SEASONAL VARIATION OF MICROPLASTIC ACCUMULATION IN LAKE SEDIMENTS
Shahina Karim Master of Science thesis Master´s Degree Program in Environmental Health and Technology University of Eastern Finland Faculty of Science and Forestry Sciences Department of Environmental and Biological Sciences 21.06.2021
UNIVERSITY OF EASTERN FINLAND, Faculty of Science and Forestry Master’s degree programme in Environmental Health and Technology
Shahina Karim: Seasonal variation of microplastic accumulation in lake sediments MSc thesis 42 pages, (42 pages)
Supervisors:
Arto Koistinen PhD, Research Director Jussi Kukkonen PhD, Professor
Saija Saarni PhD, Postdoc researcher
Emilia Uurasjärvi MSc, Early-stage researcher 21 June, 2021
________________________________________________________________________
keywords: Microplastic, sediment, FTIR spectroscopy, freshwater, Lake Kallavesi
ABSTRACT
Microplastic contamination is an emerging environmental issue. The majority of studies have focused on the microplastic pollution occurring in marine water, however, there is a lack of knowledge about the presence of microplastics in the freshwater system. The aim of this study was to understand the seasonal variation of microplastic accumulation in lake sediments.
Sediments were collected from the lake Kallavesi by using the sediment trap technique and trap monitoring of sediments lasted for two years (winter 2016 - summer 2018). Microplastics were isolated from sediments by using the heavy-liquid density separation method and then visually picked under a stereo microscope for Fourier-transform infrared spectroscopy (FTIR) analysis. Obtained spectra from FTIR were analyzed by siMPLe software. Microplastic concentrations were high in winter samples as compared to summer samples indicating spring flooding as a main source of microplastic in the lake. Overall, the winter 2016-2017 samples had the highest concentration (65.32 items/g) of microplastic, whereas the summer 2018 sample had the lowest (8.66 items/g). A total of five polymer types (PP, PS, PE, PET, and PA) were detected in the size range of 28.2 µm to 828.6 µm. PP was the predominant polymer in all samples while PA was identified in only the winter 2017-2018 sample. The fragment was the most prevalent shape found in our samples, suggesting that microplastics originated from secondary sources. Nevertheless, few fibres were also detected. In conclusion, seasonality should be considered while studying microplastic abundance in water bodies since it influences the presence of microplastics in aquatic systems.
Table of Contents
ABSTRACT ... i
LIST OF ABBREVIATIONS ... iv
TABLES AND FIGURES ... v
1 INTRODUCTION ... 1
2 BACKGROUND ... 2
2.1 Introduction to environmental problems caused by plastics ... 2
2.1.1 Definition and history of plastic/microplastic ... 2
2.1.2 Chemical nature ... 3
2.1.3 Sources of microplastic ... 3
2.1.4 Degradation of plastics ... 4
2.2 Effects of microplastic on aquatic organisms ... 5
2.2.1 Effects by ingestion of microplastic ... 5
2.2.2 Effects of leaching chemicals ... 6
2.2.3 Accumulation of other compounds ... 7
2.3 Microplastics impact on human health ... 8
2.4 Occurrence of microplastic in freshwater system ... 8
2.4.1 Microplastic in surface water ... 8
2.4.2 Microplastics in sediments... 9
2.5 Factors influencing accumulation of microplastics to sediments ... 13
2.5.1 Seasonal variation ... 14
2.5.2 Physical Factors ... 14
2.6 Sediment trapping ... 15
3 MATERIAL AND METHODS ... 15
3.1 Study site ... 15
3.2 Sampling and sample processing ... 16
3.3 Visual inspection of microplastics ... 17
3.4 Identification and analysis of microplastics ... 17
3.5 Data Reporting ... 18
3.6 Contamination control ... 18
4 RESULTS... 18
4.1 Concentration of microplastics ... 18
4.2 Characteristics of Microplastics (Sizes, morphology, and colours of microplastics) ... 19
4.3 Chemical composition of microplastics ... 20
4.4 Contamination control ... 21
5 DISCUSSION ... 22
6 CONCLUSION ... 25
7 ACKNOWLEDGEMENT ... 27
8 REFERENCES ... 28
LIST OF ABBREVIATIONS
ABS Acrylonitrile butadiene styrene DDT Dichlorodiphenyltrichloroethane FPA Focal Plane array
FTIR Fourier transform infrared spectroscopy HDPE High-density polyethylene
LDPE Low-density polyethylene
PAHs Polycyclic aromatic hydrocarbons PAN Polyacrylonitrile
PBDEs Polybrominated diphenyl ethers PCBs Polychlorinated biphenyls PET Polyethylene Terephthalate PMMA Polymethyl methacrylate PP Polypropylene
PPE Personal Protection Equipment PS Polystyrene
PUR Polyurethane PVC Polyvinyl chloride
WWTPs Wastewater treatment plants
TABLES AND FIGURES
TABLE 1.SOME FRESHWATER ORGANISMS REPORTED TO INGEST PLASTIC PARTICLES. ... 6
TABLE 2.DENSITY AND CRYSTALLINITY OF POLYMERS ADAPTED FROM WAGNER AND LAMBERT,2018. ... 7
TABLE 3.SUMMARY OF DIFFERENT STUDIES REGARDING MICROPLASTIC ABUNDANCE IN RIVER SEDIMENTS. ... 11
TABLE 4.SUMMARY OF DIFFERENT STUDIES REGARDING MICROPLASTICS ABUNDANCE IN LAKE SEDIMENTS. ... 12
TABLE 5.AVERAGE SIZE DISTRIBUTION OF PARTICLES IDENTIFIED BY FTIR. ... 19
FIGURE 1.STUDY SITE (NATIONAL LAND SURVEY OF FINLAND) ... 16
FIGURE 2.CONCENTRATION OF MICROPLASTICS PER GRAM IN SEDIMENT SAMPLES. ... 19
FIGURE 3.MORPHOLOGY OF MICROPLASTICS PRESENT IN EACH SAMPLE FROM SEDIMENTS. ... 20
FIGURE 4.IMAGES OF MICROPLASTICS IN OUR SAMPLES TAKEN BY STEREOMICROSCOPE. ... 20
FIGURE 5.POLYMER TYPES OF MICROPLASTICS OBSERVED IN SEDIMENT SAMPLES. ... 21
FIGURE 6.POLYMER TYPES OBSERVED IN BLANK SAMPLE. ... 22
1 INTRODUCTION
The onset of microplastic contamination of water bodies is pervading around the globe (Pan et al., 2020). Plastic pollution in the marine ecosystem was first studied in the 1970s, and since then, many studies have revealed the consistent presence of plastic debris in aquatic ecosystems such as streams, rivers, lakes, sea, and oceans (Woodall et al., 2014; Van Cauwenberghe et al., 2013; Eerkes-Medrano et al., 2015; Vianello et al., 2013). Plastic particles have also been investigated in the far-flung corners of the globe which have limited human activities such as Arctic Sea ice and Antarctica (Bergmann et al., 2017; Isobe et al., 2017; Cincinelli et al., 2017).
