Bachelor thesis
The fate of microplastics in Lake Geneva
The accumulation of primary and secondary microplastics concerning climate changePhoto by Till Jentzsch on Unsplash
Name: Isabelle Schut
Student number: 11902078
Supervisors: Antonia Praetorius &
Renske Hoondert
University of Amsterdam
Date: 30-05-2021
Wordcount: 4366
Abstract
Microplastics are a big component of micro-sized litter found everywhere, from the surface of the sea to deep ocean floors. Freshwaters, like rivers and lakes, are sources and ways of transport for microplastics to the oceans. Because most of the studies about microplastics focus on oceans, the situation regarding lakes remains unknown. Primary and secondary microplastics differ in shape, size and structure. Fibers from fishing nets (primary
microplastics) and degraded water bottles (secondary microplastics) were chosen for this study. Lake Geneva was chosen as a lake. Measured data on microplastics was already present about this lake. Because of this available data, further investigation could be done.
By use of an existing model, which is adapted from a model for engineered nanoparticles in surface waters, parameters were changed in order to investigate the
differences in fate between primary and secondary microplastics. As an output, microplastics could accumulate to one of the four compartments. The water surface, epilimnion,
hypolimnion and the surface sediments.
The fate of primary and secondary microplastics was also investigated concerning climate change. By increasing the temperature of the lake and adjusting more parameters that were influenced by climate change, this scenario was realized.
The first scenario without climate change obtained useful outputs. 80% of the primary microplastics accumulated to the epilimnion and 20% to the surface water. Because of the smaller sizes the primary microplastics have, the primary microplastics have lower weights which causes them to float. Almost all secondary microplastics accumulated to the
hypolimnion.
The second scenario investigated the fate of microplastics concerning climate change. Because climate change is not (yet) adapted in this model, more parameters had to be adjusted in order to obtain results. 99% of the primary microplastics accumulated to the epilimnion. Almost all secondary microplastics accumulated to the hypolimnion, which was also seen in the scenario without climate change.
Future research should focus on the impact of this destination on the food web in lakes and on the impact this destination has on the water quality.
Introduction
Plastic is used all over the world, and the consumption rates are increasing. 335 million tons of plastic were produced globally in 2016 (Boucher et al., 2019). Many plastics are not managed well as they are not recycled or end up as litter. Therefore, they will end up in the environment as waste. As an increasing number of plastics enter lakes, rivers and oceans, this is of big concern. Not only because of its impacts on biodiversity and ecosystems, but also on human health as we swim in waters and might ingest it either directly or via our diet (Boucher et al., 2019). The presence of a new type of contaminant, microplastics, has become a great concern for government authorities (Li et al., 2018). Microplastics are a big component of micro-sized litter. They are everywhere, from the surface of the sea to deep ocean floors (Lehtiniemi et al., 2018). In the paper of Li et al (2018), a comprehensive definition of
microplastics is given: “Microplastics are widely defined as synthetic polymers with an upper size limit of 5 mm and without specified lower limit”.
Microplastics can occur as primary or secondary plastics (Li, Liu & Chen, 2018). Primary microplastics are microplastics which are originally and intentionally produced to be smaller than 5 mm. Examples of products with primary microplastics are body care products, textiles, fibers and medicines. Transported by rivers, these primary microplastics can end up in fresh waters or seawaters (Li, Liu & Chen, 2018). Secondary microplastics originate from the fragmentation of larger plastics. Objects from households, plastic bottles and other plastic waste are the origins of secondary plastics (Li, Liu & Chen, 2018).
When microplastics end up in lakes, this is often not their final destination.
Freshwaters, like rivers and lakes, are sources and ways of transport for microplastics to the oceans (Wang et al., 2018). And when microplastics end up in all aquatic environments, these microplastics can be harmful to the organisms living there. Because of the small size of these particles, organisms might confuse microplastics with food particle sizes some aquatic organisms prefer. When small organisms get eaten by bigger organisms, and those by even bigger ones, microplastics end up all throughout the food-web (Wang et al., 2018). To accentuate the problem of microplastics, a study was performed that searched for
microplastics in the deep sea (Cauwenberghe et al., 2013). The authors found that at a depth of 1100 to 5000 meters in sea, plastic particles were present. On average, they found 1 microplastic per 50 cm2. This means that plastic pollution has spread throughout oceans and seas across the whole world, even in the unknown deep sea (Cauwenberghe et al., 2013).
