Redox sed.	1.4	0.2482	0.08	0.4	0.7591	0.01	0.78

). Environmental variables accounted for r² = 0.58 (DistLM adjusted r²) of the variation in species composition, and of these, depth (14% of variation) and distance from the wreckage (11% of variation) explained the most variation (50% of the cumulative variation in sequential test).

Due to the lower number of stations (7), marginal tests could not be performed for the chemical variables; therefore, only the BEST (BIO-ENV) routine was used to investigate the relationship between macrozoobenthos taxonomic composition and chemical variables. The analysis suggests that from 15 chemical variables, a combination of four variables (copper (Cu), zinc (Zn), nickel (Ni), and cadmium (Cd) concentrations) best explains the observed patterns in macrozoobenthos taxonomic composition (

Table 5. BEST (BIO-ENV) results from PRIMER 7 showing which environmental variables (up to 5 variables) best explain the observed patterns in macrozoobenthos taxonomic composition.

No. Vars	Corr. Selections
1	0.605 Cu
2	0.629 Cu, Zn
3	0.630 Cu, Ni, Hg
4	0.634 Cu, Zn, Ni, Cd
5	0.634 Cu, Zn, Ni, Cd, TBT

). No other significant relationships between biological data and chemical environment variables were found.

3.4. Assessment and Criteria of Contamination in Sediments and Mytilus Trossulus Soft Tissues

The analysis of bottom sediments from the area of the S/s Stuttgart wreck (2023) showed that the concentrations of heavy metals, PAHs, and PCBs did not exceed the limit values specified in the Polish regulations on the recovery of dredged material [61]. Comparison with international standards (OSPAR) [87] showed that PCB concentrations were below the alarm thresholds, but the concentrations of PAHs (benzo(a)anthracene, benzo(a)pyrene, dibenzo(a,h)anthracene, indeno(1,2,3-cd)pyrene) exceeded the BAC values (0.016; 0.03; 0.08; 0.103 mg·kg⁻¹), which indicates anthropogenic pollution (Figure 13). Moreover, the concentrations of Hg and metals (Cu, Cd, Pb) exceeded the BAC values (0.07; 27; 0.31; 38 mg·kg⁻¹ DW) at several points, which requires further monitoring.

The analysis of metals and PCBs in M. trossulus tissues showed that PCB, Cd, and Pb concentrations were below the limits for organisms intended for consumption. However, Hg concentration exceeded the EQS threshold value (0.0005 mg·kg⁻¹ WW), indicating the need for further monitoring of this element. Concerning HELCOM and OSPAR standards, Cd, Pb and Hg concentrations in 2023 indicated exceedances of the limit values (0.11; 0.02; 0.160 mg·kg⁻¹ WW), suggesting subGES for these metals in the study area. PAH concentrations also exceeded EQS values (fluoranthene—0.030 mg·kg⁻¹ WW, benzo(a)pyrene—0.005 mg·kg⁻¹ WW), indicating contamination. Comparison with OSPAR limits showed that metal and PAH concentrations in mussel tissues did not exceed these standards. Conclusions indicate the need to monitor contaminants, especially Hg and heavy metals, which exceeded international standards in the area of the S/s Stuttgart wreck.

3.5. Ecotoxicological Assessment of Sediments

The assessment of the ecotoxicity of bottom sediments and their water extracts conducted in 2016 and 2023 showed that most samples were non-toxic or had a very low level of toxicity. The exception was the sample from 2016 taken from an area heavily contaminated with fuel (V_EK7, Figure 1), which showed high toxicity towards V. fischeri (89%) and 100% mortality of H. incongruens. In 2023, only the sample from point 1 showed increased toxicity but did not exceed the 50% acute toxicity limit. It showed a 20% immobilization of D. magna and a 22% slowdown in root growth of S. alba, which could be related to the highest concentration of MBT, a compound with antifouling activity. The results of ecotoxicity tests did not show any correlation with laboratory chemical analyses, and the sediments with the highest concentration of pollutants were not the most toxic to indicator organisms. This may indicate the limited bioavailability of some substances in the sediments or the effectiveness of their degradation and sorption processes.

