This study is based on analyses of two sediment cores collected during an offshore sampling campaign operated by the Swedish Geological Survey in 2020 (campaign MOS20). The two cores were collected at two stations included in the Swedish National Environmental Monitoring Program, in the central Baltic Sea: station SE-05 (N 58° 58′ 38.16′′, E 20° 18′ 25.42′′, northern Baltic Proper, water depth: ca. 175 m, Figure 1) and station SE-08 (N 58° 40′ 41.25′′, E 18° 21′ 24.45′′, western Gotland Basin, water depth: ca. 404 m; Figure 1). Station SE-08 is situated closer to the coastline and the Swedish capital of Stockholm, 34 km from the coastline and 72 km from the Stockholm city center. Station SE-05 is located in the middle of the open sea, approximately 110 km, 167 km, and 100 km from the Swedish, Finnish, and Estonian coasts, respectively. Both stations are situated in areas with high sediment accumulation rates (1.03 cm yr^-1) and anoxic bottom water conditions (Josefsson, 2022; Ketzer et al., 2024).

A GEMAX-type twin gravity corer equipped with two transparent acrylic liners (70 cm in length, 9 cm in inner diameter) was used for sediment core collection. Parallel cores were collected from each site to enable both sedimentological characterization and geochemical analyses. Previously published work on the cores reported that there were no signs of bioturbation, with the sediment comprising black to dark brown, massive to laminated mud, and no visible marine fauna (Ketzer et al., 2024). The cores used for MP analyses from stations SE-05 and SE-08 measured 62 cm and 60 cm in length, respectively. They were sliced by a clean metal blade at two-centimeter intervals, resulting in a total of 31 and 30 samples. All samples were immediately frozen at -20 °C after slicing.

The methods used to investigate MP abundance in this study were modified from the NOAA guideline (Masura et al., 2015), with reference to the standardized protocol (Frias et al., 2018) from JPI-Oceans (Joint Programming Initiative Healthy and Productive Seas and Oceans). Upon receiving the 2 cm thick (15 ± 11 g-DW) pre-sliced frozen sediment samples, a series of laboratory steps were performed, including sample drying, H₂O₂ treatment, sieving, density separation, centrifugation, filtration and microscopic examinations. The samples were dried in a 90 °C oven for 24–48 hours until a constant weight was achieved. To remove organic matter, each sample was slowly mixed with 100 mL of 10% H₂O₂ and left for 24 hours. The mixture was then passed through a stainless steel sieve (mesh size: 20 µm), and both the beaker and sieve were thoroughly rinsed with distilled water to ensure complete transfer of residual solids. This sieving step also served as a preliminary filtration to remove particles smaller than 20 µm, thereby reducing the total solid mass and minimizing interference in subsequent separation and analysis steps.

Density separation was carried out by adding saturated NaCl solution (density: 1.2 g/cm³) at a volume at least three times that of the residual solids, allowing MPs to float in the supernatant. Centrifugation of the supernatant at 4300 rpm for 5–10 minutes (Tang et al., 2020; Grause et al., 2021; Xu et al., 2021) was used to enhance separation and obtain a cleaner fraction for subsequent filtration and analysis. The supernatant was filtered using Munktell glass fiber filters (pore size: 8 - 12 µm), and the retained MPs were examined under a stereomicroscope (Olympus SZX10) and a compound microscope (Olympus BX50). MPs were identified and described based on color, size, and shape. Following the JPI-Oceans protocol (Frias et al., 2018), MPs were classified into two categories: (1) fibers – long, thread-like particles (also referred to as filaments), and (2) fragments – irregularly shaped, broken-off pieces. Film particles (thin, flat, and sheet-like) were included in the fragment category.

Many recent studies (Pervez and Wang, 2022; Khan et al., 2023; Izlal et al., 2024; Souza et al., 2024) have employed the same or similar analytical methods as those used in the present study to determine the MP abundance in the environment, due to considerations such as cost-effectiveness and limited access to advanced equipment. However, this methodology carries certain limitations that may affect the interpretation of the results and should thus be taken into account.

