Lake Sediments as Microplastic Sink: The Case of Three Lakes from Northern and Central Poland

Plastic, whose mass production began in the 1950s, is an increasingly common environmental pollutant (Xiong et al. 2018). Due to its properties, it is one of the main materials used in the production of packaging, the building and construction industry, the household goods industry or the electronics and electrical industry (Cole et al. 2011, Hidalgo-Ruz et al. 2012, Eerkes-Medrano et al. 2015). It is estimated that >9 billion tons of plastic have been produced worldwide to date. Globally, the total weight of plastic products (without products resulting from the plastic recycling process) was about 367 million tons in 2020. In Europe alone, it was about 55 million tons, but it should be noted that since 2017, when 64.4 million tons were produced, production volume has been slowly declining (PlasticsEurope 2021).

The mass use of plastic items with improper planning and process of recycling has caused them to become ubiquitous in the natural environment (Harley-Nyang et al. 2023). As a result, plastic pollution has become dangerous to both living and non-living components of the environment (Iroegbu et al. 2021). Objects made of plastic, as a result of improper disposal, end up in the environment through various routes, where they begin to degrade under the influence of factors such as UV radiation, high temperature, mechanical fragmentation during the flow or rippling of water, or biodegradation processes (Weinstein et al. 2016). As a result, they break down into smaller particles, and based on size, they can be divided into macroplastics (>25 mm), mesoplastics (5–25 mm), microplastics (1 μm–5 mm) and nanoplastics (<1 μm) (Hartmann et al. 2019). Currently, the most frequently discussed issue is environmental pollution with microplastics (MPs). In the literature, one can also encounter a classification according to which MPs are particles from 1 μm to 1 mm (Hartmann et al. 2019, Ivleva 2021), but the most widely used classification is the one according to which MPs are particles from 1 μm to 5 mm. MP can appear in the environment in primary or secondary form (Mintenig et al. 2017). The primary form is particles of MP, contained in various types of cosmetics such as creams, scrubs, toothpastes and lipsticks, and in detergents such as powders, gels, laundry capsules, detergents or dishwasher tablets, which are supposed to increase their effectiveness. After human use, they travel through sewer systems to wastewater treatment plants, where, unfortunately, not all of them are caught and end up in the environment. The secondary form of MP is the aforementioned degradation, directly in the natural environment, of plastic objects into smaller particles (Andrady et al. 2022).

In scientific research, the issue of plastic pollution was first addressed when clusters of plastic waste floating on the surface of the Sargasso Sea were described (Carpenter, Smith 1972), while the first study of plastic pollution in lakes was on Lake Huron in North America (Zbyszewski, Corcoran 2011). Pollution of the natural environment by MP is a pervasive problem today (Vaughan et al. 2017). In addition to common examples, such as pollution of seas, rivers, lakes and soil, MP is also found in the air (Gasperi et al. 2015, 2018), at high altitudes (Allen et al. 2019), in the digestive systems of animals (Gurjar et al. 2021) and their faeces (Santillán et al. 2020), in brewed tea (Hernandez et al. 2019) and bottled water (Welle, Franz 2018), in human lungs (Jenner et al. 2022) and blood (Leslie et al. 2022) or even in the bodies of newborn babies (Sripada et al. 2022). MP also enters the environment as a result of everyday human activity involving, for example, opening bottled water or other plastic packaging (Sobhani et al. 2020).

Research on the occurrence of MPs in the environment has for a long time focussed mainly on marine areas, but over the last few years, knowledge about the presence of MPs in freshwater ecosystems has been significantly expanded (Talbot, Chang 2022). By 2021, 98 lakes in the world have been described that are identified with MP pollution (Dusaucy et al. 2021). This number is systematically increasing (D’Avignon et al. 2022, Yang et al. 2022, Nava et al. 2023, Dimante-Deimantovica et al. 2024). In Poland, MP pollution in lakes has so far been the subject of only a few publications (Kaliszewicz et al. 2020, Rogowska et al. 2021, Nava et al. 2023, Pol et al. 2023) and none of them analysed the presence of MP in bottom sediments. Therefore, the research aimed to determine the concentration of MP particles in the bottom sediments of lakes lying outside urban areas. Combined with data on average sedimentation rates in so-called ‘Danish’ traps, this survey was carried out to determine whether MP could be used as an indicator of sedimentation rates in lakes.