Because of the huge utilization demand of plastic products and low recycling practices, plastic contamination has become a severe environmental problem (Shahul Hamid et al., 2018).
Persistency of plastics in the environment is another important factor in excerbating plastic pollution globally.
A category of plastic fragments that remain smaller than 5 mm in length are represented by the term microplastics (Daniel et al., 2020). The plastics that are produced are classified as either primary or secondary microplastics. Primary plastics are utilized in personal care products, cosmetics, and other direct uses while secondary microplastics are formed due to the breakdown of larger plastic particles by environmental forces such as UV light, wind, and waves. Several studies have documented the ingestion of microplastics by aquatic organisms. Microplastics can readily cause a variety of environmental problems by acting as a vector for the transport of organic contaminants and heavy metals due to their large surface area to volume ratio and hydrophobic nature (Ding et al., 2019; Barboza et al., 2018).
Although there has been extensive research on microplastic pollution in the marine environment, studies on inland water systems are still diminutive (Lebreton et al., 2017; Iannilli et al., 2020; Akdogan and Guven, 2019). Microplastics appear to sink more frequently in the lacustrine environment under stagnant water conditions and a longer water residence time.
Numerous studies have reported that the accumulation of low-density plastic polymers in the sediments might be due to biofouling and plastic fillers (Iannilli et al., 2020). The presence of microplastics in the sediments of Lake Kallavesi is merely explored, thus microplastic in the sediments must be studied to gain a complete picture of the microplastic pollution in the Lake Kallavesi.
The aim of this study was to investigate the seasonal variation of microplastic accumulation in lake sediments by studying the quantities and characteristics of microplastics.
2 BACKGROUND
2.1 Introduction to environmental problems caused by plastics
The production of plastic has increased dramatically from around 0.35 million tons in the 1950s to 359 million tons in the year 2018 (Govender et al., 2020) . This production rate included majority of the plastic being non reusable. Since this era of the global pandemic is demanding more plastic to produce personal protection equipment (PPE), packaging and medical tools, a huge rise in plastic production is expected (Govender et al., 2020). The rate of plastic entering the environment is higher than the recovery rate therein, coastal, and oceanic systems are suffering from a rapid plastic pollution. Consequently, the probability of microplastic pollution is also increasing in these sites (Govender et al., 2020). Freshwater bodies play a critical role in the transitioning of microplastic from land to sea as they are potential pathways as well as potential hotspots when surrounded by human residency. Factors promoting the plastic accumulation in freshwater bodies such as lakes are: vicinity to human population, water residence time and stagnant water conditions (Iannilli et al., 2020). Majority of studies have been done in the microplastic pollution in coastal and marine environment while research on fresh water is still limited (Iannilli et al., 2020).
2.1.1 Definition and history of plastic/microplastic
The development of rubber technology after 1800s is the primary cause of excessive usage of plastic materials today (Thompson et al., 2009). Bakelite, the very first synthetic polymer, was introduced by a Belgian chemist namely Leo Baekeland in 1907. In the upcoming decades, many more plastic were also produced. Later, after the 1940s and 1950s, the actual mass production of plastic used in everyday life started. Development of polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyurethane (PUR), and polystyrene (PS) occurred in 1930s.
1950s is considered as the era of development of high-density polyethylene (HDPE) and polypropylene (PP). Synthetic plastic materials were produced without using natural resources in 1960s due to the advancement in material sciences (Thompson et al., 2009; Wagner and Lambert, 2018).
World War Ⅱ accelerated the production of plastic and in 1950s annual production was about 5 million tonnes (Napper and Thompson, 2020). The properties of plastic such as light weight, strong, durability, corrosion-resistant, and cheap production cost further promote the production of plastic. Hence, the production of plastic boosted from 30 million tonnes in 1998 to 359
million tonnes in 2018 (Napper and Thompson, 2020). Plastic production will surge in upcoming years due to its benefits and imagining of modern society is not easy. (Napper and Thompson, 2020; Dong et al., 2020).
Smaller pieces of plastic having diameter less than 5mm are termed as microplastics, however globally accepted definition for microplastic does not exist yet (Pan et al., 2020; Hartmann et al., 2019). According to Hartmann et al. (2019) the size of nanoplastics ranges from 1 to <1000 nm, microplastics ranges from 1 to >1000 µm, mesoplastics ranges from 1 to <10 mm, and macroplastics is larger than 1cm. Presence of microscale particles were identified in marine ecosystem in 1970s and plethora of studies have revealed the occurrence of microplastics in marine system since then (Li et al., 2020). Thompson et al. (2004) first introduced the term
“microplastic” and research on microplastic in fresh water bodies was first initiated by (Zbyszewski and Corcoran, 2011). To date, studies on microplastics in freshwater bodies has been done by not more than 23 countries, with the majority of them being in North America and Europe as well as China, India, Mongolia and other countries. (Ma et al., 2019).
2.1.2 Chemical nature
Fossil fuel is mainly used for the synthesis of plastic, but biomass is sometimes used as feedstock (Andrady and Neal, 2009). The most prevalent plastic materials, polypropylene (PP), Polyethylene (PE), polyvinyl chloride (PVC), polyethylene terephthalate (PET), and polystyrene (PS) altogether make around 90% of overall production of plastic in the world (Andrady and Neal, 2009). These polymers contain high molecular weight and are nonbiodegradable. Therefore, they are also consistently existing in the environment. The usage of these resins; PE, PP, PVC, PS, PET, and PUR is 29, 19, 12, 8, 6, and 7% respectively in the global production (Rani et al., 2015). Nearly all plastics contains carbon and hydrogen as their key component while PVC also contains chloride as main part with carbon and hydrogen. A large variety of additives such as thermal stabilizers, inorganic fillers, plasticizers, UV stabilizers, and fire retardants are also added to plastics to increase their performance (Andrady and Neal, 2009).
