1 Introduction

Potentially toxic elements (PTEs) such as Cd, Co, Cu, Fe, Ni, Mn, Pb, and Zn are hazardous contaminants with environmental persistence and high bioaccumulative potential, which commonly receive worldwide attention (Kabata-Pendias 2010; Alloway 2012). The amount of PTEs entering the marine environment as a result of human activity is constantly increasing. Huge amounts of pollutants are transported daily to the seas and oceans, where they undergo biochemical transformations and eventually accumulate in bottom sediments (Pempkowiak et al. 1999; Ansari et al. 2004; Virtasalo et al. 2020). There are also several sea-based activities that significantly contribute to the release of contaminants into the marine environment, such as shipping, mariculture, offshore oil and gas industry, offshore renewable energy infrastructure, seabed mining, dredging/dumping of dredged materials, and shipwrecks (Szefer 2002; Tornero and Hanke 2016).

Being in the food chain, PTEs can have a strong negative impact on marine ecosystems and human health (Revis et al. 1989; Ansari et al. 2004; Jokinen et al. 2020). For this reason, in the European Union, for example, monitoring the pollution status of marine waters and marine sediments and preventing the release of harmful substances at their source are among the priorities listed in the Marine Strategy Framework Directive (EU 2008).

The waters of the Baltic Sea are heavily polluted. Regional environmental reports (e.g., HELCOM 2018) usually list Hg, Cd, or Pb among the most important pollutants. The accumulation of metals in the Baltic Sea is mainly driven by anthropogenic loading associated with unsustainable settlement and high industrialization in the sea catchment area, as well as redox conditions of the environment. Hypoxia (oxygen deficiency in bottom waters), which regularly occurs in the Baltic Sea due to limited water exchange with the North Sea, as well as anthropogenic eutrophication, can significantly contribute to higher concentrations of elements such as Cd, Pb, and Zn in sediments (Carstensen et al. 2014; Shahabi-Ghahfarokhi et al. 2021). The effects of climate change cannot be overlooked either. Progressive climate warming can have a positive effect on the spread of anoxic areas (Kabel et al. 2012).

The metal content in the bottom sediments of the Baltic Sea shows a high spatial variability, which is mostly linked to sediment properties (texture and organic matter content), and the intensity of human impact (Zalewska et al. 2015). As reported by Uścinowicz et al. (2011), the highest concentrations of metals in surface silt–clay sediments were recorded in the northern part of the Baltic Proper region (Cd, Cr, Cu, Pb, and Zn), the Gulf of Finland (Cr, Zn), as well as the Bay of Lübeck, Cape Arkona, Bothnian Bay, and the Gdańsk Basin (Pb, Zn). High input of PTEs is associated with river waters, especially the Vistula and Oder rivers, which flow through densely populated and industrial regions (Szefer et al. 1999; Szefer 2002; Majewski et al. 2003; Robakiewicz 2010; GIOŚ 2014; HELCOM 2018; Remeikaitė-Nikienė et al. 2018). In the northern Baltic Sea region (Gulf of Bothnia), rivers drain the anthropogenically disturbed acid sulfate soils and transport large amounts of Cd, Co, Cu, Mn, Ni, and Zn to the coastal zone. It was found that metal enrichment in marine bottom sediments is visible up to 25 km seaward of river mouths (Wallin et al. 2015; Virtasalo et al. 2020). Metal accumulation in potential acid sulfate soils within the southern Baltic coast (Szczecin Lagoon) is significantly lower (Hulisz et al. 2022). Groundwater discharge, which in some regions can be higher than that delivered by river runoff, may also have some influence on the content of metals in the Baltic Sea. Astrup and Thunholm (2001) estimated the annual contribution of heavy metals by groundwater from the Stockholm agglomeration to Lake Mälaren, Baltic Brunsviken. The total fluxes for Cd, Co, Cr, Ni, Pb, and Zn were 0.2, 4.9, 3.1, 34, 28, 2.2, and 118 kg year−1, respectively. Metals fluxes to the Bay of Puck via groundwater inflow were similar (Kłostowska et al. 2020).

