3.1. MPs level in Swat River
An average concentration of MPs detected in water samples (305.79 ± 289.66 MPs/m3), fish (12.54 ± 8.02 MPs/individual), and sediments (588.29 ± 253.95 MPs/kg; Figure. 2). The highest concentration was observed among water samples at Mingora city and the lowest at the confluence point of the rivers near Charsadda being 753.71 ± 330.08 MPs/m3 and 57.64 ± 31.98 MPs/m3, respectively. MPs concentrations in the sediment samples were also the highest at Mingora city (834.0 ± 367.21 MPs/kg), but the lowest concentrations were recorded in sediments collected from Chakdara (215.0 ± 20.0 MPs/kg). Among the fish samples, Shizothorax plagiostomus contained the highest while Wallago attu contained the lowest MPs concentrations corresponding to 17.08 ± 8.27 MPs/individual and 5.0 ± 2.36 MPs/individual, respectively (Figure. 3).
The present concentrations of MPs in high altitude surface water (2000 m at Kalam and 980 m at an average altitude of Swat River) were comparable with other high-altitude studies. In a recent study from China, Mao et al. (2020) reported 360 MPs/ m3 in one of the three gorges in Yulin River at an altitude of 1084 m. Feng et al. (2020) described 584.82 MPs/ m3 in freshwater lakes and rivers from Qinghai Tibet Plateau, China, located at 4000 m above sea level. The similarity in the average MPs concentrations at higher altitude are more likely due to tourism activities being the primary source of pollution in these areas, as commonly these areas are less populated (Comakli et al. 2020). The increasing number of tourists in these areas leaves behind many plastic food wrappers, disposables, plastic bags, and other types of waste plastics. At higher altitude, the large volume of plastic packed food and different kinds of plastic carried by the tourists. This process supported by a study from Mount Everest where Napper et al. (2020) reported 1000 MPs/m3 in one of the streams. Comparable results also have been reported from other regions of the world, such as 400 MPs/m3 from the Trent River, UK, and 400 MPs/m3 from an Australian freshwater ecosystem (Stanton et al. 2020; Nan et al. 2020). The present concentrations of MPs are lesser than the previous studies from Pakistan, where 2074 ± 3651 MPs/m3 reported from Ravi River, Lahore, and 1420 MPs/m3 (originally 0.142 items/0.1 L) from Rawal Lake, Rawalpindi (Irfan et al. 2020a; Irfan et al. 2020b). The higher concentrations of MPs in these reports are related to higher population density and those water bodies compared to Swat River. The MP concentrations from the Ravi River were also higher than Rawal Lake and swat river because Lahore is the second highly populated metropolitan city and an industrial hub of the country.
The MPs concentration in fish also found to be comparable with the findings of Pegado et al. (2018), who reported an average concentration of 12.8 MPs/individual and 7.8 MPs/individual in Bagre bagre and Bagre marinus from Amazon River, respectively. Pazos et al. (2017) reported a higher concentration than the present study with 19 MPs/individual from the Argentinean freshwater ecosystem. Recent studies from China have reported lower concentrations of 0.2 ± 0.5 MPs/individual from the Beijing River and 4.8 MPs/individual from the Pearl River (Wang et al. 2020; Roch et al. 2019). A similar pattern of MPs was highlighted by Sun et al. (2020) in river water. In contrast, Zhang et al. (2020) described the concentration of 4.25 MPs per individual and 0.6 MPs per individual from other Chinese regions. The variation in reported concentrations with the present study may be due to different species with different feeding habits. Carnivore species will get higher concentration as feeding on fishes with ingested MPs compared to herbivores fish species. Further, the habitat of fishes also affects the concentration of MPs consumed by fishes because the availability of MPs varies in different zones of an aquatic ecosystem.
Similarly, MPs concentrations in the sediment samples were also comparable to the concentrations reported from surrounding areas. There were 403 MPs/kg reported from Dongting Lake and 90–550 MPs/kg in Yongjiang River (Yin et al. 2019; Zhang et al. 2019). High concentrations reported from Elbe River, Germany, containing 3350 MPs/kg at an altitude of 1,386 m above sea level (Scherer et al. 2020). The possible reason for this higher concentration in the Elbe River, which 25 Million inhabitants inhabit as compared to the Swat River inhabited by 2.31 million people. Hence have a direct impact on the pollution profile of the river. Due to high population density, higher concentrations reported from Poyang Lake, China, and the Northern-Tunisian freshwater stream with 1936 ± 121 MPs/kg and 2340 ± 227.15 MPs/kg, respectively (Jian et al. 2020; Toumi et al. 2020). Several studies reported the low concentrations of MPs in comparison to the current study; viz; Veeranam Lake, India (309 MPs/kg), Yongfeng River, China (26 ± 23 MPs/kg), Citarum River, Indonesia (166 MPs/kg), Shuangtaizi River, China (170 ± 96 MPs/kg) and Diliao River, China (237 ± 129 MPs/kg). These concentrations are lower than the present study due to multiple geographic and demographic factors (Srinivasalu et al. 2020; Sembiring et al. 2020; Xu et al. 2020). Furthermore, a comparison of some recent studies on concentrations of MPs given in Table 1.