2.1.3 Sources of microplastic
Microplastics enter the environment through different sources. Primary and secondary microplastics are two categories of microplastics (Horton et al., 2017). Microplastics which are intentionally produced in micrometre size such as microbeads and pellets are known as primary microplastics. Microbeads are manufactured to use them in personal care products such as
exfoliating scrubs, lotions, toothpaste while pellets are used by plastic industries to produce plastic goods. Secondary microplastics are formed through the degradation of large pieces of the plastic due to the environmental conditions and abrasion of synthetic fibres during laundry (Horton et al., 2017). According to Browne et al. (2011) estimation, one washing cycle can release more than 1900 fibres.
The major mechanisms behind the formation of microplastic are mechanical, chemical, biological and UV degradation of larger fragments (Horton and Dixon, 2018). There are numerous routes for the entrance of microplastics to the environment and they can be different for different regions. For instance, wealthy regions may experience more primary microplastic from personal care products than less wealthy region (Wu, Zhang and Xiong, 2018).
Wastewater treatment plants (WWTPs) are considered as a major pathway for the transport of microplastics since microbeads and synthetic fibres are released into them. Sludge from wastewater treatment plants can also be the source of microplastic pollution in aquatic system through runoff when applied to the agricultural land (Horton et al., 2017). Other sources for microplastic entrance include overflow of sewage system due to heavy rain, abrasion from tyres, and land litter (Eriksen et al., 2013; Andrady, 2011).
2.1.4 Degradation of plastics
Degradation is the chemical transformation of the structure of polymers by decreasing their molecular weight (Andrady, 2011). The most important feature of synthetic polymers is their strong resistance to environmental conditions which increases their residence time and minimize degradation in the environment. Conversion of polymers into smaller molecular units such as oligomers and monomers occur during the degradation process. Degradation of synthetic polymers can be categorised into two types: biotic and abiotic. (Eubeler et al., 2009).
UV-B radiation from the sun is the primary cause of Low-density polyethylene (LDPE), high- density polyethylene (HDPE), PP, and nylons photo oxidative degradation. This degradation process can be further carried on by thermo-oxidation (Andrady, 2011). Extensive degradation causes the embrittlement of the plastics, which results in micro and nano sized plastics. These micro and nano sized fragments further undergo degradation by microorganisms. Polymers containing carbon can be converted into carbon dioxide and integrated into biomass by microorganisms during biodegradation (Anderson, Park and Palace, 2016). Estimated time needed for complete mineralization of plastic is from hundred to thousand years (Barnes et al.,
2009). Plastic degradation is slower in aquatic than terrestrial systems due to the less exposure to UV radiations and oxygen (Corcoran, Biesinger and Grifi, 2009).
Many factors which affect the degradation of microplastics are environmental conditions, polymer properties (density and crystallinity), and the type and number of additivities added to polymers. Crystallinity of polymers is a key feature which influence degradation by affecting the permeability and density of polymers. The addition of additives for instance, antioxidant and antimicrobial agents extends the plastic life. On the other-hand, biological ingredients enhance microplastics degradation (Wagner and Lambert, 2018).
2.2 Effects of microplastic on aquatic organisms
To date, majority of studies have been done on the ecotoxicity of microplastics on marine life and very less attention has been given to the toxicity of microplastic on freshwater organisms (Ma et al., 2019). Microplastics in the aquatic ecosystem can affect the death rate, growth, development, food intake, and reproductivity of the organisms (Ma et al., 2019).
2.2.1 Effects by ingestion of microplastic
A variety of aquatic organisms are prone to the ingestion of microplastics. They can intake microplastic through direct ingestion or through their dermis (Wagner and Lambert, 2018). Au et al. (2015) exposed Hyalella Azteca (freshwater amphipod) to polypropylene fibre and polyethylene particles and indicated the impacts of microplastic ingestion on the growth, reproduction, and digestive system of the organism. This study also showed that micro fibres are more toxic than microplastic particles may be due to their long residence time in the intestine of organism. Another study was carried out by Lei et al. (2018) to evaluate the impact of microplastic on the pelagic and benthic organisms in fresh water. This study showed the intestine impairment such as cracking of villi and splitting of enterocytes of Zebra fish (Dernio rerio) and nematode (Caenorhabditis elegans) due to microplastic aggregation. The ingestion of microplastics by freshwater organisms has been shown by different studies (Table 1).
Benthic organisms are more susceptible to the microplastic pollution as they may not be able to distinguish between plastic particle and food (Blarer and Burkhardt-Holm, 2016). Particle size of plastic is important in posing risk to organisms because smaller particles are more hazardous than large particles (Lei et al., 2018). Moreover, attachment of plastic particles can be affected by their shape, for instance needle shape particles can be easily attached to the inner and outer side of the organism’s body. Small particles with angular shape may be difficult to remove than particles with smooth and spherical shape. This results in the blockage of the
digestive tracts as well as the gills (Ogonowski et al., 2016). Generally, ingestion does not cause direct mortality of organisms, however it can cause chronic effects such as oxidative stress and starvation which in turn can cause mortality (Bellasi et al., 2020).
Table 1. Some freshwater organisms reported to ingest plastic particles.