First, visual identification of MPs under a stereomicroscope can introduce both overestimation and underestimation. For instance, natural fibers, such as cotton and wool, may be misidentified as MPs, inflating the counts (Xu et al., 2021). On the other hand, smaller MPs, which are difficult to detect visually, may lead to underreported abundances. Advanced techniques, such as Fourier-transform infrared spectroscopy (FTIR), Laser Direct Infrared (LDIR) and Pyrolysis Gas Chromatography–Mass Spectrometry (py-GCMS), provide greater precision in quantifying MP abundance and determining their chemical composition (Dehaut et al., 2020; Cheng et al., 2022; Uaciquete et al., 2024). In some studies (Uaciquete et al., 2024; Villanova-Solano et al., 2024; Watson and Turner, 2024), this issue is addressed by referring to particles identified under the microscope solely as ‘anthropogenic particles’ unless they are validated as plastics. Furthermore, as mentioned earlier, transparent and white materials were used in the experiment. The white glass fiber filter, used as a surface for depositing the plastic particles before microscope examination, makes it difficult to identify anthropogenic particles of similar color. The proportion of white or transparent anthropogenic particles varies significantly across sampling locations (Sui et al., 2020; Sayed et al., 2021). Consequently, this study may exhibit significant bias related to the detection of such particles, and the variability in color distribution across sites further complicates efforts to assess their significance.

Another limitation is the reliance on density separation using a saturated NaCl solution. While a cost-effective method commonly adopted in previous studies (Van Cauwenberghe et al., 2015; Harris, 2020), this approach can only extract MPs with a density below 1.2 g/cm³, potentially overlooking denser polymers. Nevertheless, it effectively extracts the most common polymers found in the environment, such as polypropylene (PP), polystyrene (PS), and polyethylene (PE) (Frias et al., 2018; Phuong et al., 2021; Li et al., 2023). A previous study (Esiukova et al., 2020) conducted in Baltic Proper (same as our two study sites) sediments found that 70.3% of identified MPs had densities below 1.2 g/cm³.

Out of the plastic particles retrieved from the studied cores, 99% were of MP size (<5 mm). Most of the plastic particles (ca. 70% for SE-05 and 67% for SE-08) are smaller than 1mm (Figure 2), concurring with results found in other studies dealing with MPs in sediments (Fok et al., 2017; Wang et al., 2020; Li et al., 2021). The high abundance of smaller MPs is commonly attributed to secondary MPs caused by the fragmentation of MPs into smaller pieces in the environment (Auta et al., 2017). Additionally, the smaller-sized MPs may selectively sink to the seafloor via biofouling due to high surface to volume ratio (Kaiser et al., 2017).

The color of the analyzed MP varies significantly, encompassing blue, black, white, red, brown, green, yellow, translucent, purple, and orange (Figure 3). The most frequent colors are blue (67% of all samples, 76% of SE-05 samples, and 62% of SE-08 samples), and black (12% of all MPs, 9% of SE-05 samples, and 13% of SE-08 samples). Blue MPs are the most prevalent in both sediment cores of this study, which is in accordance with some (Stolte et al., 2015; Zheng et al., 2019; Zhou et al., 2021) but not with all previous studies (Jiang et al., 2018; Yaranal et al., 2021; Chubarenko et al., 2022). Plastic products used in household life or industry come in a variety of colors. Colors of plastic may play an important role in the plastic photoaging, and bluish plastics have been found to break down more easily than plastics in other colors (brown, red, orange, yellow, green, and purple) due to higher light energy transmitted to the blue plastic (Zhao et al., 2022). This observation explains the phenomenon that small-sized plastics in environment are commonly blue (Zhao et al., 2022).

Regarding the shape of the MP particles, it was found that fiber is the dominant form in both study sites (87% of SE-05’s samples and 89% of SE-08’s samples, shown in Figure 3). This finding is consistent with previous research in Baltic Sea sediments (Chubarenko et al., 2022; Zhou et al., 2021), and from other locations (Willis et al., 2017; Kooi and Koelmans, 2019) too. Microfibers have been reported as a major source of the MPs found in oceans, with domestic laundering being recognized as a major contributor (Singh et al., 2020). In just one wash, for instance, a single garment can produce more than 1900 microfibers (Browne et al., 2011), and it is estimated that 100–600 quadrillion pieces of microfibers are released from the washing of clothing in Europe every year (Hann et al., 2018). Once fibers are released into the environment, they tend to experience biofouling and increase settling velocity, resulting in sinking and deposition on the seafloor (Elizalde-Velázquez and Gómez-Oliván, 2021). The deep sea is statistically proven to be the ultimate terminus for synthetic fibers, implied by a set of data that spans from the coastal zone to central bottom depressions in the Baltic Sea (Chubarenko et al., 2022).