The present study will research the fate of microplastics in Lake Geneva, Switzerland. There are a couple of reasons for the choice of Lake Geneva. Firstly, measured data on
microplastics originating from this specific lake was already present (Datalakes, 2020). This literature could be used for validating the model of this study. Second of all, the Swiss government is really up to date with their data about this lake. Every day, an update is posted about the water temperatures, the depths and more factors that could be used to make this model (Gerdeaux, 2011). Because most studies focus on microplastics in ocean, microplastics regarding lakes remain largely unknown (Faure, 2015).
Lake Geneva has a surface of almost 8000km2 and around the lake the population is about one million people, 10% French and 90% Swiss (Boucher et al., 2019). The main way out from Lake Geneva is through the Rhone River. This river is located at the south western part of Lake Geneva. In the study of Faure et al. (2015), they investigated the plastic
abundance in different Swiss lakes, including Lake Geneva. Using a floating net, lake surface transects were sampled around the year of 2013. Evidence for pollution was in Lake Geneva because all sorts of microplastics were found in the obtained samples. Also, the birds and fish ingest microplastics. This can be toxic for the organisms which can result in death (Faure et al., 2015).
As mentioned above, primary microplastics are plastics which were produced
originally and intentionally smaller than 5 mm. In the present research, fibers in fishing nets will be chosen as primary microplastic. These fibers are likely to end up in this lake because of the high fishing activity that is taking place in Lake Geneva (Boucher et al., 2019). The fishing nets consist of microfibers which are tiny threads of nylon released from processes of laundering or from fishing nets and the diameter is less than 0.01mm (Mishra et al., 2019). Microfibers are complex and they often align horizontally. Settling depends on diameter only (Praetorius, nd). Secondary microplastics are particles that result from the breakdown of larger plastics, in this study water bottles. Water bottles end up in Lake Geneva because of tourism at the many beaches surrounding this lake (Filella & Turner, 2018). Water bottles consist of PET, polyethylene terephthalate, which are mostly found larger than 0.2 mm (Lehtiniemi et al., 2018).
From 1970 up until 2010, a global warming trend was seen in Lake Geneva. According to the book of Lemmin and Amouroux (2013), the warming of this lake can be related to the warming of the atmospheric boundary layer which can be seen as climate change. The expectance for 2100 were drawn up on the basis of four IPCC scenarios. With these future scenarios, the fate of the primary and secondary microplastics concerning climate change will also be investigated.
To conclude the previous section, primary and secondary microplastics differ in size and structure. Because of these differences, it is possible that the way of transport of these types of microplastics will also differ. Besides the type of plastic, another topic that must be taken into account in order to determine the behavior of microplastics is climate change. The fate of primary and secondary microplastics with both scenarios will be investigated.
Research questions
The main research question will therefore be:
To what extent is there a difference in microplastics behavior between fibers from fishing nets (primary) and degraded plastic bottles (secondary) in Lake Geneva, and will climate change influence this?
This research question will be answered according to the following two subquestions: Subquestion 1: In which compartment will fibers from fishing nets and degraded plastic bottles end up when released in Lake Geneva?
Subquestion 2: Will the relative distribution of fibers from fishing nets and degraded plastic bottles differ as a consequence of climate change?
Figure 1 gives an overview of the previous drawn up research questions.
Figure 1: Overview of the research questions. The fate of microplastics is divided into primary and secondary microplastics. Besides this, with a temperature rise, the fate of both primary and secondary microplastics will be investigated independently in Lake Geneva.
Methodology
For this study, an existing model is used (Full-Multi Microplastics Model1). This model estimates the chemical fate of microplastics. By changing parameters like size, structure and shape, the type of microplastic can be specified. The type of lake can be varied by changing parameters like the depth of the lake, the temperature of the lake and concentration of natural organic matter. The latter can affect sedimentation behavior of the microplastics which might have a different result on climate change. Appendix 1 contains the data repository. A link to the raw data and used files has been given in order that this study can be reproduced.
Firstly, this lake was subdivided into two sections. These sections are situated next to each other. This is visualized in Figure 2 (Meteolakes, nd; Bekker, 2021). These sections were divided because these parts of the lake differ from each other. Besides this, each section was divided into four compartments where microplastics can transport to are considered: the main water body (epilimnion & hypolimnion), water surface and surface sediments (Praetorius, nd). Figure 3 visualizes the four compartments to which microplastics can accumulate.