Magnetic susceptibility is commonly used as an indicator of environmental contamination by PTEs (Hoffmann et al. 1999; Desenfant et al. 2004; Schmidt et al. 2005; Magiera et al. 2006; Oldfield et al. 2015). Serving as a fast and non-destructive technique, it is measured in a variety of matrices, such as soils (Magiera and Strzyszcz 2000), aerosols (Hunt et al. 1984; Muxworthy et al. 2003; Sagnotti et al. 2006), industrial dust (Magiera et al. 2011; Gołuchowska et al. 2012; Oldfield et al. 2015; Rachwał et al. 2015), or biotic elements such as lichen or plant fragments (Moreno et al. 2003). In the bottom sediments of the Gulf of Mexico, hundreds of kilometers away from potential sources of industrial emissions, Doyle et al. (1976) revealed the presence of spherical magnetic particles of technogenic origin. Studies of bottom sediments in the Gulf of Elefsis, to a depth of 1 m near Athens and in the vicinity of iron and steel works, showed high values of magnetic susceptibility in the upper layers, decreasing with distance from the sources of pollution (Scoullos et al. 1979). Similar results were obtained by Puffer and Gussel (1980) who studied sediments in the vicinity of New York. Numerous studies (Chan et al. 1998; Hatfield 2014) have shown that magnetic susceptibility is also a useful environmental indicator to assess matrix saturation with metals for marine sediments. This seems to be very important due to the significant role of marine sediments in the transport or accumulation of PTEs (Moore and Ramamoorthy 1984; Tornero and Hanke 2016). In the case of the Baltic Sea, this issue remains under-researched. However, measurements of magnetic properties of the Baltic Sea sediments were used in paleoenvironmental studies (Reinholdsson et al. 2013) and spatial analysis of PTE contamination of sandy beach sediments along the Curonian Spit (Karlonienė et al. (2021). Apart from single studies (Nawrocki et al. 2000; Gwizdała et al. 2016), there are no comprehensive studies from the Polish coastal zone. Therefore, the purpose of this study was to analyze the spatial distribution of magnetic susceptibility and the content of potentially toxic elements (PTEs) in the surface layer of bottom sediments in the Vistula Canal (Gdańsk Gulf, Poland) in relation to local sedimentary conditions. Finally, we also assessed the usefulness of magnetic susceptibility measurements for the identification of changes in sediment properties caused by anthropogenic (technogenic) factors. Given the high dynamics of processes of deposition and redeposition of alluvial and marine sediments in this area, as well as long-term human impact (Graniczny et al. 2004; Hulisz et al. 2015; Kusza et al. 2018), a high site-specific spatial variability in sediment properties was expected.

2 Materials and methods

2.1 Study area

The Vistula Canal (54° 21′ 48″ N; 18° 57′ 08″ E) is an artificial estuary of the river to the Baltic Sea, built in 1890–1895. This construction is considered the largest flood control investment and the most important hydrographic change in the modern history of the Vistula River (Makowski 1995; Franz et al. 2005). The canal is about 3000 m long, 400 m wide, and up to 10 m deep. After the canal was completed, the formation of the delta cone has begun (Graniczny et al. 2004; Koszka-Maroń and Jegliński 2009; Hulisz et al. 2015). This increased the risk of ice jams and caused some problems with navigation. Therefore, further regulation works were carried out through the construction of breakwaters, which continue to be upgraded and extended.