The Czechowskie and Głęboczek lakes are located in the Tuchola Forest area in northern Poland. The two reservoirs are located within a single catchment, drained by the Struga Czechowska River flowing through the lakes, about 1.5 km apart. Lake Czechowskie has an area of 73 ha and a maximum depth of 32 m, while Lake Głęboczek has an area of 7 ha and a maximum depth of 18 m (Kordowski et al. 2014). Lake Czechowskie is surrounded by forest on the eastern and southern sides, while the northern and western sides are arable land. On the northern shore of the lake, there is a resort open in the summer season, while a small beach is located on the eastern shore. Summer buildings are scattered around the lake. Lake Głęboczek is entirely surrounded by forest, and there are several fishing spots on its banks. In the entire direct catchment area of the two lakes, >60% of the area is forest, about 25% is agricultural land, meadows and pastures and <10% is loose buildings (Fig. 1A).

Fig. 1.

Lake Gościąż is located in central Poland within the Płock Basin, in the Gostynin-Włocławek Landscape Park. The lake itself, due to its annually laminated sediments, has been protected as a nature reserve since 2001. The area of the lake is 43 ha and its maximum depth is 22 m (Fojutowski et al. 2021). The lake is entirely surrounded by forest, dominated by Querco roboris-Pinetum communities in wetter habitats and Peucedano-Pinetum in drier areas (Kępczyński, Noryśkiewicz 1998). The catchment area is almost 95% covered by coniferous and mixed forests, while the remainder is mainly small lakes (Fig. 1B).

Lake sediment cores were taken during field surveys in 2021 and 2022 near the deepest locations. The UWITEC gravity probe with a diameter of 90 mm was used for the sampling. Cores of lengths 93 cm (Lake Czechowskie), 50 cm (Lake Głęboczek) and 80 cm (Lake Gościąż), respectively, were cleaned and stripped of the outer layer to minimise the risk of secondary sediment contamination, and then divided into 5 cm long sections. From each section, 50 g of wet sediment was taken and transferred to a glass flask, to which 100 ml of 25% NaCl was then added. In this way, the density of the solution was 1.2 g · cm⁻³. The samples thus prepared were placed on a magnetic stirrer and stirred at high speed for 5 min (Duis, Coors 2016). The samples were left to stand for 24 h to allow the entire suspension to settle at the bottom. After this time, the supernatant was carefully decanted and 10 ml of 30% H₂O₂ was added to it. The samples were then heated to 70°C and left at this temperature until the organic matter oxidation was complete. After cooling, the samples were filtered through glass-fibre filters and then dried on glass Petri dishes at 60°C for 4 h (Klein et al. 2015). Only porcelain spatulas and glass pipettes washed with distilled water each time were used to transfer the samples. Before starting work, all working surfaces were cleaned with ethanol, and the work was performed using blue nitrile gloves. Protective clothing of a solid white colour was used to catch any possible ingress of fibres of this colour into the samples to be tested. The prepared samples were examined under a Zeiss Axio Scope A1 microscope to identify MP particles (Fig. 2). The identified particles were photographed and classified by form, size and colour.

Fig. 2.

The sediment material used to determine the average sedimentation rate, and then, to build an age-depth model (Fig. 3), was collected from ‘Danish’ traps placed in the deepest parts of the studied lakes, approximately 3 m above the bottom. This type of trap consists of four cylinders (each with a diameter of 75 mm, length of 450 mm) and is able to collect all material floating in the depths of the lake. Traps were emptied several times throughout the year. The amount of material was measured in millimetres and then converted to g · m⁻² · day⁻¹. The data was gathered between 2017 and 2021.

Fig. 3.

Maps illustrating the catchment areas were created based on land cover data from the Corine Land Cover database for the year 2018 (EEA 1995).

The highest number of MP particles were identified in the sediments of Lake Czechowskie. A total of 20 particles were found, of which 17 were fibres and the remaining 3 were fragments. The size of the particles was predominantly in the range of 0.1–1 mm (nine identified particles). In the 93 cm long core, contaminants were found at a depth of 0–60 cm. In the 50 cm long core from Lake Głęboczek, 2 MP particles were identified. One was a fragment with a diameter of 2 mm in the upper part of the core (0–5 cm), while the other was a transparent fibre 2.1 mm long found at a depth of 20–25 cm. In the core from Lake Gościąż, which was 80 cm long, 7 MP particles were identified, found in layers from 5 cm to 45 cm deep. In order to systematise the obtained results, the number of identified MP particles per kilogram of wet sediment from which they were extracted was calculated. Thus, there are 21 MP particles in Lake Czechowskie, 4 MP particles in Lake Głęboczek and 9 MP particles per kilogram of wet sediment in Lake Gościąż.