Figure 2: This map indicates how the lake was subdivided into two parts. The left picture shows the intersection of the lake. The right picture shows the top view of the lake. The left part is the shallower section, this part is 22km. The right part is the deeper section, this is 51 km (Meteolakes, nd; Bekker, 2021).
Figure 3: This figure describes the general microplastic multimedia fate and transport model (Praetorius, nd). The four layers of the lake can be visualized. From top to bottom: water surface, epilimnion, hypolimnion and surface sediment. These layers indicate where the microplastics can accumulate to.
The first subquestion was answered by identifying in which compartment the primary and secondary microplastics were mainly accumulated. However, it is often hard to find exact input parameters for these properties. Therefore, the model has been run with different
scenario representing a range of properties that are realistic for the two different microplastic types.
To answer the second subquestion, besides looking at the type of microplastic, the parameter temperature was changed in the simulations. The temperature was increased, to simulate the process of climate change in this lake. To answer this subquestion, future
scenarios must be taken into account. The expected global temperatures for 2100 were drawn up on the basis of four IPCC scenarios; The Represenetative Concentration Pathways (RCPs). These RCPs describe four possible scenarios of greenhouse gas emissions, atmospheric concentrations, air pollutant emissions and land use. They include a scenario with very high greenhouse gas emissions (RCP8.5) (IPCC, 2014). RCP8.5, the most extreme scenario has been used for this study in order to predict the most extreme future change. Under the RCP8.5 scenario, temperature rises with 4.3 degrees Celsius (Andrade et al., 2021).
The chosen mean temperature of 2020 was 11.5 degrees Celsius (= 284.15 K), which was used in the model (Weather Spark, nd).
Table 1 & Table 2 show the obtained values which are used to answer the research questions. Both tables also give a description of the parameter. The parameter MP1 is used as primary microplastic and MP2 as secondary microplastic.
Choice of variables Microplastic variables
Primary microplastic used in this study (fibers from fishing nets), have the average size of 0.01 mm. The secondary microplastics (degraded water bottles), which are spheres, are on average 0.2 mm. The density of the primary microplastic is 980 kg/m3 and the density of the secondary microplastic is 1380 kg/m3.
The variable size can be changed in the main file. For primary microplastics, the sizes "1um", "5um","10um", "50um", "100um" were chosen. For secondary microplastics, the sizes “10um", "50um","100um", "500um", "1000um” were chosen.
Lake Geneva variables
The variable ‘G’ is the shear rate of the lake. This indicates the turbulence in the water. The shear rate of lakes is lower than the shear rate in rivers. Therefore, the variable ‘G’ is lower than the default number. The variable ‘Depth’ will be different for each compartment. For the surface water layer and the sediment layer, the default values of the model will be used. For the epilimnion and the hypolimnion, the values were derived from Meteolakes (nd). Because of the little changes due to different seasons, an average value of the year 2020 was taken.
The ‘vFlow (m/s)’ variable was determined according to the study of Razmi et al. (2013). The average of four flow measurements in Lake Geneva at four different times were taken. The article of Poulier et al. (2019) calculated the suspended particulate matter (SPM) on the Rhone River. The water flows out of Lake Geneva via the Rhone river. That is why these calculations are the most valid.
For the climate change scenario, the temperature of the lake was increased by 4.3 degrees. There were some more variables that had to be adjusted in the model because the model does not (yet) adjust automatically with higher temperatures. The suspended particular matter (SPM) concentration was increased. The depth of the epilimnion and hypolimnion also changed due to climate change. The value of the alpha, which represents the attachment efficiency for heteraggregation of the microplastics with natural SPM particles, is influenced by the presence of organic matter in the water. There will be more organic matter (Chen et al., 2012) which leads to a lower alpha value. The degradation of microplastics is faster at higher
temperatures (Ceccarini et al., 2018), so the half-life time would be shorter. The t_half_d value was decreased. The time for biofilm growth, t_biof_growth_d, was increased (Villanueva et al., 2011).
At last, a sensitivity analyses was performed to investigate which variables that were changed during the climate change scenario caused the biggest changes in outputs. All variables mentioned in the previous paragraph were taken into account. One variable was changed at a time when the other variables were kept on default. Table 3 shows the parameters that were taken into account when realizing the climate change scenario.
