Plastic contamination is prevalent in lake Kallavesi, according to our results. It is challenging to compare the concentration of microplastics found in the Lake Kallavesi sediment to that found in other freshwater studies for various reasons. Firstly, it is due to the usage of various sampling methods by researchers. Samples taken from different sediment environments such as shore sediments and bottom sediment with different techniques cause difficulty in comparison. Secondly, the difference in sample processing techniques for extracting microplastics from sediments is a crucial step for microplastic analysis (He et al., 2020). The third reason for the difficulty in comparison is the utilization of different units for expressing results by different studies. Some researchers express their results in items/kg of dry sediments while others prefer to express in items/m2 (Campanale et al., 2020). However, we used items/g of dry sediment for reporting our results. The fourth reason for difficulties is that different types of environments (e.g., sea, lake, river) may not be comparable.
Based on FTIR analysis, microplastic concentration with respect to dry sediment weight in our samples of summer 2017 (15.09 items/g), winter 2017-2018 (8.66 items/g), and summer 2018 (4.75 items/g) are similar to the concentration of microplastics in the growing season (10.2 items/g) and winter season (4.2 items/g) studied by Saarni et al. (2021) in Huruslahti Bay, Finland. However, the concentration of microplastics reported by Saarni et al. (2021) was higher in summer samples while our results showed the highest concentration in winter samples especially in winter 2016-2018 samples (65.32 items/g).
Seasonality can have an impact on the microplastic accumulation to the sediments by influencing the microplastic availability in the catchment. Frozen and snow-covered soil holds microplastics during winter and releases them to water bodies during spring flooding.
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Moreover, microplastics can also be released by lake ice during its melting period since the ice can trap the microplastics into its structure during freezing period (Saarni et al., 2021; Peeken et al., 2018). Higher levels of microplastic in our winter samples could be because of spring flooding and lake ice melting.
The approximate deposition time of plastic polymers to the Baltic Sea sediments is about two weeks in calm conditions while almost 50 days in turbulent conditions (Schernewski et al., 2020). According to the snow depth data of the Finnish Meteorological Institute, the snow melted completely on 1st May 2017 and on 8 May 2018 (Finnish Meteorological Institute, 2021). Our samples were taken after 33 days of snowmelt in winter 2017 and 18 days after snowmelt in winter 2018. Plastic particles might have taken 4 weeks to finally reach the bottom sediment of lake Kallavesi. This is contributed by the strong currents running beneath the Kallansilat bridge as mentioned by Uurasjärvi et al. (2020). Furthermore, the highest abundance of microplastic found in the winter 2016-2017 samples collected after the 33 days of snowmelt could be due to microplastics took up to 4 weeks to reach the lower sediment layers. Less number of microplastics in summer samples as compared to winter samples can be resuspension of sediments during fall overturn in lake prior to our sample collection. Lake water turns over from top to bottom during spring and fall due to temperature change which in turn triggers the resuspension of sediments (Apolinarska et al., 2020).
Using density separation as an extraction method and FTIR as analyzing tool, Haave et al.
(2019) reported a microplastic concentration of 12-200 items/g in sediments of urban harbour of Norway. This concentration is considerably higher than the concentration of microplastics in the current study as this harbour receives microplastics from wastewater treatment plants with only primary treatment while our sampling area is far away from the wastewater treatment plants. In addition, wastewater treatment plants in Finland use several treatment steps before discharging water into lakes (Talvitie et al., 2015). Moreover, sampling sites were near the city center in western Norway, in a comparatively higher human activity spot than our sampling site.
The abundance of microplastics found in this study is comparatively higher than in other freshwater studies carried out in highly populated regions. Eo et al. (2019) identified 1.97 items/g in the sediments of Nakdong river, South Korea and the concentration were higher in the wet season. Moreover, reported microplastic abundance in the sedimnets of lake Bolsena and lake Chiusi are 0.12 item/g and 0.23 items/g respectively with size ranged from 300 µm to
5000 µm (Fischer et al., 2016). The prevalence of microplastics has been linked to the vicinity of highly inhabited areas in numerous studies (Liu et al., 2019). Although lake Kallavesi is located in a less populated area, the higher concentration of microplastics can be due to reporting smaller particles (< 1000 µm) in our samples.