The Vistula River is the most important source of sediment inflow to the Baltic Sea. It is estimated that it transports from 0.4 to 1.4 million tons of dredged material and 1.5 to 2.2 million tons of suspended material (solids) annually (Cyberski and Mikulski 1976). Due to the intensive industrialization and urbanization, as well as the presence of ports, the area adjacent to the river mouth has been subjected to severe anthropogenic pressure. Along with sedimentary material, the waters of the Vistula River discharge pollutants from its large catchment directly into the Baltic Sea, including nutrients, organic substances, heavy metals, and many others, which significantly affect changes in the geochemical properties of sediments in the southern Baltic basin (Szefer et al. 1999; Cyberski et al. 2006; Uścinowicz et al. 2011; Winogradow et al. 2019).

2.2 Field work

The study was carried out in the summer season of 2014 in the area of the Vistula Canal during a cruise aboard the “Oceanograf 2” ship and an inflatable boat. Sampling sites were selected for different distances from the river mouth, sediment dynamics, and bathymetry. A total of 27 surface bottom sediment samples (0–10 cm) were collected using the Van Veen sampler in four areas: delta plain (to a depth of approx. 5 m; D1–D9), delta front (to a depth of 16 m; D10–D15), prodelta (to a depth of approx. 20 m; D16–17), and open water (beyond the reach of direct river water outflow or from a depth of more than 20 m; O1–O10; Fig. 1; Supplementary Material, Table S1–1).

Fig. 1
figure 1

Location of the study area (A), sampling sites (A), and transects (A, B)

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The detailed examination of the effects of sediment textural and geochemical properties on magnetic susceptibility and PTEs content was carried out along three transects grouping the following sampling sites in the Vistula estuary region: transect I (along the central axis; D1–D2–D3–D12–O4–O7–O10)—16.6 km long, transect II (on the western side; D9–D10–O2–O8)—9.6 km long, and transect III (on the eastern side; D4–D15–O4–O6)—10.9 km long (Fig. 1). The location of the transects accounted for the asymmetry of water flow in the estuary area related to the local hydrodynamic conditions. Three subsamples were collected from each site (surface area of about 0.2 m2) and then mixed into one composite sediment sample. All samples were placed in plastic bags and transported to the laboratory.

2.3 Laboratory analysis

Air-dried, homogenized, and passed through a 2-mm sieve, the samples were submitted to the granulometric analyses using the sieve (standardized set of nylon sieves) method (Łęczyński and Szymczak 2010) combined with the pipette method. Sediment grain sizes were defined according to Wentworth (1922). The organic matter content (LoI) in the sediments was determined using the loss on ignition method at 550 °C. The content of carbonates (as CaCO3) was determined by Scheibler’s method. The pH and electrical conductivity were measured in 1:5 soil to water extract (EC1:5) using an Elmetron® CPC-411 pH/conductivity meter (Ostrowska et al. 1991). The total content of Cd, Cr, Cu, Fe, Ni, Pb, and Zn was analyzed using the AAS (Atomic Absorption Spectrometry) method with the Perkin Elmer (model AAnalyst 300, Perkin–Elmer, Waltham, MA, 02451 USA) after mineralization of the lyophilized samples in a Milestone Ethos One microwave apparatus with concentrated HNO3:HF (9:1) according to the US EPA 3052 method (US EPA 1996). Specific (mass) magnetic susceptibility (χ) was determined in the freeze-dried samples using an MS2 Bartington® laboratory magnetic susceptibility meter, equipped with a dual frequency MS2B sensor—low frequency (κlf) is 0.47 kHz, and high frequency (κhf) is 4.7 kHz. Magnetic susceptibility (κ) was expressed in dimensionless SI units. Values of specific (mass) magnetic susceptibility (χ) were given in m3 kg−1 (Thompson and Oldfield 1986). The complete raw data for all 27 sampling sites are included in the electronic supplementary material S1 (Tables S1–2 and S1–3).

2.4 Statistical data analysis

Statistical analysis of selected grain size distribution characteristics included indicators such as mean grain diameter (Mz) and sorting (σ1) expressed in phi units (Folk and Ward 1957). Diagrams of the relationship between Mz and σ1, and C/M (Passega 1964; Passega and Byramjee 1969) were made using the GRADISTAT program (Blott and Pye 2001).