Statistical analyses
In order to test this study on its validity, it is necessary to perform statistical analyses on the model. Because this Full-Multi Microplastics Model is still under development and early access has been given, one of the options for statistical analyses was a scenario analysis. The best possible (most likely) case and the extreme cases were tested in this model. This gave three different results: the most likely case, the extreme high values and the extreme low values. This scenario analyses was performed on the sizes of the microplastic particles. In Table 1, the extreme values that are being researched are showed in the third column.
Table 1: Parameters of the microplastics that will be included in the model. The name of the parameter is given, the description, the value and if necessary, the extreme values.
Parameter microplastic
Description parameter
Value Extreme values
Primary microplastic Density_MP1 (km/m3) The density of the microplastic 980 -
Shape_MP1 The shape of
the
microplastic
Fiber -
SizeBin_MP1 (mm) The size (diameter) of the microplastic 0.01 0.001, 0.005, 0.05, 0.1 Composition_MP1 Type of material the microplastic consists of PE - Secondary microplastic Density_MP2 (kg/m3) The density of the microplastic 1380 -
Shape_MP2 The shape of
the
microplastic
Sphere -
SizeBin_MP2 (mm) The size (diameter) of the
microplastic
0.1 0.01, 0.05, 0.5,
Composition_MP2 Type of material the microplastic consists of
Table 2: Parameters of Lake Geneva that will be included in the model. The name of the parameter is given, the description, the value and if necessary, the climate scenario value.
Parameter lake Description parameter
Value Climate scenario
value Depth (m)
Section 1
The depth of the four different layers of the lake. This will change during different seasons. The average has been taken.
Layer 1 (water surface): 0.1 m Layer 2 (epilimnion): 11 m Layer 3 (hypolimnion): 89 m Layer 4 (sedimentation): 0 m - Depth (m) Section 2
The depth of the four different layers of the lake. This will change during different seasons. The average has been taken.
Layer 1 (water surface): 0.1 m Layer 2 (epilimnion): 11 m Layer 3 (hypolimnion): 299 m Layer 4 (sedimentation): 0.02 m - Length (m) Section 1
The length of the lake in the upper section
22000 -
Length (m) Section 2
The length of the lake in the lower section
51000 -
G Shear rate: the
turbulence in the water
10 -
Temp (K) The water temperature in Kelvin
284.15 + 4.3
vFlow (m/s) The speed of the water flow in the lake.
Layer 1 (water surface): 0.047
Layer 2 (epilimnion): 0.047
Layer 3 (hypolimnion): 0 -
Table 3: The parameters that were changed to realize the climate change scenario. The name of the parameter is given, the description, the default value and the value given for the climate change scenario.
Layer 4 (sedimentation): 0
Parameter Description Default setting Climate change SPM (mgL) Suspended particulate
matter: it represents natural particles that the microplastics can heteroaggregate with. The more SPM, the more organic matter.
38 50
Alpha value The attachment
efficiency of the MP to SPM
0.01 0.001
t_half_d in days The degradation half-life of MP in days
5000 2500
t_biof_growth_d The time for the biofilm coverage to growon the Mp surface in days
Results
Scenario under present circumstances
Figure 4 and Figure 5 show the relative abundance of primary (Figure 4) and secondary (Figure 5) microplastics over a year (in %). These figures were produced in RStudio on the basis of Table 4 and Table 5 (Appendix 2). The primary microplastics show an increase in the surface water and a decrease in the epilimnion over the year. However, the relative abundance in the epilimnion is still the highest. In the hypolimnion and in the sediment layer, little to no microplastics were found after a year.
The relative abundance of secondary microplastics is shown in Figure 5. Most of the microplastics ended up in the hypolimnion. The percentage of secondary microplastics in the epilimnion decreases. The biggest difference between the two types of microplastics is that the primary microplastic accumulate for 80% to the epilimnion layer and for 20% to the surface water. The secondary microplastic will mainly accumulate to the hypolimnion.
Figure 4: The relative abundance of primary microplastics over 360 days. Most of the primary microplastics are present in the epilimnion layer. The blue line shows the surface water, the red line the epilimnion, the green line the hypolimnion and the black line the sediment. This figure was produced in Rstudio.
Figure 5: The relative abundance of secondary microplastics over 360 days. Most of the secondary microplastics were present in the hypolimnion. The blue line shows the surface water, the red line the epilimnion, the green line the hypolimnion and the black line the sediment This figure was produced in Rstudio.