Most of the studies conducted to date on plastic pollution in lake ecosystems concern lakes located in or near urban areas. The lakes studied by the author belong to a very small group of so-called rural lakes for which such analyses have been carried out (Dusaucy et al. 2021). Even fewer studies have been conducted on the bottom sediments of these lakes. There are only a few such studies, and direct comparisons of the results are difficult due to the different units of measurement. For example, in a small, shallow lake on Spitsbergen, sediment was collected from a rocky bottom for research purposes and 400 MP particles per square metre were identified (Gonzalez-Pleiter et al. 2020). It is therefore difficult to relate these findings to the results obtained by the author. In the deep (maximum depth 34 m) mountain lake Sassolo in Switzerland, 514 MP particles per kilogram of wet sediment were identified in the deepest part of the lake and 34 MP particles per kilogram of wet sediment at the lake outflow (Negrete Velasco et al. 2020). Although the latter value is the smallest, it is >60% higher than the pollution levels found in Lake Czechowskie. Such large amounts of MP in locations far from urbanised and industrialised areas indicate that atmospheric input is an important source of MP input to lakes (Evangeliou et al. 2020, Huang et al. 2021). Brahney et al. (2020) report that in the U.S. 1000 tons of wind- and rain-borne MP fall per year into areas that are protected. Hence, the establishment of an area as a protected area does not eliminate the problem of plastic pollution. This is also confirmed by the results obtained in the sediments of Lake Gościąż, where, despite the establishment of a nature reserve in 2001, a total of 9 MP particles per kilogram of wet sediment were identified (Fig. 4). However, this is a relatively small amount compared to Lake Pinku in Latvia, where, despite its location in a protected area, up to 4.1 MP particles per gram of dry sediment were identified (Dimante-Deimantovica et al. 2024), which corresponds to 4100 particles per kilogram. This also raises the question of direct comparison of the quantities determined, as the measurement here is based on dry sediment. Assuming that the sediment from the lakes investigated by the author was also dried, the number of particles identified would still be significantly lower than in Lake Pinku.

Fig. 4.

The number of MP particles in the drill core from Lake Czechowskie is similar to the values determined in the coastal sediments of Lake Ełk, which were collected in the vicinity of urbanised and tourist areas (Rogowska et al. 2021). In contrast, significantly fewer MP particles were identified in the sediments of Lake Głęboczek compared to Lake Czechowskie, although the lakes are only 1.5 km apart. Lake Głęboczek is completely surrounded by forest and has only one small tributary, which does not serve as a source of MP inflow into the lake. It can therefore be assumed that the MP particles reached the lake via atmospheric deposition. The immediate surroundings of Lake Czechowskie include not only forests but also agricultural areas, a vacation resort and a small beach, which is used in the summer season. The lake is also used by anglers. Therefore, MP can also enter the bottom sediments through rain runoff and the fragmentation of macroplastics from the coastal sediments.

The size and colour of microplastic particles

In zooplankton-rich lakes, MP particles of 4–5 mm in size proportionally predominate, while in zooplankton-poor reservoirs MP particles of <1 mm in size dominate (Karpowicz et al. 2020, Pol et al. 2023). In the lakes studied, the majority (76%) of the identified MP particles represent sizes <2 mm (Fig. 5). Such a structure may suggest that in these reservoirs the amount of zooplankton is relatively low, or only a small amount of MP consumption by zooplankton occurs (Bellasi et al. 2020, Kvale et al. 2021). As many as 30% of the MP particles found during the study were red in colour (Fig. 6). Marti et al. (2020) suggest that a higher consumption of red particles by aquatic organisms, which confuse them with food, is responsible for the small percentage of this colour in pollution. Thus, such a significant amount of red MP particles in these lakes suggests that these plastics are not, to date, significantly consumed by aquatic organisms.

Fig. 5.

Fig. 6.