These results allowed for determining the conditions of the sedimentary environment in the Vistula Canal, being helpful in interpreting the spatial variability of sediment properties.

The software package Statistica, version 13.3, was used for statistical analysis (StatSoft Inc. US 2021). As the data set did not follow a normal distribution, basic statistical parameters such as median, minimum, and maximum, as well as Spearman’s rank correlation coefficients (rs) were calculated. The nonparametric Mann–Whitney U test was used to test the significance of differences between median values. Differences at p < 0.05 were considered significant.

3 Results

3.1 Lithodynamics of the study area

The studied sediments were characterized by greatly varying values of granulometric parameters, especially regarding the two groups, which are deltaic and open-water sediments (Table 1). The percentage of the finest fraction (ø < 0.063 mm) ranged from 0.0 to 30.1 (median < 0.1%) and from 0.0 to 49.8 (median 1.4%), respectively. Less variability was recorded for median values of grain diameter (Mz) and sorting (σ1), i.e., Mz 1.5, σ1 0.7 phi for deltaic sediments, and Mz 2.3, σ1 0.7 phi for open-water sediments. The outliers of the analyzed parameters were found in samples D13 and D15 taken from the delta front, as well as samples from points O7 and O10 located farthest from the mouth of the Vistula (12.5 and 16.6 km, respectively). As shown in Table 1, the differences between the two groups of sediments were statistically significant (with p < 0.05) except for σ1.

Table 1 Minimum (min), median, and maximum (max) values of sediment parameters
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In general, the delta plain and the delta front consisted of moderately-to-moderately well-sorted medium-grained sands, while the riverbed (the main distributary channel with a jetty on either side) was dominated by moderately sorted coarse-grained sands. The diameter of the sediments decreased with distance from the river mouth. Consequently, the open-water sediment samples were dominated by fine-grained sands enriched with moderately and moderately well-sorted clay and silt fractions (Fig. 2).

Fig. 2
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Relationship between the mean grain diameter (Mz) and the sorting index (σ1) expressed in the phi scale

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The highest proportion of the fine fractions was observed below the 55 m isobath and accounted for over 35% of all fractions (samples O7 and O10) in transect I (Fig. 3). In the prodelta, sediment granulometry differed slightly in the western part (transect II) compared to the eastern one (transect III). Medium-grained sands dominated there, and the content of the < 0.063 mm fraction did not exceed 2.75% (Fig. 3), while in the eastern area, in addition to medium-grained sands, there were also fine and very fine-grained sands, and the percentage of the silt–clay fraction increased to 30% (Fig. 3).

Fig. 3
figure 3

Granulation of sediment samples along transects I, II, and III

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The varying sediment granulometry corresponded to the conditions of the sedimentary environment in the Vistula Canal as evidenced by the C/M relationship diagram (Passega 1964; Passega and Byramjee 1969). The first percentile of the grain size distribution (C) and the median (M) parameters clustered the sediment samples in fields I, II, IV, V, and VII (Fig. 4). Within the delta area (delta plain, delta front, and prodelta), the sediments were deposited by rolling with some grains transported in suspension in a highly dynamic environment (fields I and II; samples D1–D5, D7, D11–D14, and D16–D17). The depositional environment on the western side of the river mouth was more turbulent and was dominated by deposition processes from the gradation suspension (field IV; D6, D8–D10, O2, and O9). Sediment deposition due to the graded suspension regime dominated toward open waters, with some grains transported by rolling under moderate turbulence conditions (field V; O3–O5 and O8). On the other hand, deposition from homogeneous suspension occurred below a depth of 55 m in the environment with reduced dynamic activity (field VII; O7 and O10). Similar depositional conditions occurred in the shallower delta zone on the eastern side of the river mouth (sample D15; Fig. 4; Table S1–2).