Figures 6a & 6b and Figures 7a & 7b show the distribution of the primary and secondary microplastics with different sizes. Figure 6a shows the shallower and smaller section of the lake and Figure 6b the deeper section. No difference between the two sections was observed with primary microplastics. Extreme sizes of primary microplastics were taken into account in order to decrease uncertainties. In this case between the lines with 0.001 (blue line) mm and 0.1 mm (green line). A negligible amount of primary microplastics is present in the sediments. The largest chosen sizes of primary (0.1, 0,05, 0,01 mm) microplastics were present in the hypolimnion. All sizes of primary microplastics were present in the epilimnion and in the surface waters. More smaller sizes of the primary microplastics were found in the epilimnion and the surface water.
Most of the secondary microplastics (Figure 7a & Figure 7b) were present in the sediments and the hypolimnion. The difference in sizes do not show significant differences. Extreme sizes of 0.01, 0.05, 0.5 and 1 mm were investigated. Figure 7a represent the left and shallower section of the lake. More secondary microplastics are seen in the epilimnion in this part of the lake compared to the deeper section of the lake seen in Figure 7b. The different sizes do not show any differences in accumulation.
Figure 6a: the concentration in number/m3 of primary microplastics in Lake Geneva (lake section 1) with different sizes. The sizes that are chosen are (from bottom to top): 1, 5, 10, 50, 100 micrometers. The average is 10
micrometers, so extreme values are taken into account. This is the left and shallower part of the lake, consisting of a length of 22 km.
Figure 6b: the concentration in number/m3 of primary microplastics in Lake Geneva (lake section 2) with different sizes. The sizes that are chosen are (from bottom to top): 1, 5, 10, 50, 100 micrometers. The average is 10
micrometers, so extreme values are taken into account. This is the right and deeper part of the lake, consisting of a length of 51km.
Figure 7a: the concentration in number/m3 of secondary microplastics in Lake Geneva (lake section 1) with
different sizes. The sizes that are chosen are (from bottom to top): 10, 50, 100, 500, 1000 micrometers. The average is 100 micrometers. This is the left and shallower part of the lake, consisting of a length of 22 km.
Figure 7b: the concentration in number/m3 of secondary microplastics in Lake Geneva (lake section 2) with
different sizes. The sizes that are chosen are (from bottom to top): 10, 50, 100, 500, 1000 micrometers. The average is 100 micrometers. This is the right and deeper part of the lake, consisting of a length of 51km.
Scenario climate change
After adjusting all the right parameters to realize the climate change scenario (Table 3), results were obtained. Figure 8 and Figure 9 show the relative abundance of primary (Figure 8) and secondary (Figure 9) microplastics over a year (in %). These figures were produced in RStudio on the basis of Table 6 and Table 7 (Appendix 2). The primary microplastics
abundance (Figure 8) in the surface layer is very low, in comparison with the scenario without climate change. The relative abundance in the epilimnion is almost 100%. This has increased by 20% in comparison with the scenario without climate change. The climate change scenario did not change the secondary microplastics abundance compared to the secondary
microplastics without climate change. The highest abundance of secondary microplastics is in the hypolimnion (Figure 9).
Figure 8: The relative abundance of primary microplastics over 360 days with climate change. Most of the microplastics are present in the epilimnion. The blue line shows the surface water, the red line the epilimnion, the green line the hypolimnion and the black line the sediment. This figure was produced in Rstudio. .
Figure 9: The relative abundance of secondary microplastics over 360 days with climate change. Most of the secondary microplastics are present in the hypolimnion. The blue line shows the surface water, the red line the epilimnion, the green line the hypolimnion and the black line the sediment. This figure was produced in RStudio.
Figure 10a & Figure 10b (Appendix 2) show the concentration of primary microplastics in Number/m3 concerning climate change. It shows that in the epilimnion, even the smallest primary microplastics (0.001mm) were present. This size of primary microplastic was not present in the epilimnion without climate change. In the hypolimnion, the primary
microplastics of 0.01 mm were present. Only bigger sizes of primary microplastics were present in the hypolimnion without climate change. The two different lake sections do not show any differences in concentration of microplastics. To conclude, the smallest sizes of microplastics are now seen in the epilimnion layer and in the surface layer instead of in the surface layer only.
In the climate scenario with secondary microplastics (Figure 11a & Figure 11b), the concentration in the sediments and in the hypolimnion stayed the same. The concentration of secondary microplastics was higher in the epilimnion in the first, shallower section of the lake in comparison with the second, deeper section. This was also seen in the scenario without climate change. Sizes did not show any significant changes.