In Sweden, textile laundering has been identified as a major contributor to MP fiber emissions (Swedish Environmental Protection Agency, 2021). A study (Magnusson et al., 2016) of three Swedish wastewater treatment plants found that influent water can contain over 20,000 MP fibers per cubic meter. Although most fibers are retained during treatment, effluent still contains between 150 and 3,300 fibers per cubic meter. Wastewater treatment plants, while generally effective, may still serve as pathways for microfibers into the marine environment (Magnusson et al., 2016). Given the dense network of wastewater treatment facilities along the Baltic Sea coast (Schernewski et al., 2021), especially in the Stockholm region, it is plausible that urban effluents contribute significantly to the fiber-rich MP profiles observed in our sediment cores, particularly at SE-08.

The raw MP counts can be found in the Supplementary Data 1. No clear correlation was found between the depth and size, color or shape of plastic particles. They are very diverse, and a selection of MPs is shown in Figure 4 to illustrate the variety of colors and morphologies.

While visual identification under a microscope provided valuable insights into the size, color, and shape of MP particles, it is inherently limited in confirming polymer composition. To strengthen the reliability of MP identification, future studies could incorporate polymer characterization techniques, such as FTIR spectroscopy, on a subset of visually identified particles. This would reduce uncertainty in visual identification and offer a deeper understanding of polymer types and their environmental behavior.

In this study, SE-08 exhibited higher levels of MP contamination than SE-05. Considering both of the study sites are with similar sedimentation rates and characterized by fine-grained sediments and high organic matter content (Josefsson, 2022), one plausible explanation is the location of the sampling sites: SE-08 is closer to the shoreline and to Stockholm, whereas SE-05 is situated in the middle of the open sea (Figure 1). In addition, a major surface current passes over SE-08, transporting water from the Stockholm coastal area and potentially carrying plastic particles from urban sources (Karimova and Gade, 2016). Additional factors, such as the influence of bottom currents or shipping activities, may also contribute to the observed variations. However, due to practical constraints, these factors could not be thoroughly investigated in this study. Furthermore, analyzing additional sediment cores would be necessary to draw more robust conclusions about large-scale deposition patterns. Despite these limitations, the findings of this study provide valuable preliminary insights into the historical deposition of MPs in the central Baltic Sea.

In the study area, MPs were found at maximum burial depths of 33 cm in core SE-05 and 47 cm in core SE-08. The MPs found in the samples below those depths are considered to come from the operation (see Methods). The concentration of MP particles increases towards the seafloor (Figure 5; Supplementary Data 1), from zero to more than 2000 and 4000 pcs kg-DW^-1 of sediment at the top 2 cm in cores SE-05 and SE-08, respectively, revealing their progressive accumulation over time. With a sediment accumulation rate of 1.03 cm yr^-1, we assess that the base of the cores was deposited in the early 1960s, and that the MP accumulation started ca. 1974 in the western Gotland Basin (SE-08), ca. 1988 in the northern Baltic Proper (SE-05), and peaked at present-day in both sites (Figure 5). However, due to natural variability in sedimentation rates and potential sediment dynamics, the estimated onset of MP accumulation should be interpreted with caution. This trend is different from the observed for metal pollutants in the Baltic Sea in the same study locations, where Cd, Pb, and Zn concentrations peak in the 1980s and decrease till present-day owing to efforts to control emissions of those pollutants in the region (Shahabi-Ghahfarokhi et al., 2021b). This result supports the potential of MPs as stratigraphic markers in sediment profiles (Bancone et al., 2020).

The best fitting curves of MP concentrations (pcs kg-DW^-1) with depth for both SE-05 and SE-08 are shown in Figure 6a. To account for the potential influence of human activities on MP pollution in our study area, the Swedish population was incorporated as a factor in our analysis instead of the total population of the Baltic Sea region. This is attributable to the fact that the total population of the entire Baltic Sea region grew at a relatively low rate of just 1.2% between 1990 and 2018 (ESPON, 2019), whereas the populations of Sweden and Stockholm increased by 19.1% and 42.8%, respectively, during the same period. This suggests that any increases in MP abundance, if linked to population size changes, are more likely to be associated with demographic shifts in Sweden rather than in the broader Baltic Sea region.