Phylum Species Plastic size References
Chordate Hoplosternum
littorale
Mesoplastic, microplastic
Silva-Cavalcanti et al., 2017
Mollusca Anodonta anatina Microplastic Berglund et al., 2019 Arthropoda Daphnia magna Nanoplastics,
microplastics
Rosenkranz et al., 2009
Mollusca Potamopyrgus
antipodarum
Microplastic Duis and Coors, 2016
Arthropoda Notodromas
monachal, Bosmina coregoni
Microplastic Duis and Coors, 2016,
Wagner and
Lambert, 2018
Annelida T. tubifex Microplastic Hurley, Woodward
and Rothwell, 2017
2.2.2 Effects of leaching chemicals
Microplastics can be source of harmful chemicals because additives are used in the manufacturing of synthetic polymers to enhance their properties (Bellasi et al., 2020). The most common additives incorporated to plastics are phthalates, nonylphenol, bisphenol A, and brominated substances. Since these chemicals are not covalently bound to the polymers, they can eventually leach out of the plastics and pose a risk to the aquatic organisms (Bellasi et al., 2020). Direct exposure of aquatic organisms to the additives can occur because of the large surface area to volume ratio of microplastics in the guts. These ingested chemical additives can act as endocrine disruptors which can further affect the mobility, reproduction, and development of aquatic life. Moreover, plasticizers can also induce carcinogenesis in aquatic biota (Barnes et al., 2009).
2.2.3 Accumulation of other compounds
Hydrophobic nature of microplastics having large surface areas, help them to sorb organic contaminants such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenylethers (PBDEs), dichlorodiphenyltricholoroethane (DDT) (Anderson, Park and Palace, 2016). Turner and Holmes. (2015) showed the accumulation of metals such as Cd, Co, Cr, Cu, Ni, Pb to the microplastics in different pH and salinity levels.
This study also found that metal adsorption to microplastics is significant in freshwater than seawater. Aging of plastics enhances its porosity, and polarity which makes them susceptible to accumulate metals easily and transport them in aquatic environment (Ashton et al., 2010).
Turner and Holmes. (2015) investigated that the adsorption of metals (Ag, Al, Cd, Co, Cr, Fe, Hg, Mn, Ni, Pb, and Zn) was higher in weathered pellets than fresh plastic pellets.
Polymer types and its properties including density and crystallinity play a vital role in the adsorption rate of metals (Table. 2) (Teuten et al., 2007; Wagner and Lambert, 2018).
Moreover, pollutant type and surrounding water environment are also considered as a key factor in the adsorption process. Plastics particles having amorphous areas are more prone to accumulation and diffusion of hydrophobic pollutants than crystalline areas as these areas contain strong polymer chain (Wagner and Lambert, 2018). Microplastics may act as a vector for adsorbed contaminants to reach the food chain via accidental ingestion by aquatic organisms, potentailly amplifying the bioacumulatio of pollutants (Qu et al., 2018).
Table 2. Density and crystallinity of polymers adapted from Wagner and Lambert, 2018.
Polymer Type Density (g/cm3) Crystallinity Low- density polyethylene 0.91 – 0.93 45 – 60%
High-density polyethylene 0.94 – 0.97 70 – 90%
Polystyrene 0.96 – 1.05 Low
Polypropylene 0.85 – 0.94 50 – 80%
Polyvinyl Chloride 1.38 High
Polyethylene terephthalate 1.34 – 1.39 High
Polyoxymethylene 1.41 70 – 80%
Polycarbonate 1.20 Low
Polylactic acid 1.21 – 1.43 37%
Polyamide 1.12 – 1.14 35 – 45%
Natural rubber 0.92 Low
2.3 Microplastics impact on human health
Human health can be jeopardized by the introduction of toxic substances and microplastics into the food web (Cole et al., 2013). Different studies showed that microplastics are ingested by variety of marine species which in turn are directly eaten by humans (Cole et al., 2013).
Microplastics has also been investigated in the gastrointestinal track of freshwater fish which are consumed by humans (Biginagwa et al., 2016). Nonetheless, human health risk from microplastics depends on the level of microplastic absorption by intestine cells and their translocation into the tissues of aquatic organisms. Microplastics containing additives that are considered carcinogenic and mutagenic can be leached into human bodies if aquatic organisms exposed to microplastics are consumed (Wright and Kelly, 2017; Auta et al., 2017).
Microplastics act as substrate for the growth of microorganisms due to its hydrophobic nature and they can be transferred to humans via consumption of aquatic organisms (Zettler et al., 2013). Moreover, scientific studies have demonstrated the existence of microplastic in some aquatic species and speculated the possible detrimental consequences on human beings, so far no research have either validated or refused this risk (Rist et al., 2018).
2.4 Occurrence of microplastic in freshwater system
According to recent studies microplastic is ubiquitous in different compartments of freshwater environments, including sediments (Wagner and Lambert, 2018). The presence of micro and macro plastics were first evident in lakes in Switzerland (Faure et al., 2012) and in subalpine lake in Italy (Imhof et al., 2013). As a result, many studies were conducted and published regarding the deterioration of water bodies including lakes, estuaries, and rivers, addressing macro and micro plastic contamination. Nevertheless, quantification of microplastic has been carried out by using different methods in different studies which in turn cause difficulty in comparison of results.
2.4.1 Microplastic in surface water
The occurrence of microplastics has been studied in the surface water, especially in lakes (Faure et al., 2012). The density of microplastics found in Lake Geneva was 48146 particles/km2 (Faure et al., 2012). Correspondingly, microplastics were also investigated in the surface water of Laurentian Great Lakes (Lake Huron, Lake Superior, Lake Erie). Average microplastic concentration reported in these lakes was 43.157 particles/km2 with overall concentration ranged from 0 to 450000 particles/km2. Among the three lakes, Lake Erie had the highest
concentration (90%) of microplastics due to the vicinity of coal power plants (Eriksen et al., 2013). Free et al. (2014) evaluated the concentration of microplastics in Lake Hövsgöl, a remote mountain lake located in Mongolia. According to this study, average microplastic density was 20264 particles/km2 which is higher than the average concentration of microplastics in Lake Huron and Superior.
Surface water of lakes in China contained the highest concentration of microplastics ranging from 1660 to 15000 particles/m3. While the concentration of microplastic in North America, Europe and Mongolia were much less (0.06 to 3.02 particles/m3). This high value of microplastics can be the overestimation due to methodological differences used for sampling (Wang et al., 2018; Su et al., 2016). Microplastic concentration in surface water of Lake Kallavesi, Finland was investigated by Uurasjärvi et al. (2020) using manta trawled and pump filtered methods. The average abundance of microplastics reported by using the manta trawled method was 0.27 ± 0.18 microplastics/ m3 while the results for pump filtered method were 1.8
± 2.3 (> 300 µm), 12 ± 17 (100-300 µm), and 155 ± 73 (20-100 µm) microplastics/m3. This study detected the highest concentration in the vicinity of the wastewater treatment plant, harbour, and snow dumping area.