Sensitivity analyses climate change scenario
The parameters concerning climate change (Table 3) were changed one at a time, when the others were kept default in order to investigate which parameter has the biggest influence on climate change. When changing the suspended particulate matter (SPM), no change occurred for both primary and secondary microplastics in comparison with the scenario without climate change. When changing the Alpha value, there were seen some changes. More smaller sizes of the primary microplastics were detected in the epilimnion. Besides the primary
microplastics, also the secondary microplastics showed some changes. The concentration of secondary microplastics increased in the hypolimnion. For both parameters ‘t_half_d’ and ‘t_biof_growth_d’ no significant changes were visualized for the primary microplastics. The change of parameter ‘t_biof_growth_d’ caused an increased in concentration of secondary microplastics in the hypolimnion and the epilimnion.
Discussion
A new concept of environmental fate modelling for engineered nanoparticles (ENPs) was developed by Praetorius et al. (2012). This new fate modelling framework was made to be highly flexible which can be adjusted to different ENPs and various environmental settings. An advantage of this new environmental fate model for ENPs is the possibility to predict the fate as well as the transport behavior of these particles in many cases. The parameters used in this model can be modified to represent many different aqueous systems, like lakes
(Praetorius et al., 2012).
There were some important assumptions and simplifications made in this modelling study, but besides this, the results are representative for a large range of cases. According to
Praetorius et al. (2012), when looking at future modelling studies, water parameters can be adapted for each freshwater segment to represent specific rivers or lakes.
Therefore, an advantage of modelling is the flexibility to change different scenarios. Different scenarios are always possible. Because of the flexibility of this model, the existent River Model was changed to a Lake Model for Lake Geneva.
There are a couple of ways in which the methodologies can be improved. As mentioned before, this model was originally developed for modelling microplastics in rivers. For this study, input files were changed in order to work for a lake. The base of the model remains the same which means that the base of the model is still originally meant to use for rivers. This may cause different outputs because the mechanism in rivers still works differently than in lakes. To reduce this uncertainty, a model has to be developed for lakes only.
Second of all, all input values for Lake Geneva were gathered using recently published scientific sources. To reduce the uncertainties, extreme values for some input parameters were also taken into account. However, there is not a 100% certainty that the chosen values are correct, but every obtained value is substantiated with a scientific source.
Third of all, the output figures and tables were obtained over the period of one year. There is a possibility some patterns start to show after more years. This means that it is uncertain what the fate of microplastics is after the examined 365 days. This uncertainty can be reduced to look at bigger periods of time. Future research could investigate the fate of primary and secondary microplastics over periods of 5, 10 or maybe 50 years from now.
Besides this, the climate change scenario is still not yet directly implemented in this model. Parameters other than temperature needed to be changed that are influenced by climate change. The suspended particulate matter (SPM), the degradation half-life of microplastics,
the attachment efficiency of the MP to SPM & the time for the biofilm coverage to grow on the microplastic surface in days are changed as well. By only changing the temperature of the whole lake by 4.3 degrees, no change in the fate of both types of microplastics were found. By changing the previously mentioned parameters as well as the temperature of the lake, some differences were seen. More primary and secondary microplastics accumulated to the epilimnion layer. There were smaller sizes of primary microplastics in the epilimnion. The change of the parameter ‘Alpha-value’ did have the biggest influence on the outputs of the climate change scenario. The ‘Alpha-value’ is the attachment efficiency of the MP to SPM. This value has been decreased for the climate change scenario. Future research should focus on why this value has the biggest impact on the climate change scenario.
Although the data are scarce because it is not monitored daily, results were still obtained which answered the formulated research questions. The differences in the fate of primary and secondary microplastics has been investigated and the output gave many results. Whereas primary microplastics mostly ended up in the epilimnion, secondary microplastics
accumulated to the hypolimnion. This can be explained by the size of the different microplastics. On average, the primary microplastics are smaller than the secondary microplastics.