3.6. Limitations and Future Directions of the Study

This study provides valuable insights into the problem of sediment contamination and its potential biological effects in the area surrounding the S/s Stuttgart wreck. However, certain limitations should be acknowledged. The relatively small number of sediment and macrozoobenthos samples constrains the ability to fully capture spatial variability and contamination gradients. Additionally, the patchy distribution of pollutants in sediments complicates the interpretation of contamination trends and associated environmental risks.

The observed discrepancies between contaminant levels in sediments and Mytilus trossulus tissues underline the complexity of bioaccumulation processes. For PAHs, despite elevated sediment concentrations at certain stations (e.g., station 3), tissue concentrations did not follow the same pattern, likely due to differences in compound-specific bioavailability, organism metabolism, and environmental factors such as water exchange or degradation rates. Similarly, the lack of a clear correlation between sediment metal content and mussel tissue concentrations reflects the species’ epibenthic lifestyle, with primary exposure occurring through the water column rather than direct sediment contact. Metal bioaccumulation depends not only on total concentrations but also on bioavailable fractions, influenced by chemical speciation, hydrodynamics, and biological regulation mechanisms.

At station REF_1, elevated concentrations of heavy metals, particularly Hg, detected in the soft tissue of mussels, as well as high Cu and Cd concentrations in sediments, are likely to contribute to the poor ecological status of the macrozoobenthos and the low condition of M. balthica even beyond the study area, confirming the strong anthropogenic pressure on the Gulf of Gdańsk.

To better understand the long-term impacts of the wreck, continued biological and chemical monitoring is necessary. Special attention should be given to PAHs, particularly fluoranthene and benzo(a)pyrene, as well as Hg, due to their elevated concentrations in both sediments and Mytilus trossulus tissues. Long-term observation of macrozoobenthos composition and biomarker responses in indicator species could further clarify the ecological consequences of chronic exposure to contaminants originating from the wreck.

4. Conclusions

In studies conducted in 2016 and 2023 in the area of the S/s Stuttgart wreck, the taxonomic composition of the macrozoobenthos community was typical of a muddy bottom habitat with an admixture of sand, with a predominance of opportunistic species. In 2023, an increase in the total number of macrozoobenthos taxa was noted, while its abundance decreased, but at most stations, the biomass was comparable to 2016, and the ecological quality in the area of the wreck remained poor. Three separate assemblages of macrozoobenthos (near the wreck and to the east of it, to the north and south of the wreck, and at a considerable distance from the wreck) were found in 2023. Statistical analysis showed that these groups were distinguished primarily depending on the depth and distance from the wreck.

Chemical tests revealed locally elevated concentrations of heavy metals, including copper, zinc, cadmium, and lead, with the highest values recorded at a station located approx. 2 km northeast of the wreck. Despite the detection of significant concentrations of pollutants, statistical analysis did not indicate any clear correlations between the chemical composition of sediments and the structure of macrozoobenthos but proved that a combination of four variables (Cu, Zn, Ni, Cd) best explains the observed changes in the taxonomic composition of macrozoobenthos.

While the concentrations of heavy metals and PCBs in M. trossulus tissues remained within acceptable limits, mercury and selected PAHs exceeded established threshold values. At the same time, the condition indices of M. balthica were lower than in unpolluted areas. This suggests a negative impact of elevated concentrations of bioavailable metals on the functioning of benthic organisms and the formation of the ecosystem around the wreck. The results of ecotoxicological studies did not show a direct relationship between the toxicity of the samples and the highest concentrations of harmful substances, which emphasizes the need to combine chemical analyses with biological tests in assessing the impact of pollution.

Moreover, our findings indicate that contaminant accumulation in mussels does not always directly correspond to sediment contamination, due to the role of bioavailability, biological regulation, and species-specific exposure routes. Particularly, M. trossulus, as an epibenthic filter feeder, primarily accumulates contaminants from the water column rather than from sediments themselves.

Additionally, the unexpected increase in mercury concentrations at the reference station (REF_1) suggests possible diffuse or atmospheric sources of pollution, and raises the possibility that REF_1 was not appropriately selected or that local pollution sources were overlooked

Although the area of the S/s Stuttgart wreck remains under the influence of pollution, its impact on benthic organisms is complex and not always clear, which indicates the need for further monitoring, including both biological and chemical parameters.