The distribution of microplastics into the different compartments of the water body can be affected by the density of particles (Ballent et al., 2016). PE and PP being less dense polymers than water were common in our sediment samples. The possible reason for the frequent occurrence of these polymer types in the sediment of lake Kallavesi can be biofouling. Higher surface area to volume ratio of microplastics accompanied by hydrophobic nature stimulates the adsorption of organic materials which in turn promotes the microbial colony on its surface.
Biofouling can increase the density of polymers and leads them to sink to the benthic sediments (Kaiser et al., 2017). Moreover, the addition of inorganic substances to the polymers during manufacturing can also increase the density of less dense polymers which can influence the distribution pattern (Ballent et al., 2016). Nevertheless, we did not study inorganic fillers in our samples during FTIR analysis. Low-density polymers such as PE and PP have also been found in inland water sediments in other studies. For instance, high quantities of PE and PP were discovered in Lagoon sediments of Venice, Italy (Vianello et al., 2013). Frère et al. (2017) also investigated PE and PP as dominant polymers in the sediments of Bay of Brest, France.
Confining microplastics to specific origins is challenging because they have a fragmented nature and small size, and numerous possible sources (Ballent et al., 2016). The predominancy of PS followed by PP, PE, and PET in all of our samples is probably due to their extensive use.
PP, PE, PET, and PS are used in a variety of consumer products such as packaging materials, bottles, and construction materials as a result they are ubiquitous in the environment (Vianello et al., 2013). PA could be uncommon in our sampling area because we found PA in only winter 2017-2018 samples. Furthermore, if biofilm develops on surface of PA it can be difficult to identify it with FTIR (Uurasjärvi et al., 2020). Previous study also reported PS, PE, and PP, as well as minor quantity of PA and PVC in the sediments of Lake Garda, Italy (Imhof et al., 2013). Our samples were kept in centrifuge tubes made of PP and ethanol was used for rinsing purpose. Centrifuge tubes and ethanol may have contaminated our samples and results in high abundance of PP in all of our samples. Nonetheless, PP particles less than 100 µm were excluded from our results.
The predominance of fragments over fibres in sediment samples of lake Kallavesi indicates that the major source of microplastic is from secondary sources such as the breakdown of larger plastic particles or debris. The wastewater treatment plant is considered as the main source of fibres input into the water bodies (Murphy et al., 2016), this supports the low abundance of fibres in our samples because there is no wastewater treatment plant located near our sampling site. The presence of fibers in sediments could be due to the shed of fibres from the clothes of people while performing sports activities on a frozen lake as well as from sports equipment such as fishing lines. A previous study also mentioned clothes and fishing lines as a major source of fibres (Campanale et al., 2020). Stream flowing from north and entering the Kallavesi lake near Kallansillat bridge can be the potential source of microplastic entrance into the lake.
Furthermore, surface runoff during rain or snowmelt can transfer microplastic to the lake. In addition, a Pulp mill in the proximity of the Kallansillat bridge and litter thrown out from vehicles might be a possible source of microplastics input to the lake.
During microplastic sorting under stereo microscope, we noticed a huge number of black particles in our samples which can be from vehicle tires as our samples were collected from under highway bridge which experiences heavy traffic all the time. However, we did not identify any styrene-butadiene rubber (SBR) during FTIR analysis. Tire rubber cannot be identified by FTIR in both transmission and reflectance mode because of the presence of carbon black in the tires (Haave et al., 2019; Wagner et al., 2018). Plastic particles from tire wear and road paints might readily be swept away from the highway to the lake during rainstorms or snowmelt.