There is a considerable lack of knowledge on microplastics contaminating freshwater ecosystems (Imhof et al., 2013). However, there already is a little research on the fate of microplastics in lakes. The study of Fonte et al. (2016) studied the fate of microplastics at increasing temperatures. They discovered that at 20 °C, as the control group, fish mortality of 8% occurred. At 25 °C, so with a temperature rise of 5 °C, the increase in fish mortality was 33%. With a rise of 5 degrees, the mortality increased from 8%-33%. This is because the temperature increases the toxicity of microplastics (Fonte et al. (2016). The capability of fish to get food is lower at high water temperature. Mortality of fish has not been researched during this study but is important to take into account because it is a result of climate change. The results of this study do not show differences in the fate of microplastics due to climate change, but climate change does change the food web. Plastics become more toxic when water temperature rises. However, this present study discovered that both primary and secondary microplastics are more present in the epilimnion layer of the lake after a temperature rise. And it turns out that the fish, which ingest microplastics, swim in this epilimnion layer the most (Matechak, 2021). This means that with a temperature rise more microplastics are in the area of the fish that might ingest them. So, according to Fonte et al.
(2016) microplastics become more toxic due to a temperature rise, but according to this study, the microplastics become more available for those fish to ingest. This means that the risks posed by microplastics increase, both because of increased hazard and increased exposure.
According to the study of Xiong et al. (2018), small microplastics (0.1-0.5 mm) are dominating the lake surface waters. Because of the smaller size and therefore lower weight, those plastics will float. Larger microplastics however, are more abundant in the samples taken from rivers (Xiong et al., 2018). In this present study it is also seen that the smaller microplastics (primary) are on top of the larger microplastics (secondary). Another study (Mao et al., 2020) discovered that 98.2% of the microplastics are smaller than 2 mm in one of the biggest lakes in China. This corresponds with the results of this study. The secondary microplastics are bigger than the primary microplastics in this study but even the biggest size of the secondary microplastics chosen in this study was 1 mm. This means that all researched microplastics are smaller than 1mm which corresponds with this study of Mao et al (2020). The same study discovered that the number of microplastics in all water samples decreased with increasing sizes. This is also seen in this model of Lake Geneva.
A couple of studies were already performed about Lake Geneva. Primary microplastics in Lake Geneva indicate that these microplastics sources can be found in upstream oceans. The reason that secondary microplastics can be found in Lake Geneva is that the residence time of the plastics is long enough for plastics to degrade. The microplastics found in Lake Geneva are most likely to travel from the Mediterranean through the Rhone River. During the journey, plastics are likely to degrade and become secondary microplastics (Alencastro, 2012). This study corresponds with the study of Boucher et al. (2019), where they investigated that Lake Geneva acts as a sink for plastics. Because Lake Geneva acts as a sink, plastics are likely to degrade to secondary microplastics from their sources. This study also suggests the potential role of big lakes like Lake Geneva as a barrier to prevent more plastics from reaching oceans (Boucher et al., 2019). These studies correspond with this study. The amount secondary microplastics, which was investigated in this study, are present because this lake acts as a sink.
Future research is encouraged to focus on multiple topics. First of all, the investigated microplastics mainly accumulated to one of the four compartments of the lake. Future
research should focus on the impact of this destination on the food web, on the climate and on the water quality. Second of all, a standardized freshwater protocol for sampling should be made in order to improve the research on microplastics (Cera et al., 2020). Lastly, the sources of contamination as well as transport routes of microplastics should be investigated. In this
way, the detection of microplastic will be increased (Cera et al.,2020). Controlling these sources might decrease the quantity of incoming plastics (Alencastro, 2012).
Conclusion
This study investigated the fate of primary and secondary microplastics. Primary and
secondary microplastics could accumulate to four different compartments. The first scenario investigated the fate without climate change and the second scenario the fate with climate change. This first scenario obtained useful outputs. 80% of the primary microplastics accumulated to the epilimnion and 20% to the surface water. Almost 100% of secondary microplastics accumulated to the hypolimnion.
The second scenario investigated the fate of microplastics with climate change. 99% of the primary microplastics accumulated to the epilimnion. Almost all secondary
microplastics accumulated to the hypolimnion, which was also seen in the scenario without climate change. The changed parameter ‘Alpha-value’, caused the biggest influence on the climate change scenario.
Future research is encouraged to investigate the consequences of the destination of primary and secondary microplastics.
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Acknowledgements
I would like to thank Antonia Praetorius for helping and supporting me during the process of this bachelor thesis. She helped a lot with the understanding of the existent model and tried to help with the best suitable simulations to the model. I would also like to thank Prado
Domercq for sharing the model, which is still under development, with me. Last, but not least, Renske Hoondert was of huge help giving feedback multiple times in order to make this thesis the best as possible.