Fig. 4
figure 4

C/M diagram after Passega (yellow—delta samples, blue—open water samples)

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3.2 Effects of sediment textural and geochemical properties on magnetic susceptibility and PTE content along the studied transects

Open-water sediments were richer in organic matter (LoI 0.6–8.6%, median 2.6%) and more saline (EC1:5 0.4–8.1 dS m−1, median 1.2 dS m−1) than deltaic sediments (LoI 0.2–1.4%, median 0.6%; EC1:5 0.7–4.0 dS m−1, median 0.7 dS m−1; Table 1). The opposite trend was observed for pH and carbonate content (pH 7.6–8.6, median 8.4; CaCO3 0.2–8.2%, median 0.5% for deltaic sediments and pH 7.1–8.3, median 7.6; CaCO3 0.2–3.2%, median 0.4% for open-water sediments). The highest median total content of Cr and Fe was found in deltaic sediments (2.4 and 2310 μg g−1 d.m., respectively), while the open-water sediments were characterized by the highest median content of Cu (2.1 μg g−1 d.m.), Ni (1.8 μg g−1 d.m.), Pb (5.1 μg g−1 d.m.), Zn (18.1 μg g−1 d.m.) and the highest median value of magnetic susceptibility (χ 6.6 10−8 m−3 kg−1; Table 1). Statistically significant differences (at p < 0.05) between the two groups of sediments were found only for LoI, pH, as well as Cu, Cd, and Pb content.

The results of Spearman’s rank correlation analysis showed that there was a moderate-to-strong and positive correlation between fine-grained fraction (f < 0.063 mm) and parameters such as LoI (0.71), EC1:5 (0.53), χ (0.69), Cd (0.54), Cu (0.63), Pb (0.56), and Zn (0.43; Table 2). The LoI was also positively and significantly correlated with χ (0.46), Cd (0.61), Cu (0.71), Ni (0.52), Pb (0.67), and Zn (0.50). Magnetic susceptibility showed a moderate correlation with Cd, Cu, Pb, and Zn (rs between 0.40 and 0.61). The correlations between χ-Cr and χ-Fe were very weak and statistically insignificant.

Table 2 Spearman’s rank correlation (rs) between sediment properties
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The distribution of selected PTEs along the studied transects was correlated with the percentage of the fine-grained sediment fraction (f < 0.063 mm) and mass magnetic susceptibility (χ) (Fig. 5). We observed the highest variability between the minimum and maximum values of these parameters in transect I. The distribution of the content of selected metals in relation to the granulometric parameters, the dominance of the fine-grained sediments in the samples, and the values of mass magnetic susceptibility showed a good correlation with the sediment deposition processes. The riverbed sediments and the cone sediments during intense hydrodynamic flow were characterized as coarse- and medium-grained with a low content of detected metals and a low value of mass magnetic susceptibility. Sediment samples O7 and O10 collected at 62 m b.s.l., and 68 m b.s.l. were characterized by the dominance of fine-grained fraction (< 0.063 mm) and the maximum Pb values 50.1 and 86.1 μg g−1 d.m., respectively. In this area, we also observed the highest mass magnetic susceptibility of 19.6 10−8 m3 kg−1 (Fig. 5; Table S1–S3). Transects II and III represented sediments deposited under varying hydrodynamic conditions and bottom morphology formed by the impact of the western and eastern beds. The western area along transect II, with the dominance of well-sorted medium sands, was characterized by low values of determined metals below 18.0 μg g−1 d.m. Low values of the fine-grained fraction (f < 0.063 mm), ranging from 0 (sample D9) to 2.2% (sample O8), correlated with low values of mass magnetic susceptibility, ranging from 0.8 to 4.4 10−8 m−3 kg−1 (Fig. 5). The eastern area of transect III is a deposition area of sediments with varying granulometric characteristics. The medium and fine sands were enriched with silt and loam fractions. This observation was also reflected in the distribution of metals studied in this area and the mass magnetic susceptibility. Among the metals, Cr and Pb dominate, with the values for sample D15 of 7.6 and 11.7 μg g−1 d.m., respectively (Table 1). We assumed that this result should be correlated with the significant proportion of fine-grained fraction (f < 0.063 mm) of 30.1% and the mass magnetic susceptibility of 10.6 10−8 m3 kg−1 for this sample (Fig. 5). The conditions of sediment deposition on the seabed within the Vistula Canal also affected the distribution of organic matter content (expressed in % and denoted as LoI). The highest LoI values were recorded in samples O8 (8.6%), O10 (6.1%), and O6 (6.0%) collected in the outer sediment deposition area (Figs. 1 and 5).