All models used in the GAM analysis have k-index values greater than 1, indicating that the number of basis functions chosen is appropriate and there is no significant risk of overfitting. The GCV scores and edf values are also within reasonable ranges, further supporting the stability and reliability of the models (Supplementary Data 1). GAM plots (Figures 6b, c) further explain the variance of MP concentration distribution in the samples through splines of MP concentrations to each explanatory variable at both SE-05 (Figure 6b) and SE-08 (Figure 6c) according to the Generalized Additive Model (Kanhai et al., 2017; Liu et al., 2021; Meyerjürgens et al., 2022) (GAM, smooth functions; see Methods). The populations of both Sweden and Stockholm have stronger capacities to explain the variance of MP concentration than global plastic production and cumulative plastic production, contributing more to the variance in MP concentration at both sampling sites. Specifically, the deviance explained by the population of Sweden is 75.6% (p < 0.001) at SE-05 and 79% at SE-08 (p < 0.001). For the Stockholm population, the deviance explained is 74.9% at SE-05 (p < 0.001) and 83.9% at SE-08 (p < 0.001). In contrast, the deviance explained by global plastic production is much lower or unsignificant, at 22.4% at SE-05 (p > 0.05) and 14.3% at SE-08 (p < 0.005). Similarly, cumulative plastic production explains 23.4% of the deviance at SE-05 (p > 0.05) and 66.5% at SE-08 (p > 0.05).

Some studies have reported similar exponential patterns of increasing MP concentrations towards the surface layers of sediment profiles, for example, in the Kuwait Bay (Uddin et al., 2021), the Sakurada-bori Moat in Japan (Matsuguma et al., 2017), the Gulf of Thailand (Matsuguma et al., 2017), the Santa Barbara Basin in California (Brandon et al., 2019), and the Jiaozhou Bay in China (Zheng et al., 2020). Previously, the higher MP concentration in near-seafloor, present-day sediments has been correlated with the historical increase in plastic production (Brandon et al., 2019; Martin et al., 2020). Data from both Kuwait Bay (Uddin et al., 2021), Santa Barbara Basin in California (Brandon et al., 2019) as well as South China Sea (Chen et al., 2020) indicate that MPs started accumulating at these sites during the 1970s-1980s, which is roughly the same time as this study in the central Baltic Sea. Although plastic production began around 1950, it was not until the 1980s that global production started to accelerate (Geyer et al., 2017). Furthermore, synthetic fibers have been identified as a significant source of MPs in the ocean (Boucher and Friot, 2017), and the demand for synthetic fibers began to rise rapidly from 1970 onwards, eventually surpassing that of natural fibers by 2000 (Mills, 2011). Our findings on the historical MP accumulation in Baltic Sea sediments are strongly consistent with the development of the synthetic fiber market, with MPs first appearing in the Baltic Sea sediment in the mid-1970s (SE-08) and late 1980s (SE-05), and concentrations beginning to increase significantly during the 2000s-2010s.

There are also several publications (Nel et al., 2017; Brandon et al., 2019; Jorquera et al., 2022) which demonstrate how MP accumulation trends in sea sediments correlate with increased population densities. For example, the study in the Santa Barbara Basin (Brandon et al., 2019) points to a close relationship between the increase in coastal population and plastic accumulation in sea sediments. Similarly, in our study, the Stockholm County population contributes to the variance of MP concentration to a large extent (75% at SE-05, 84% at SE-08; Figures 6b, 6c). Numerical modelling work corroborates our findings by indicating that local onshore sources are important for the MP accumulation in the deep Baltic Sea. A recent study found, for instance, that different types of MPs, notably from tire wear and some specific household MPs derived from effluents of wastewater treatment plants, can efficiently be transported from coastal to offshore areas and reach the bottom of deep Baltic Sea basins (Murawski et al., 2022).

Based on the fitting equations of both study sites (Figure 6a), we estimated the future MP concentration in the surface sediments (Methods). It assumes that i) there will be no effective prevention or treatment measures induced in plastic management and production, ii) that the population growth will continue along the present-day trend, and iii) that the sedimentation rate will be the same. By 2030, for instance, the MP accumulation in the top 2 cm are projected to reach up to 9930 pcs kg-DW^-1 at SE-05 (northern Baltic Proper) and an alarming 72,924 pcs kg-DW^-1 at SE-08 (western Gotland Basin). Station SE-05, even located in the middle of the open sea, will eventually reach present-day contamination levels of one heavily polluted beach site in the South China Sea (Qiu et al., 2015). The prediction serves as a warning to society and provides a compelling rationale for increasing policy efforts to control MP discharge and to support more scientific research in this area.