Microplastic as a potential source of information on the age of bottom sediments

Lake sediments serve as a kind of archive in which human activities and their impact on the environment are recorded. Therefore, testing the bottom sediments of lakes for plastic pollution not only allows to determine the concentration of MP but can also become one of the tools to help in determining the sediment accumulation rate. Using additional methods, such as in this case, the average sediment accumulation rate in sedimentation traps, we can determine the moment of potential MP appearance in these lakes. In the case of the three lakes analysed, we can clearly see the boundary below which MP was not found in the sediments. In Lake Czechowskie it is 60 cm, in Lake Głęboczek it is 25 cm, while in Lake Gościąż it is 45 cm. The average sediment accumulation rate for 2017–2021, calculated on the basis of the amount of sediment deposited in sedimentation traps located in the deepest locations of the three studied lakes, is 5 mm · a⁻¹ for Lake Czechowskie, 4.3 mm · a⁻¹ for Lake Głęboczek and 14.4 mm · a⁻¹ tor Lake Gościąż (Fig. 3). Assuming that a similar sediment accumulation rate would have persisted over the past few decades, and juxtaposing this with the depths at which MP particles were found, the beginning of plastic accumulation in the lakes studied is 1986 in Lake Gościąż, 1963 in Lake Głęboczek and 1901 in Lake Czechowskie, respectively (Fig. 7). For Lake Gościąż, the age of the first MP particles found corresponds to the start of polyvinyl chloride production at the Anwil nitrogen plant in Włocławek, located about 25 km northwest of the lake, whose installations were built in 1976–1985. Given the prevailing wind directions in the area (Marosz 2016) and the relatively short distance separating the two locations, these pollutants could have been transported to the lake area just by atmospheric means. In Lake Głęboczek, the first MP fragment found was associated with the start of mass production of plastics in Poland, which dates to 1957 (Kochanowski 2017). Here too, in the absence of potential sources of plastic pollution in the immediate vicinity of the lake, it should be expected that they were transported by the atmospheric route. In contrast, in the case of Lake Czechowskie, where the first MP particles were identified at a depth representing 1901, we can speak of intense sediment compaction or potential movement of MP particles into the depths. Dimante-Deimantovica et al. (2024) indicate that MP particles can migrate deep into the sediments, which they supported by dating the sediments using ²¹⁰Pb. According to these analyses, the first MP particles found in Lake Pinku would date to 1733. Assuming such a scenario, the first appearance of MP in the sediment would not necessarily mean that it dates from the 1950s (the beginning of the mass production of plastic) or later. Given the density of most plastics, which ranges from 0.92 g · cm⁻³ to 1.6 g · cm⁻³ (Hidalgo-Ruz et al. 2012), their ability to penetrate deep into the lake sediment, whose density, depending on its composition and physical properties, ranges from 0.96 g · cm⁻³ to 1.85 g · cm⁻³ (Sekellick et al. 2013, Sowiński et al. 2023), seems to be quite limited, but not out of the question. Especially if the MP particles are rounded fragments rather than fibres and are small in size (Dimante-Deimantovica et al. 2024).

Fig. 7.

Identification of MP particles in cores taken from all three studied lakes allowed us to confirm that MP is currently a problem not only for marine and freshwater environments with a high impact of anthropopressure but also for small lakes of only local importance, where the impact of anthropopressure is negligible or moderate. The variation in the amount of MP particles in the lakes studied is mainly due to the nature of the immediate surroundings of the lakes. In two of the lakes, which are surrounded by forests, the main source of MP delivery is atmospheric transport. In the third lake, the pollution originates mainly from agricultural and tourist activities. In each of the sediment cores, a clear boundary becomes apparent, below which MP contamination no longer occurs. Juxtaposed with data on sediment accumulation rates in sediment traps, an attempt was made to determine the date of the first appearance of MP in the lakes. The results obtained are consistent with the possible impact of human activities in the case of two lakes, while in the third lake, the question of the possible sinking of MP particles in the sediments remains open, which complicates the interpretation of the age of these sediments. Additional comparative methods are needed to make MP a reliable tool for determining chronology. The studies carried out were of a pilot nature and the results presented are a prelude to further, more extensive analyses. These future analyses will include denser core samples, precise dating and analysis of the identified polymer types by FTIR. In addition, analyses of MP concentrations in the water and coastal zones of these lakes will be conducted.

The process of work implementation has brought dozens of important questions, such as: Can MP migrate deeper into lower layers of bottom sediments, what are the sources and mechanism of its delivery to lakes and what’s the seasonal variability of these sources? We suppose that well-planned further research will provide answers to these questions.