Fig. 5
figure 5

Variability of magnetic susceptibility and the content of trace metals in relation to the content of organic matter (LoI) and fine-grained fraction (< 0.063 mm) along the studied transects

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4 Discussion

Waters from the Vistula Canal are distributed mainly along two channels, i.e., the western and eastern channels, which are located between the delta plain and the deposited rock debris at the extension of the jetties (Wróblewski et al. 2015). Both the relief of the Vistula Canal and the nature of the sedimentary environment are related to the wave regime and the current system occurring in the Gulf of Gdansk. Wave approaches from westerly and northwesterly directions (Ostrowski et al. 2009; Pruszak and Szmytkiewicz 2015), periodic erosion, and destruction of the western part of the estuary, and what is more, the greater intensity of longshore sediment transport in an easterly direction than in a westerly direction (Pruszak 1989), cause river waters to spread around the estuary with some asymmetry in an easterly and northeasterly direction (Cyberska and Krzymiński 1989), affecting the relief of the cone and the distribution of sediment, both in the delta plain and in the prodelta (Gajewski and Rudkowski 1997; Zajączkowski et al. 2010; Pruszak and Szmytkiewicz 2015). The main morphological contours of the cone of the Vistula Canal, delineated by isobaths of 5, 10, and 15 m, corresponding to the delta plain/delta front outline, the delta front/prodelta, and the prodelta range, respectively, show spatial and temporal stability. The largest changes occur on the delta plain (to a depth of 5 m) and within the distributary channels, as manifested by the variation in the location of depressions and shallowing forms. The cone’s erosion occurs mainly in the distributary channels and at the front of the river mouth, on the western side (Wróblewski et al. 2015).

The morphodynamics of the estuary corresponds to the changing dynamics of the sedimentary environment and, consequently, to the nature of the sediments that form the cone. As evidenced by our research, the sedimentary environment in the western part of the estuary (where erosion occurs) is turbulent, and silt and clay fractions are washed away by waves and currents, while on the eastern side, the dynamic activity of the environment is reduced, allowing deposition of the fractions from suspension (Figs. 2, 3, and 4). Outside the estuary, in the open water zone, the sedimentary environment is turbulent and is associated with the periodic hydrodynamic activity of storm waves. At depths below 55 m, the dynamic activity of the environment decreases, promoting deposition from suspended solids. Our study showed that these processes were reflected in significant spatial heterogeneity of sediment properties. This was mainly confirmed by the extremely high values of analyzed parameters recorded in spots, many times higher than the median values (Table 1). It indicates high dynamics of changes in the chemistry and granulometry of the sediments, both in the delta plain (Cd, Cr, Cu, Pb, f < 0.063 mm, CaCO3, and EC1:5) and for open waters (Cd, Cu, Ni, Pb, f < 0.063 mm, CaCO3, and EC1:5).