Appendix 1
Data repository
The model consists of one main file called “MainMPsRiver.py”. This file contains the entire script of the lake model. There are three input files that are used to change parameters in order to answer the research questions. “MicroplasticsSizeClass.txt” is used to change the type of microplastics that were being investigated. The file “compartmentsGenericRiverSec_prop.txt” consist of parameters that show the characteristics of the lake. The file
“process_paramRiver.txt” can be used to change processes that happen in lakes or rivers. The link to the used data is: https://github.com/Isabelleschut/Microplastics-River-Model---Isabelle-Schut.git. The raw data is visible, and this study could be reproduced.
Appendix 2
Table 4: The relative abundance of primary microplastics over a year divided over the four (surface water, epilimnion, hypolimnion and sediment) layers (in %).
Timepoint (days)
Surface water % Epilimnion % Hypolimnion % Sediment %
30 2.01 97.92 0.07 0 60 3.61 96.25 0.14 0 90 5.36 94.42 0.22 0 120 7.15 92.57 0.29 0 150 8.9 90.75 0.35 0 180 10.6 89.01 0.4 0 210 12.23 87.33 0.44 0 240 13.79 85.74 0.47 0 270 15.29 84.21 0.5 0 300 16.72 82.76 0.52 0 330 18.09 81.37 0.54 0 360 19.41 80.03 0.56 0
Table 5: The relative abundance of secondary microplastics over a year divided over the four (surface water, epilimnion, hypolimnion and sediment) layers (in %).
Timepoint (days)
Surface water % Epilimnion % Hypolimnion % Sediment %
30 0 1.56 98.4 0.04 60 0 0.85 99.12 0.03 90 0 0.58 99.4 0.02 120 0 0.45 99.53 0.02 150 0 0.38 99.61 0.02 180 0 0.33 99.65 0.01 210 0 0.3 99.69 0.01 240 0 0.28 99.71 0.01 270 0 0.26 99.73 0.01 300 0 0.25 99.74 0.01 330 0 0.24 99.75 0.01 360 0 0.23 99.76 0.01
Table 6: The relative abundance of primary microplastics over a year divided over the four (surface water, epilimnion, hypolimnion and sediment) layers with climate change (in %).
Timepoint (days)
Surface water % Epilimnion % Hypolimnion % Sediment %
30 0.03 99.91 0.06 0 60 0.05 99.83 0.12 0 90 0.07 99.75 0.18 0 120 0.09 99.67 0.24 0 150 0.11 99.59 0.3 0 180 0.13 99.51 0.36 0 210 0.14 99.43 0.43 0 240 0.16 99.34 0.49 0 270 0.18 99.26 0.56 0 300 0.2 99.18 0.62 0 330 0.22 99.09 0.69 0 360 0.24 99.01 0.75 0
Table 7: The relative abundance of secondary microplastics over a year divided over the four (surface water, epilimnion, hypolimnion and sediment) layers with climate change (in %).
Timepoint (days)
Surface water % Epilimnion % Hypolimnion % Sediment %
30 0 0.6 99.4 0 60 0 0.3 99.7 0 90 0 0.21 99.79 0 120 0 0.16 99.84 0 150 0 0.13 99.87 0 180 0 0.11 99.89 0 210 0 0.1 99.9 0 240 0 0.09 99.91 0 270 0 0.08 99.92 0 300 0 0.07 99.93 0 330 0 0.07 99.93 0 360 0 0.06 99.94 0
Figure 10a: the concentration in number/m3 of primary microplastics in Lake Geneva with different sizes with a temperature rise of 4.3 degrees. The sizes that are chosen are (from bottom to top): 1, 5, 10, 50, 100 micrometers. This is the left and shallower part of the lake, consisting of a length of 22 km.
Figure 10b: the concentration in number/m3 of primary microplastics in Lake Geneva with different sizes with a temperature rise of 4.3 degrees. The sizes that are chosen are (from bottom to top): 1, 5, 10, 50, 100 micrometers. This is the right and deeper part of the lake, consisting of a length of 51km.
Figure 11a: the concentration in number/m3 of secondary microplastics in Lake Geneva with different sizes with a temperature rise of 4.3 degrees. The sizes that are chosen are (from bottom to top): 10, 50, 100, 500, 1000
Figure 11b: the concentration in number/m3 of secondary microplastics in Lake Geneva with different sizes with a temperature rise of 4.3 degrees. The sizes that are chosen are (from bottom to top): 10, 50, 100, 500, 1000 micrometers. This is the right and deeper part of the lake, consisting of a length of 51km.