To assess the present-day level of MP abundance, we compared the concentration observed in the top 2 cm of our two cores with findings from other publications from the Baltic Sea area and other marine environments, as shown in Figure 7 (more details are included in Supplementary Data 1). While we primarily focused on studies involving sediments less than 3 cm in depth, we also included some publications reporting MPs in deeper sediments, owing to the lack of relevant data from certain regions, such as the Baltic Sea region (Chubarenko et al., 2022) (covering depths of 2–7 cm) and Arctic Ocean (top 5 cm) (Bergmann et al., 2017; Tekman et al., 2020; Fang et al., 2022). It is important to note that various factors may contribute to differences in MP concentrations reported across studies. Differences in sampling strategies, analytical methods, and sediment depths can significantly influence the results. Additionally, the sampling times of the comparative studies differ, spanning from 2012 to 2020, which may also limit the interpretation of the comparison.

The MP abundance level in our near-seafloor sediment cores (2143 pcs kg-DW^-1 at SE-05 and 4450 pcs kg-DW^-1 at SE-08) is comparable in magnitude to the MP concentrations identified in most other studies from the Baltic Sea, as illustrated in the left-hand side of Figure 7. The variability between individual studies and samples is large, though. For instance, one study (Chubarenko et al., 2022) that examined 53 samples of the upper 2–7 cm sediment from the Gotland, Gdansk, and Bornholm Basins in 2015 - 2016, showed an average MP concentration of 863 ± 1371 pcs kg-DW^-1, but with a min-max span that covers 2 orders of magnitude; 103 – 10,179 pcs kg-DW^-1. The MP concentration in 37 out of the 53 samples was between 100 and 1000 pcs kg-DW^-1, and 50 of the samples contained below 2000 pcs kg-DW^-1. Another study (Zhou et al., 2021) that sampled the top 3 cm of sediment at 11 sampling stations in 2019 across the whole Baltic Sea found an average MP concentration of 800 pcs kg-DW^-1, ranging from 200 to 1500 pcs kg-DW^-1. In a study (Strand et al., 2013) that examined the top 2 cm sediments from 21 stations in the Danish waters of the Baltic and North Seas during 2012 - 2013, highly variable MP concentrations were reported, ranging from 60 to 3600 pcs kg-DW^-1, with no mean value indicated. Most of the MP concentrations found (in 16 of the 21 stations) were below 500 pcs kg-DW^-1. While some examples in the literature suggest low MP accumulation in recent sediment deposits in certain areas of the Baltic Sea, the lowest concentration reported (15 ± 10 pcs kg-DW^-1) comes from a study (Graca et al., 2017) that analyzed sediments (top 2.5 cm) from four shallow and two deep-water stations along the Polish coast of the Baltic Sea in 2014. The study revealed minimum and maximum values of 0 and 27 pcs kg-DW^-1, respectively.

The right-hand part of Figure 7 shows data from other oceans and seas worldwide, and it also points to a ubiquitous MP abundance of today’s marine environment, but to various degrees according to different studies. While the average number of MP pieces found per kilogram of dry sediment is generally below ten thousand, concentrations exceeding one hundred thousand have been reported. The very high MP concentration, 20,5859 pcs kg-DW^-1, was found in the top 1 cm sediment from the urban fjord in Norway in 2017 (Haave et al., 2019), with 24,953 pcs kg-DW^-1 larger than 50 μm. The study (Haave et al., 2019) on the Norwegian Continental Shelf shows how the top 1 cm sediment of 35 cores contains MPs in the order of 4900 ± 6200 pcs kg-DW^-1, ranging from 180 - 31,000 pcs kg-DW^-1. Even in the studies (Bergmann et al., 2017; Tekman et al., 2020; Fang et al., 2022) in Arctic Ocean during 2016 - 2020, MPs have accumulated in the upper 5 cm sediments with concentrations ranging from 0 to 13,331 pcs kg-DW^-1. Moreover, the MP concentrations comparison also shows that the MP concentrations in the upper 2 cm of our samples are comparable in magnitude in a wider geographical context.