6 AIMS OF THE STUDY
7.1 Sampling
Lake Kallavesi is located in Eastern Finland and is the tenth largest lake in Finland.
It has a surface area of 478.1 km2, mean depth of 9.7 m, and maximum depth of 75 m (Miettinen and Lindholm, 2018). It contains over 1900 islands, is fed from a drainage basin of 16 270 km2, and is covered by ice on average 170 days/year. Lake Kallavesi provides drinking water to the city of Kuopio (population: 118 000) and is important commercial and recreational fishing area. Lake Kallavesi was chosen as the study site, because it represents typical large dimictic northern European lake, which had not been studied for MP concentrations before this study. In dimictic lakes, water body is completely mixed during spring and autumn, when the temperature of the water body is constant. Concentrations, behaviour, and fate of MPs was hypothesized to be different in dimictic, seasonally ice-covered lake compared to the ones in warmer climate regions. Moreover, as the City of Kuopio is surrounded by
6 AIMS OF THE STUDY
The aims of this study were:
• Investigation of microplastic concentrations in Finnish marine and freshwater environments from water and fish samples.
• Development and validation of FTIR microscopic methods to quantify microplastics from environmental samples.
• Selection and development of quality control measures for pre-treatment and FPA-FTIR measurement of microplastics.
7 METHODS
This thesis started with sample collection in 2016, and since publication I, the methods for quantifying MPs from environmental samples have developed remarkably worldwide. The methods used in this thesis, listed in Table 6, represent recent progress of the research. This chapter focuses on the laboratory methods, because sampling methods were adapted from previous studies or literature and their development was not the aim of this thesis.
Table 6. Pre-treatment and measurement methods and validation measures used in original publications. UEPP = Universal enzymatic purification protocol (Löder et al., 2017), SDS = sodium dodecyl sulphate.
Publication Pre-treatment Controls Recovery test Analysis method
I SDS, NaOH Only air fallout No
Light microscope, manual selection,
point FTIR measurements
II UEPP:
SDS, H2O2, chitinase Procedural, n=3 Yes, n=2 FPA-FTIR
III UEPP:
SDS, protease, chitinase, H2O2
Procedural, n=10+3 Yes, n=3 FPA-FTIR
7.1 SAMPLING
Lake Kallavesi is located in Eastern Finland and is the tenth largest lake in Finland.
It has a surface area of 478.1 km2, mean depth of 9.7 m, and maximum depth of 75 m (Miettinen and Lindholm, 2018). It contains over 1900 islands, is fed from a drainage basin of 16 270 km2, and is covered by ice on average 170 days/year. Lake Kallavesi provides drinking water to the city of Kuopio (population: 118 000) and is important commercial and recreational fishing area. Lake Kallavesi was chosen as the study site, because it represents typical large dimictic northern European lake, which had not been studied for MP concentrations before this study. In dimictic lakes, water body is completely mixed during spring and autumn, when the temperature of the water body is constant. Concentrations, behaviour, and fate of MPs was hypothesized to be different in dimictic, seasonally ice-covered lake compared to the ones in warmer climate regions. Moreover, as the City of Kuopio is surrounded by
the lake, it has many potential plastic pollution hotspots on the lakeshore, which were targeted in the sampling design. Both surface water and fish samples were collected from Lake Kallavesi.
Surface water samples were collected for publication I from the Lake Kallavesi with a manta trawl equipped with 333 µm net and a pump filtration system. Manta trawling was towed from eight transects in autumn (Figure 4). Moreover, pump filtration was conducted from six sites. The filtration equipment was a cascade-like tube with three screw joints to hold three filters. Filter mesh sizes were 300, 100, and 20 µm. Manta samples were collected from open lake areas, whereas pumping was done from small and shallow bays also, which were not practical for towing.
Additionally, fish samples were collected from the lake. Small perch (n=51) were caught from lakeshore with a beach seine. Small vendace (n=45) were acquired from a local fisher, who caught them with a trawl from open lake areas.
Figure 4. Sampling sites at Lake Kallavesi, Finland.
Besides the lake, the Baltic Sea was chosen for sampling site, because it has unique geographical and hydrological features. It is a brackish inland sea, which receives saline water only occasionally from the North Atlantic via the Danish Straits (Leppäranta and Myrberg, 2009). These salt pulses bring dense saline water, which sink to the bottom. Meanwhile, freshwater flows from rivers and precipitation and stays on the surface of the sea. Thus, the water body has strong stratification meaning that salinity, temperature, and density of water change as a function of depth.
Halocline is a depth area, where salinity changes rapidly, and similarly in thermocline temperature changes rapidly. Water samples were collected from these depth ranges to examine the possible vertical accumulation of MPs by the density of water (Figure 5). Hypothetically MPs sink and accumulate at least for a while to the depths, where density changes rapidly and exceeds the density of MPs.
the lake, it has many potential plastic pollution hotspots on the lakeshore, which were targeted in the sampling design. Both surface water and fish samples were collected from Lake Kallavesi.
Surface water samples were collected for publication I from the Lake Kallavesi with a manta trawl equipped with 333 µm net and a pump filtration system. Manta trawling was towed from eight transects in autumn (Figure 4). Moreover, pump filtration was conducted from six sites. The filtration equipment was a cascade-like tube with three screw joints to hold three filters. Filter mesh sizes were 300, 100, and 20 µm. Manta samples were collected from open lake areas, whereas pumping was done from small and shallow bays also, which were not practical for towing.
Additionally, fish samples were collected from the lake. Small perch (n=51) were caught from lakeshore with a beach seine. Small vendace (n=45) were acquired from a local fisher, who caught them with a trawl from open lake areas.
Figure 4. Sampling sites at Lake Kallavesi, Finland.
Besides the lake, the Baltic Sea was chosen for sampling site, because it has unique geographical and hydrological features. It is a brackish inland sea, which receives saline water only occasionally from the North Atlantic via the Danish Straits (Leppäranta and Myrberg, 2009). These salt pulses bring dense saline water, which sink to the bottom. Meanwhile, freshwater flows from rivers and precipitation and stays on the surface of the sea. Thus, the water body has strong stratification meaning that salinity, temperature, and density of water change as a function of depth.
Halocline is a depth area, where salinity changes rapidly, and similarly in thermocline temperature changes rapidly. Water samples were collected from these depth ranges to examine the possible vertical accumulation of MPs by the density of water (Figure 5). Hypothetically MPs sink and accumulate at least for a while to the depths, where density changes rapidly and exceeds the density of MPs.
Figure 5. Example of halocline (green), thermocline (blue), and sampling depths in publication II, marked with dashed lines (Uurasjärvi et al., 2021).
Water samples were collected from five sites (F62, NAR2, 9F5, Arus, 6P) with a WP2 net, mesh size 100 µm (Figure 6). During the sampling, WP2 net is lowered to the start depth, then towed towards surface, and closed in the end depth. The sampler filters all > 100 µm particles from the sampled depth range. In this study, the sampled depth range was between thermoclines and haloclines, and the filtered volume was 8–67 m3. Additionally, a Jussi water sampler was used for collecting samples from thermoclines and haloclines at seven sites (F62, UUS23, LL9, LL7, NAR2, 9F5, Arus).
Jussi is a large Limnos-type sampler, which collects 30 L water in a desired depth at once. Samples were thereafter pre-filtered with a 50 µm pore sized filter.
Figure 6. Sampling sites at the Baltic Sea (Uurasjärvi et al., 2021).