Microplastic contamination of surface water in Rhine river has been studied by Mani et al., (2015). An average concentration of microplastics taken from eleven location was 892777 particles/km2 and the most common shape of microplastic recorded in this study was sphere.
Moore et al. (2011) reported the microplastic presence in surface water of Los Angeles and Gabriel rivers. The abundance of plastic particles in these rivers ranged between 0.01 to 12.9 particles/L. The most common type of plastic particles found in both rivers were foam based.
The deterioration of Dnube river by microplastic pollution was studied for two years (2010- 2012) and a mean concentration of 316.8 ± 4664.6 items/1000m-3 was reported in surface water (Lechner et al., 2014).
2.4.2 Microplastics in sediments
The recorded values of microplastic pollution in beach and bottom sediment are significantly higher than surface water. This is in line with the fact that, both the bottom sediment and drift line, which are mostly sampled to measure microplastic abundance on beaches are considered as heavy microplastic aggregation zones (Browne et al., 2015; Imhof et al., 2018). Furthermore, microplastic presence has also been studied in the abyssal sediments of marine environments, indicating the microplastic contamination of isolated habitats (Martin et al., 2017).
2.4.2.1 Lakeshore and riverbank sediment
Plastic deposition in lakes can occur due to the transportation of plastic waste generated in the catchment area of lakes (Yang et al., 2021). The highest reported concentration of microplastics were in the lakeshore sediment of Dongting Lake in China. Polystyrene (PS) and polyethylene terephthalate (PET) were the abundant microplastic type in both West and South Dongting Lake. The reason for the highest microplastic concentration can be the location of the lake near an overpopulated city and the large tourist influx to this area (Jiang et al., 2018). A tremendous amount of microplastics were found in the lakeshore sediment in Norway, with concentrations ranging from 12000 to 200000 items/kg and particle size less than 100 µm. This study associated the highest concentration of microplastics with reporting smaller sizes of plastic particles (Haave et al., 2019).
Rivers are considered as one of the most dominant route for the transportation of microplastics from land to oceans (Yang et al., 2021). Microplastics have been found in the freshwater sediments all over the globe, and the average concentration of microplastics ranged from zero to several thousand particles per kilogram. To date, the highest abundance of microplastics in riverbanks sediments are revealed in the Rhine River situated in Germany, with concentrations ranging between 260 ± 10 to 11070 ± 600 items/kg. Moreover, highly populated residential areas and plastic manufacturing industries located near Rhine River catchments areas can be the reasons behind the high concentration of microplastics in this river (Yang et al., 2021).
Sarkar et al.( 2019) studied the microplastic occurrence in the sediments of Ganga River, India, with abundance ranged from 99.27 to 409.86 items/kg. The most common type polymers in this study were polyethylene terephthalate (39%) and polyethylene (30%).
2.4.2.2 Lake and river bottom sediment
Since the observation of microplastics in a benthic environment in 1970, sediments are thought to be long-term sinks for microplastics (Zhang et al., 2020). Microplastics are far more abundant in the sediments as compared to surface water because of their long-term accumulation to the sediments. The environmental circumstances in the bottom sediments are more complicated, resulting in significant harm from microplastics in sediments. Nevertheless, there are limited studies regarding temporal and spatial dispersal and degradation of microplastics in benthic sediments (Zhang et al., 2020).
Occurrence of microplastics in bottom sediment of Lake Ontario was investigated by Corcoran et al. (2015). The total number of 35 microplastics were detected in bottom sediment and study
of sedimentation rate revelaed that microplastic acummulation started 38 years ago. Bottom sediments of Taihu Lake in China have been considered as highest microplastic reserviour (Su et al., 2016). Vesijärvi Lake and Pikku Vesijärvi pond in Finland were surveyed to investigate microplastic contamination. The average number of microplastics in bottom sediment were 395.8 ± 90 items/kg. Polyamide was the dominant polymer and stormwater was considered as major source of microplastic contamination (Scopetani et al., 2019). Saarni et al. (2021) studied microplastic concentration in Lake Haukivesi, Finland by sediment trap method and reported about 10000 particles/kg.
Studies has also been carried out globally regarding microplastic accumulation in river bottom sediments (Yang et al., 2021). For instance, 30.3 ± 1.59 items/kg of microplastics were found in the bottom sediments of Ciwalengke River, Indonesia. Microplastic contamination can be because of the discharge of industrial water into this river (Alam et al., 2019). 220 items/kg of microplastics were recovered from the bottom sediment of Ottawa river in Canada. Wastewater treatment plant was suggested source of plastic pollution in this river (Vermaire et al., 2017).
According to He et al. (2020) microplastic abundance in Brisbane River, Australia, ranged from 10 to 520 items/kg. Polyethylene, polyamide, and polypropylene were the predominant polymers in the samples taken from bottom sediments. Eo et al. (2019) reported the presence of microplastic in Nakdong River, with the mean concentration of 1970± 62 particles/kg. The most common type of plastic invaded the sediments of this river were polyethylene and polypropylene. Streams can also be contaminated by plastic pollution since microplastics were detected in 18 streams in and around Auckland, New Zealand. The reported concentration of microplastics in the sediments of these small streams was 80 items/kg (Dikareva and Simon, 2019). Both smaller and larger sizes of microplastics have been found in freshwater bodies all around the globe (Table 3 and 4).
Table 3. Summary of different studies regarding microplastic abundance in River sediments.