Magnetic susceptibility was strongly correlated with organic matter content, grain size f < 0.063 mm, Cd, Cu, Pb, and Zn. The strong correlation between magnetic susceptibility and concentrations of several PTEs may suggest a significant anthropogenic (technogenic) impact on magnetic properties in the top layers of the sediments, especially in the open water area. The high correlation between heavy metals may also indicate that they have originated from similar contaminant sources. The selectivity of clay minerals and hydrous oxide absorbents for a given metal may explain the relative order of correlations of these metals with magnetic susceptibility, which generally corresponds to the sequence Cu > Pb > Cd > Zn (Chan et al. 1998). However, no significant relationship was found between magnetic susceptibility values and relatively high iron content. This resulted from the lack of ferromagnetic iron forms of technogenic origin due to the presence of other iron forms, such as sulfides (paramagnetic properties), which are formed in the environment of marine sediments with a limited supply of oxygen. Redox conditions are also conducive to natural processes of enrichment with Cu, Ni, and Zn which are adsorbed on the surfaces of Fe–Mn concretions (Szefer 2002). Low values of magnetic susceptibility recorded in some parts of the study area, not exceeding 11.2·10−8 m−3 kg−1, indicate the magnetic properties of materials naturally forming the top layer of sediments in the delta plain. Magnetic enhancement, expressed by the magnetic susceptibility value almost twice as high, was found in the open-water sediments at sampling site O10. This may be related to the high chemical and mineralogical stability of ferrimagnetic iron oxides present in open-water sediments and the ability of these forms to retain and store organic carbon (Lalonde et al. 2012).

One of the sources of ferromagnetic material in bottom sediments is the deposition of atmospheric anthropogenic dust. However, considering the delta area with high dynamics in the sediment movement, this factor should be excluded as affecting the amount of pollutants. Despite the increase in the value of this parameter, it can be concluded that the impact of technogenic factors on the geomagnetic quality of the analyzed marine sediments is small. The values of magnetic susceptibility of marine sediments obtained by Doyle et al. (1976), Chan et al. (1998), and Desenfant et al. (2004) were much higher, indicating excessive accumulation of pollutants from anthropogenic sources. Based on the analyses of the spatial variability in the magnetic susceptibility in sediments and its relationship with heavy metal content, it can be assumed that this parameter is useful in assessing the amount of pollution in deep marine sediments.

Considering the low cost and fast speed of measurements, the determination of ferrimagnetic minerals in sediments can be a good indicator of chemical changes resulting from the accumulation of ferrimagnetic minerals that are carriers of heavy metals. As in the case of ferromagnetic content in marine sediments, the concentration of heavy metals is strongly related to grain size distribution. Numerous studies have shown that the more mineral grains with a diameter of < 0.063 mm, the greater the enrichment in potentially toxic elements (Pempkowiak et al. 1999; Ansari et al. 2004; Remeikaitė-Nikienė et al. 2018; Virtasalo et al. 2020; Shahabi-Ghahfarokhi et al. 2021). Studies of marine sediments indicate the importance of the surface and size of mineral particles in binding metals of technogenic origin. The concentration of heavy metals is also related to the content of organic matter. Due to their bioaccumulation in organic complexes, a significant enrichment of bottom sediments with heavy metals is observed. Research conducted on bottom sediments of lakes, rivers, and seas confirms the strong relationship between the accumulation of PTEs in clay and organic complexes in aquatic ecosystems. This is due to the diversity of organic matter transported with river inflow, both in terms of composition and quantity (Koszka-Maroń and Jegliński 2009; Remeikaitė-Nikienė et al. 2016; Baran et al. 2019).

