Microplastics in lakeshore and lakebed sediments – External influences and temporal and spatial variabilities of concentrations
Graphical abstract
Introduction
As the plastic production rose continuously in the last decade, from 250 million tons in 2009 to 368 million tons in 2019 (PlasticsEurope 2015, 2020), the environmental pollution is likewise gradually increasing. The intensified pollution is caused by the longevity and low decomposition rates of plastic items and particles in the environment (Andrady et al., 2015) as well as by ongoing accidental and intentional plastic input into the environment (Barnes et al., 2009). First presence of plastics in the environment were proven in the early 1970s (Carpenter et al., 1972; Carpenter and Smith 1972). The number of studies investigating plastic contamination in different environmental compartments strongly increased since the early 2000s century (Ivleva et al., 2017). Plastics of all sizes were found in different compartments around the world, ranging from water and ice to sediments to biota as well as atmosphere (Bellasi et al., 2020; Bianco and Passananti 2020; Rochman 2018; Ivleva et al., 2017; Dris et al., 2015; Ivar do Sul and Costa 2014; Wagner et al., 2014).
Plastics are classified according to their size, namely macro-, meso-, micro-, and nanoplastics. However, definitions vary for the individual terms (Hartmann et al., 2019). Commonly, microplastics are referred to as particles <5 mm (Arthur et al., 2009). Attention on plastics shifted from large to small ones with time and even nanoplastics (<1 μm; GESAMP 2015) are increasingly studied today (Lehner et al., 2019; Mattsson et al., 2018; Koelmans et al., 2015). Similarly, the number of studies investigating freshwater bodies rose lately (Lambert and Wagner 2018; Rochman 2018; Horton et al., 2017a; Dris et al., 2015; Wagner et al., 2014). However, Blettler et al. (2018) stated that 87% of plastic pollution studies dealt with marine environments compared to 13% of studies analyzing freshwater systems. Marine environments are supposed to be the final sink for plastics (Rochman 2018; Bergmann et al., 2017; Wagner et al., 2014; Woodall et al., 2014; Thompson et al., 2004) whereas terrestrial freshwaters are considered as transport pathways for plastics from land to the ocean (Lebreton et al., 2017; Eerkes-Medrano et al., 2015; Mani et al., 2016; Gasperi et al., 2014; Wagner et al., 2014). Studies investigating the impacts of plastic on organisms were also mainly focusing on marine environments whereas freshwater studies were underrepresented (Blettler and Wantzen 2019). Possible impacts of plastic on marine and freshwater organisms comprise the entanglement within large plastic items, the ingestion of small particles and consequently the uptake of possibly toxic, chemical compounds, added or adsorbed to microplastics (e.g., GESAMP 2015; GEF 2012; Cole et al., 2011; Teuten et al., 2009; UNEP 2005; Thompson et al., 2004; Laist 1997; Laist 1987). For an adequate risk assessment of plastics within freshwater environments, detailed information on impacts on organisms is necessary as well as knowledge concerning plastic concentrations in different environmental compartments.
Microplastic pollution in freshwater bodies was demonstrated at the water surface (e.g., Lenaker et al., 2019; LfU, 2019; Mani et al., 2016; Eriksen et al., 2013), within the water column (e.g., Lenaker et al., 2021; Tamminga and Fischer 2020; Eo et al., 2019; Dris et al., 2018), in shoreline (e.g., LfU, 2019; Fischer et al., 2016; Faure et al., 2015; Imhof et al., 2013) and bed sediments (e.g., Wilkens et al., 2020; Zobkov et al., 2020; Turner et al., 2019; Castañeda et al., 2014) in different regions of the world. Results show high variability of microplastic concentrations. Unfortunately, the comparison of results is hampered due to differences in applied methods during sampling and laboratory processing (Horton et al., 2017a; Ivleva et al., 2017; Dris et al., 2015). Still, data provided by studies investigating microplastic abundances in freshwaters is highly needed. Especially, as freshwater sediments are expected to be temporary or long-term sinks for microplastics (Li et al., 2020; Bordós et al., 2019; Turner et al., 2019; Vaughan et al., 2017; Imhof et al., 2013).
In this study, microplastic concentrations in sediments of Lake Tollense, Mecklenburg-Western Pomerania, Germany, were analyzed over a two-year period by semi-annual sampling. Lakeshore sediments from four beach segments around the lake were taken as well as lakebed sediments at several points in the center of the lake. The major aim of the study was the quantification of microplastics in freshwater sediments using the example of Lake Tollense and addressing the hypothesis that lake sediments serve as a sink for microplastics. Furthermore, spatial and temporal distribution of microplastics were analyzed as well as the factors influencing the microplastic pollution within lake sediments, including pedologic characteristics such as percentage of organic matter and grain sizes.
Section snippets
Study area
The study was conducted at Lake Tollense in Mecklenburg-Western Pomerania (MV) in the northeast of Germany (Fig. 1b). Lake Tollense covers an area of 17.9 km2, has a mean depth of 17.3 m (max. depth 31.2 m) and a catchment area of 515 km2 which is predominated by agricultural and forest areas (Nixdorf et al., 2004). Several tributaries from sub-catchments with different landcover characteristics contribute to the water and material budget of Lake Tollense. One of the major inflows is the
Quality assurance and quality control (QA/QC)
The evaluation of QA/QC measures resulted in a mean and standard deviation (SD) of 28 ± 16 microplastic fragments and 8 ± 4 fibers per sample for blank samples paralleled with lakeshore sediments. The contamination for lakebed sediment blanks was 54 ± 33 fragments and 9 ± 4 fibers per sample. Many individual laboratory processing steps might cause contamination in samples. Increased microplastic numbers in blank samples for lakebed sediment analysis may result from more extensive rinsing
Microplastics in lake sediments
Microplastic abundances in freshwater environments have been studied around the world in the past decades. However, a comparison between the individual studies is hampered due to differences in methodical approaches as no standard protocol for microplastic identification in sediments exists so far (Koelmans et al., 2019; Prata et al., 2019; Dris et al., 2018; Horton et al., 2017a; Ivleva et al., 2017; Van Cauwenberghe et al., 2015). Therefore, results of this study can only be compared to other
Conclusion
Ubiquitous and high abundances of microplastics in shore and bed sediments analyzed over a two-year period were found at Lake Tollense. The study highlighted the retention capabilities of lake sediments and with this the storage characteristics lakes provide for microplastic particles on transport pathways from land-based sources to marine environments, functioning as miniature oceans. These results underline the importance of investigating lake sediments concerning their plastic pollution
Authors contribution
Elena Hengstmann: Conceptualization, Investigation, Methodology, Formal analysis, Writing – original draft, Visualization. Esther Weil: Software, Methodology. Paul Christian Wallbott: Software, Methodology. Matthias Tamminga: Conceptualization, Investigation, Writing – review & editing. Elke Kerstin Fischer: Conceptualization, Resources, Writing – review & editing, Supervision.
Funding
This study was supported by “Evangelisches Studienwerk Villigst” and by the Deutsche Forschungsgemeinschaft (DFG; grant number: 411261467) as being part of the MICROLIM project implemented at Lake Tollense.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank the “Evangelisches Studienwerk Villigst” for their PhD grant and the DFG funding the MICROLIM project (grant number: 411261467). We further thank Grace Swanson for kindly revising the language of our manuscript. Additionally, many thanks to the student assistants supporting us during field campaigns and Ann-Kristin Deuke and Sarah-Christin Stöwer for additional help in sample processing.
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