Location Compartment Concentration Common Shape
Reference
Nakdong River, South Korea
Bottom sediment
1970 ± 62 items/kg
>50 % fragments
Eo et al., 2019
Ciwalengke River, Indonesia
Bottom Sediment
30.3 ± 1.59 items/kg
91% fibre Alam et al., 2019
Brisbane River, Australia
Bottom sediment
10-520 items/kg Films (dominant)
He et al., 2020
Ottawa River, Canada
Bottom sediment
220 items/kg N/A Vermaire et al., 2017
Pearl River, China
Bottom sediment
80-9597 items/kg
54.7% fibre Lin et al., 2018
Wei River, China
Bottom sediment
360 - 1320 items/kg
50.01% fibre Ding et al., 2019
St. Lawrence River, Canada
Bottom sediment
13832 ± 13677 items/m2
90% pellet Castañeda et al., 2014
Antua River, Portugal
Bottom sediments
2.6 – 529 items/kg
Fragments (dominant)
Rodrigues et al., 2018
Wen-Rui Tang River, China
Bottom sediment
32947 - 15342 items/kg
65% fragments Wang et al., 2018
Bloukrans River, South Africa
Riverbank sediment
160.1 ± 139.5 items/kg
N/A Nel et al., 2018
Thames River, UK
Riverbank sediment
660 items/kg 49.3%
fragments, 47.4% fibres
Horton et al., 2017
Tibetan Plateau Rivers, China
Riverbank sediment
90 – 130
items/kg
53.8% - 80.6%
fibre
Jiang et al., 2019
Rhine-Main River, Germany
Riverbank sediment
260 ± 10 – 11070 ± 600 items/kg
N/A Mani et al.,
2019
Ganga River, India
Riverbank sediment
99.27 – 409.86 items/kg
N/A Sarkar et al.,
2019
Table 4. Summary of different studies regarding microplastics abundance in Lake sediments.
Location Compartment Concentration Common Shape
Reference
Taihu Lake, China
Bottom sediment
11 – 235
items/kg
≈ 84% fibre Su et al., 2016
Vesijärvi Lake and Pikku Vesijärvi pond, Finland
Bottom sediment
395.8 ± 90 items/kg
N/A Scopetani et al., 2019
Urban Lake, London
Bottom sediment
539 items/kg >80% fibres Turner et al., 2019
Poyang Lake, China
Bottom sediment
54 – 506
items/kg
44% fibres Yuan et al., 2019
Lake Bolsena, Italy
Bottom sediment
112 ± 32
items/kg
Fibre and
fragments
Fischer et al., 2016
Chiusi Lake, Italy
Bottom sediment
234 ± 85
items/kg
Fibres and fragments
Fischer et al., 2016
Quinghai Lake, China
Lakeshore sediment
67-1292 items/m2
Fibres and sheets
Xiong et al., 2018
Tibet Plateau Lakes, China
Lakeshore sediment
4 - 1219 items/
m2
>80% fragments Zhang et al., 2016
Urban recipient, Norway
Lakeshore sediment
12,000 – 200,00 Items/kg
N/A Haave et al.,
2019 Dongting Lake,
China
Lakeshore sediment
200 – 1566 items/kg
77.4% fibre Jiang et al., 2018
2.5 Factors influencing accumulation of microplastics to sediments
To date, the dispersal of microplastics in freshwater sediments is not completely known nevertheless understanding the external forces that drive their movement is crucial to deeply know about their distribution pattern (Yang et al., 2021). Quantitative and modelling methods will be required for understanding the impact of physical forces on spatial distribution of microplastics (Yang et al., 2021). The amount of microplastics in the sediment can be influenced by several factors such as seasonal variation, biofouling, and characteristics of microplastics (Yang et al., 2021).
2.5.1 Seasonal variation
Seasonality can affect the microplastic accumulations rates in the sediments (Saarni et al., 2021). Freshwater researchers have associated high microplastic abundance in aquatic system with rain fall events (Moore et al., 2011; Corcoran et al., 2015; Faure et al., 2015; Campanale et al., 2020). Surface runoff increases the microplastic entrance into the storm drains, rivers, streams and eventually to marine environment during rainy seasons (Cheung et al., 2016). For instance, high microplastic levels were identified in the Ontario Lake sediments and in Swiss Lakes (Lake Geneva, Constance, Maggiore, and Zurich) during wet period of the year (Corcoran et al., 2015; Faure et al., 2015). In comparison to dry season, monsoon season (July to August) showed a significant increase of microplastic concentration in the coastal environment of Cochin, India (Daniel et al., 2020).
Saarni et al. (2021) analysed microplastic accumulation in both winter and growing season and reported high accumulation of microplastics in sediments during growing season. The lower concentration of microplastics in winter samples can be due to microplastics being trapped by ice cover in the lake as the structure of ice has ability to accumulate microplastics in its freezing period (Saarni et al., 2021;Peeken et al., 2018). Seasonal trends can also effect the accesibility of microplastics in the catchment areas since microplastics can be retained by frozen soil in winter times and released them during spring flooding. Thawed soil is more efficient in giving away microplastics to the environment. This can cause higher inflow of microplastics in growing seasons, amplified with additional particles freed via snow melting (Saarni et al., 2021).
2.5.2 Physical Factors
Physical forces effecting the transportation of microplastics in freshwater system are expected to be alike to the forces in coastal waters. Settling of microplastics into the sediments can be influenced by high energy forces for instance, storms (Turra et al., 2014). Currents, convergence zones and wave actions are considered as vital factors in the transportation and deposition of microplastic in the lacustrine environment (Hoffman and Hittinger, 2017; Cable et al., 2017). Presence of organic matter promotes the accumulation of microplastic into the lakeshore sediments by trapping them from water (Corcoran et al., 2015). River features such as slopes can influence the abundance of microplastics as less sloppy areas of river can act as a sink for microplastics (Mani et al., 2015). According to Naidoo et al. (2015) stratification and low flow rates in estuaries boosts the sedimentation of microplastics.
The characteristics of microplastics such as size, shape and type have greater impact on their vertical distribution and buoyancy in freshwater environment (Cable et al., 2017). Deposition of microplastics to the sediments can be increased by alteration of their density due to biofouling and mineral adsorption such as clay and quartz (Corcoran et al., 2015; Mani et al., 2015; Kowalski et al., 2016). Biofouling is the process in which hydrophobic nature of microplastics and their high surface area to volume ratio increases the adsorption of organic materials. Subsequently, microbial colonization occurs on the surface of microplastics (Kaiser et al., 2017). Particle shape has also significant impact on their sinking behaviour. Microplastics having irregular shape have unstable particle motion which cause noteable reduction in sinking velocity (Kowalski et al., 2016).