Zalewska et al. (2015) studied the distribution of potentially toxic elements content (Cd, Hg, Pb, and Zn) in the marine sediments from the offshore areas of the southern Baltic Sea: Gdańsk Deep, SE Gotland Basin, and Bornholm Deep. The authors indicated the highest concentrations of metals in the Gdańsk Deep area, which resulted from the deposition of pollutants carried directly by the Vistula and its tributaries. The analysis of historical data on the heavy metal content in the sediments of the Gdańsk Deep showed a significant increase in the inflow of pollutants, especially after 1980. A marked increase in deposited pollutants led to the detection of PTEs with the maximum level of Cd (1.99 μg g−1 d.m.), Pb (82 μg g−1 d.m.), and Zn (246 μg g−1 d.m.) in the upper layers of marine sediments (Pempkowiak 1991; Glasby et al. 2004; Zalewska et al. 2015). The reason for the rich inflow of PTEs should be attributed to the intensive industrial development in the Vistula catchment in the 1960s and 1970s. In recent years, however, there has been a reduction in atmospheric and aquatic emissions, which has been reflected in a decrease in metal concentrations in the upper layers of sediments in the Gdańsk Deep (Renner et al. 1998). In our study, the level of Pb in the open water amounted to 53.2 μg g−1 d.m. (transect I, site O10), which is lower than the concentrations recorded previously, such as 75 μg g−1 d.m. by Szefer et al. (1999). At the same time, much lower concentrations of Cd (1.1 μg g−1 d.m.) were also recorded at the same sampling site. This indicates a downward trend in the concentration of Cd and Pb in the sediments.

The increasing accumulation of PTEs in the sediments furthest from the mouth of the Vistula (O7–O10) may be strongly influenced by flocculation processes, which are often enhanced by increased pollution. When colloidal particles move downward from freshwater into the marine environment, flocculation causes larger particles to form and settle onto the seabed. Flocculation processes at the seawater–freshwater interface has been widely reported in other studies (Palanques et al. 1995; Karbassi et al. 2013; Shamkhali et al. 2013). According to Pustelnikovas et al. (2007), the technogenic sedimentation zone of the Klaipėda Strait is characterized by stagnant conditions and the low oxygen level, resulting in metal concentrations that are 5–50 times higher than in the natural sedimentation zone. Other authors also indicated elevated concentrations of Cd, Cu, and Pb in the open-water sediments of the southern Baltic Sea because of anthropogenic pollution (Galkus et al. 2012; Remeikaitė-Nikienė et al. 2018). One of the sources of PTEs is agriculture, especially its sectors that use pesticides and phosphate fertilizers, followed by their runoff to river waters, and eventually to marine waters (Alloway 2012; Baran et al. 2019). Moreover, low Ni content was found in all analyzed sediment samples, and slight Ni enrichment was recorded only in open waters at sites O9 and O10. Renner et al. (1998) showed that Ni, like Mn and Co, entered the southern Baltic due to natural erosion processes. Considering the low content of the analyzed PTEs in the surface layer of sediments, their low sorption and potentially low bioavailability for living organisms should be noted. Our research addresses the issues of the state of the Baltic Sea by providing complementary data on bottom sediments that can be used to determine the impact of pollutants on the mass balance of trace elements in the sediments of this marine ecosystem.

5 Conclusions

Sediment deposition in the Vistula fan delta is characterized by high dynamics of development, caused by the varied flow of river waters and by the presence of the impact zone of marine hydrodynamic processes. This development is reflected mainly in the granulometric structure of sediments and the dynamics of morphological changes in the formation of the fan delta. In addition, this is an area of anthropogenic activity, where hydrotechnical engineering operations are carried out.

This study demonstrated that the content of potentially toxic elements (PTEs) and magnetic susceptibility of sediments showed a very high heterogeneity within the study area, depending mainly on the location (delta vs. open waters), grain size composition, and organic matter content. A high correlation was found between the presence of the metals such as Cr and Pb in the sediment, the dominance of the fine-grained fraction with a diameter of < 0.063 mm, the content of organic matter expressed by the LOI parameter, and the magnetic susceptibility measurements.

Our results demonstrated that magnetic susceptibility analysis is an easy method that can be used to complement the identification of PTE contamination zones in the sediments and to analyze technogenic changes and interference in the marine environment. Thanks to the high sensitivity of the magnetic measurement, it is possible to find areas with even a small amount of ferrimagnetic minerals.