2.6 Sediment trapping
Accumulation rates of microplastics in sediments are not properly revealed yet (Bancone et al., 2020).Tracking and quantification of sediment components and their deposition rates have been carried out by sediment trapping for a long time. From 1970s to present, significant work has been done for the establishment of sediment trap protocols (Saarni et al., 2021). However, very few studies have used sediment trapping as a tool for estimating microplastic deposition. Sediment trapping has ability to give more precise information about the origion of microplastics and factors effecting the microplastic fate in aquatic systems.
Moreover, it can also be useful tool to detect the effect of seasonal variations in microplastics inputs (Enders et al., 2019).
3 MATERIAL AND METHODS 3.1 Study site
Lake Kallavesi is the biggest lake situated in eastern Finland, encompassing the city of Kuopio.
It is the tenth largest lake in Finland with an area of 478.1 km2, containing several sub-basins and diversified islands (Uurasjärvi et al., 2020; Johansson et al., 2019; Partanen and Hellsten, 2005). The average and maximum depth of Lake Kallavesi are 9.7 m and 75 m, respectively (Uurasjärvi et al., 2020). The lake is located in the boreal vegetation zone. The annual average temperature of this region is +3 °C while the average temperature of the coldest month, January is –10 °C and the warmest month, July is +17 °C. Around 644 mm/year of precipitation is recorded, out of which half is in the form of snow. For around 6 months from November to April, the lake remains covered with ice (Johansson et al., 2019). Lake Kallavesi is the source of drinking water for Kuopio city (Uurasjärvi et al., 2020). Samples were taken from Kallavesi lake under the Kallansillat Bridge (Fig. 1).
Figure 1. Study site (National Land Survey of Finland) 3.2 Sampling and sample processing
The sediment trap method was used for trapping samples from lake Kallavesi. On 11 November 2016, a Sediment trap was placed to the bottom of the Lake from the boat. Boat traffic and ice cover interferences were avoided, and sample representativeness was ensured by choosing maximum depth for the placement of a sediment trap. Winter 2016-2017 samples were taken on 3rd June 2017 in two collector tubes and then these collector tubes were fixed again to the sediment trap which was installed back to the bottom of the lake for summer samples. Summer 2017 samples were collected on 21st October 2017. A sample representing winter 2017-2018 was taken on 26th May 2018 and a sediment trap was slowly placed to the bottom of the lake for summer samples. Summer 2018 samples were collected on 6 Oct 2018 (Saarni et al., 2021).
Sediment properties were determined by using a sample from one collector tube while microplastic analysis was done by using the sample from another collector tube. Hydrogen peroxide (H2O2) was used for the decomposition of organic matter (from microplastic surfaces) and then the heavy-liquid density separation method was used for separating microplastics from clastic sediments. Lithium heteropolytungstate was diluted to density of 2.0 g/cm3 and used as
a heavy liquid solution for the density separation method (Saarni et al., 2021). The isolated microplastics and remaining particles were filtered on standard 12-15 µm pore size general purpose filters (Munktell Ahlstrom, size 90 mm, grade 1003) for further inspection.
3.3 Visual inspection of microplastics
The visual method was used for the selection of microplastics. Microplastics were visually isolated from filters with a pair of micro tweezers under Zeiss Stemi 508 stereomicroscope with 6.3 - 50 × magnification. Particles were photographed with a stereo microscope camera (Axiocam ERc 5s camera). Particles were picked according to the criteria set by Noren (2007):
fibre particles having an equal thickness, uniformly coloured, non-natural, and transparent particles. Picked particles were transferred to 15 ml centrifuge tubes with the help of micro tweezers and then vacuum filtered through a silver membrane filter having a pore size of 5µm and diameter of 25mm. Filtration was carried out under a vacuum hood and filtration funnel and centrifuge tubes were rinsed with deionized water. Filters were dried at room temperature in lidded glass Petri dishes, Next, dried filters were attached to glass microscope slides with the help of double-sided tape for microplastic identification and analysis.
3.4 Identification and analysis of microplastics
Microplastic identification was conducted using an imaging Fourier-transform infrared spectroscopy (FTIR). Calibration and background check of imaging FTIR was done before by putting a glass microscope slide under the imaging FTIR for analysis. Imaging FTIR utilized for the analysis of particles consists of Agilent Cary 670 spectrometer and Cary 620 microscope incorporated with focal plane array (128×128 FPA) detector. Measurements were taken in a reflection mode with a pixel size of 5.5 µm, a spectral resolution of 8 cm-1, and a spectral range of 3800-750 cm-1. Total scans taken with FTIR were 4.
siMPLe software which is produced by Aalborg University, Denmark, and Alfred Wegner institute, Germany, (Primpke et al., 2020) was used for the analysis of spectral data obtained from FTIR. This software provided information about particle size, mass, and plastic types by calculating Pearson’s correlation between samples and reference spectra. Reference spectral library consists of both open source and in-house spectra of prevalent plastic types as well as natural proteins and cellulose polymers. Common plastic types used for the reference database were polyamide (PA), polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS), polypropylene (PP), polyvinyl chloride (PVC), polyurethane (PU), acrylonitrile butadiene styrene (ABS), polyacrylonitrile (PAN), and polymethyl methacrylate (PMMA).
Raw spectra, first derivative, and second derivative were used for the calculation of correlations.
siMPLe software determined the major dimensions of the particles by calculating the longest distance between the pixels of identified plastic particles. Correlation thresholds were also set for the determination of plastics.
3.5 Data Reporting
Morphological characteristics of particles were evaluated from the data provided by siMPLe software. siMPLe calculates the major and minor dimensions (µm) of the particles and this information was used for finding the shapes of particles. Shapes were estimated by dividing the major and minor dimensions of the particles (Eq. 1). If the dimension ratio of the particle was greater than 5 then it was considered as fibre.
Particle dimension ratio = 𝑀𝑎𝑗𝑜𝑟 𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 (µm)
𝑀𝑖𝑛𝑜𝑟 𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛 (µm) (1)
The concentration of microplastics in sediments was calculated by using the number of plastic particles provided by siMPLe and the dry weight of samples (Eq. 2). The concentrations of particles were presented in items/g.
MP concentration = 𝑃𝑎𝑟𝑡𝑖𝑐𝑙𝑒 𝑛𝑢𝑚𝑏𝑒𝑟
𝐷𝑟𝑦 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒 (𝑔) (2) 3.6 Contamination control
Laboratory contamination was assessed by the control sample. Open petri dish containing clean filter was exposed to laboratory air for thirty minutes during laboratory work. Afterward, particles were picked under stereomicroscope and microplastic contamination was analyzed by FTIR in the same way as real samples. Moreover, samples were kept in closed centrifuge tubes to avoid air contamination and filtration was carried out under fume hood.
4 RESULTS
4.1 Concentration of microplastics
Microplastics were found in all sediment samples (Fig.2). Our results showed that the concentration of microplastics in winter 2016-2017 samples (65.32 items/g) was higher when compared with the concentration of microplastics in summer 2017 samples (15.09 items/g). When we analyzed the samples of the year 2017-2018, we found that the microplastic concentration in winter samples was also higher (8.66 items/g) than the microplastic concentration in summer samples (4.76 items/g). Overall, the maximum concentration of microplastics was in winter 2016-2017 samples and the least were in summer 2018.
Figure 2. Concentration of microplastics per gram in sediment samples.
4.2 Characteristics of Microplastics (Sizes, morphology, and colours of microplastics) Fibers and fragments were detected in all samples (Fig. 3). The fragment was the most common shape in samples while fibers were very few according to the dimension ratio (>5). The average microplastic size ranged between 116.964 ± 43.442 µm and 173.594 ± 121.959 µm in all samples (Table. 1). Winter 2016-2017 and Summer 2018 Samples had the largest dimension of 828.6 µm and 591.6 µm, respectively. The smallest particle size identified in Winter 2017 - 2018 was 28.2 µm while in summer 2017 samples it was 28.8 µm. The most common colours of microplastics identified during visual inspection through stereomicroscope were blue, black, green, pink, and red. Black colour was dominant among other colours (Fig. 4).
Table 5. Average size distribution of particles identified by FTIR.
Samples Average particle size (µm)
Standard Deviation (SD)
Smallest particle size (µm)
Largest particle size (µm)
Winter 2016- 2017
173.594 121.9597 30.1 828.6
Summer 2017 141.122 60.90252 28.8 335.1
Winter 2017- 2018
116.964 43.44217 28.2 182.3
Summer 2018 171.06 112.9868 35.1 591.6
0 10 20 30 40 50 60 70
Microplastics/g
Samples
Winter 2016-17 Summer 2017 Winter 2017-18 Summer 2018
Figure 3. Morphology of microplastics present in each sample from sediments.
Figure 4. Images of microplastics in our samples taken by stereomicroscope.
4.3 Chemical composition of microplastics
Overall, five different types of microplastics were confirmed by FTIR analysis. Identified polymers were Polystyrene (PS), Polyethylene (PE), Polypropylene (PP), Polyethylene terephthalate (PET), Polyamide (PA). PP was the most abundant polymer type in all samples.
In the sample representing summer 2017, PP, PET, PS and PE were identified, among them PP was the most dominant (85%) polymer type. The sample representing winter 2016-2017
0 5 10 15 20 25 30 35 40 45
Winter 2016-17 Summer 2017 Winter 2017-18 Summer 2018
Number of particles
Fibers Fragments
contained only three types of microplastics (PET, PS, PP), yet PP had the highest percentage of 81% and PS and PET were found in 12% and 7% respectively.
All five types of polymers (PP, PE, PET, PA, PS) were analyzed in a sample representing the growing season of 2018. PP being the most dominant polymer again was found in 70%, PS following with 13%, while PET and PE both had 3%. In the same sample, PA had 10%. The sample Winter 2017-18 showed the predominance of PP as 45%, while PE being 36% and PS and PA both 9%.
Figure 5. Polymer types of microplastics observed in sediment samples.
4.4 Contamination control
Total seven plastic particles were found in the control sample. Polymer types identified were polypropylene (PP) and polyethylene (PE) and PP was dominant polymers type. All the particles were fragments while fibres were missing. These results shows that our sample were not significantly contaminated.
Figure 6. Polymer types observed in blank sample.
5 DISCUSSION
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.
1
6
Blank
PE PP
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.
6 CONCLUSION
This study reveals that Lake Kallavesi is contaminated by microplastics with levels higher than those reported in other freshwater studies. This higher concentration could be due to reporting smaller sizes of microplastics (<1000 µm) which is not documented in most studies. Seasonal variation influences the abundance of microplastics in lake sediments as confirmed by our results. Winter samples showed the highest concentration of microplastics as compared to summer samples. The possible reason for highest level of microplastics in winter samples can be spring flooding and lake ice melt which has ability to accumulate plastic particles in its structure.
The morphology of microplastics found in this study was fragments and fibres. Fragments were the most prevalent shape in the samples with only a few fibres indicating microplastics input to
the lake from secondary sources. Polypropylene (PP) was the predominant polymer type following by polystyrene (PS), polyethylene (PE), and polyethylene terephthalate (PET) attributing their extensive usage in consumer products. The minimum amount of Polyamide detected in samples may be because of the difficulty of its identification by FTIR or its less abundance in the sampling site. Surface runoff, a pulp mill in the vicinity of the lake, and a stream following from north to the lake were considered hypothetical sources of microplastics in this study.
Comparison of different microplastic studies is difficult since no standardized monitoring procedure for microplastics exists internationally. Development and implementation of a standardized method for sampling and reporting microplastic results is utterly needed. Samples can be readily contaminated during sampling and laboratory work therefore more attention should be given to improve blank control sample procedure in future studies to minimize contamination. Microplastic studies on freshwater bodies located in the boreal environment are sparse. Hence, further research is needed to closely study microplastic pollution in these environments. Furthermore, sampling should be done in both winter and summer seasons as seasonality has great impact on the abundance and accumulation of microplastic in water systems.
7 ACKNOWLEDGEMENT
This study was carried out in the SIB lab which is in the University of Eastern Finland and was funded by the Academy of Finland. I am very thankful to my main supervisors Arto Koistinen PhD, Research Director and Jussi Kukkonen PhD, Professor for the research topic and for overseeing my written thesis. I would also like to thank my supervisor Saija Saarni PhD, Postdoc researcher for helping me in understanding the sampling procedure and for her helpful feedback. Special thanks to my supervisor Emilia Uurasjärvi MSc, Early-stage researcher for answering my plethora of questions, for supervising my project work and, for helping me throughout the writing process.