Table 1
Comparison of MPs concentrations reported by different studies
Medium
|
Region
|
Concentration
|
Reference
|
Water
|
Pakistan
|
305.79 ± 289.66 MPs/m3
|
Present Study
|
|
China
|
360 MPs/m3
|
Mao et al. (2020)
|
|
China
|
584.82 MPs/m3
|
Feng et al. (2020)
|
|
UK
|
400 MPs/m3
|
Stanton et al. (2020)
|
|
Pakistan
|
1420 MPs/m3
|
Irfan et al. (2020b)
|
|
Pakistan
|
2074 ± 3651 MPs/m3
|
Irfan et al. (2020a)
|
|
Australia
|
400 MPs/m3
|
Nan et al. (2020)
|
|
Nepal
|
1000 MPs/m3
|
Napper et al. (2020)
|
|
China
|
1660 ± 639.1 to 8925 ± 1591 MPs/m3
|
Wang et al. (2017)
|
|
South Africa
|
258 ± 53-1215 ± 277 MPs/m3
|
Nel and Froneman (2015)
|
Sediments
|
Pakistan
|
588.29 MPs/kg
|
Present Study
|
|
Indonesia
|
166 MPs/kg
|
Sembiring et al. (2020)
|
|
China
|
237 ± 129 MPs/kg
|
Xu et al. (2020)
|
|
3350 MPs/kg
|
Scherer et al. (2020)
|
|
China
|
1936 ± 121 MPs/kg
|
Jian et al. (2020)
|
|
India
|
309 MPs/kg
|
Srinivasalu et al. (2020)
|
|
Tunisia
|
2340 ± 227.15 MPs/kg
|
Toumi et al. (2019)
|
|
China
|
26 ± 23 MPs/kg
|
Rao et al. (2019)
|
|
Canada
|
610 MPs/kg
|
Ballent et al. (2016)
|
|
Canada
|
616 MPs/kg
|
Corcoran et al (2015)
|
Fish
|
Pakistan
|
12.54 MPs/individual
|
Present Study
|
|
China
|
4.8 MPs/individual
|
Wang et al. (2020)
|
|
China
|
4.25 MPs/individual
|
Sun et al. (2020)
|
|
China
|
0.6 MPs/individual
|
Zhang et al. (2020)
|
|
China
|
0.2 ± 0.5 MPs/individual
|
Roch et al. (2019)
|
|
Brazil
|
12.8 MPs/individual
|
Pegado et al. (2018)
|
|
Argentina
|
19 MPs/individual
|
Pazos et al. (2017)
|
3.2. Shapes and Size Of MPs
Among the water samples, the relative proportion of the fibres remained the highest (80%), and the fragments were minimum (6%). Comparable relative dominancy of fibres also observed by Jian et al. (2020), where fibres were dominant (25–50%) and the fragments were the least (< 25%) among all types of MPs. Scherer et al. (2020) reported 46.5% of fibres and 22.9% fragments, while Zhang et al. (2019) detected 73.90% of fibres and 13.8% fragments in their findings. Sutton et al. (2016) also reported the same pattern in the present study where fibres were 80%, and fragments were 17% of total MPs. Likewise, Baldwin et al. (2016) also reported fibres to be the highest (71%) among the detected MPs concentrations. In another study conducted by McCormick et al. (2014) fibres and fragments followed the same trend being 59.28% and 37.14%, respectively. Mason et al. (2016) reported fibres to be 59%, and fragments being 33% of their total MPs count. A recently published study from Pakistan also reported the dominancy of fibres in Rawal Lake, with fragments being the second abundant (Irfan et al. 2020b). However, a different trend reported in the second study from Pakistan, where fragments were the most dominant (56.1 %) and fibres were the second abundant type (38.6%) among the total MPs detected (Irfan et al. 2020a). These variations result from the inflow of untreated municipal and industrial wastewater into the Ravi River that has deformed the natural shape of the river. The contribution of foams and sheets in the present study was 12% and 2%, respectively, while beads did not observe in any samples. Swat River is not highly polluted, so most of the fibres were broken pieces of fishnets, with some of them from households and clothing.
Fibres were found the most abundant MPs type within the sediment samples contributing about 92% to the observed MPs. A recent study by Yin et al. (2020) also reported fibres as a dominant type of MPs with 41–49 to 100 % of the total types of MPs. The same highest proportions of fibres documented by Sembiring et al. (2020). In another study, Zhang et al. (2019) detected the fibres in sediments to be 60%, with fragments being 30%. The order of different types of MPs abundance detected in present sediment samples was fibers (92%) > fragments (6%) > sheets (2%) > foams and beads (0%). Irfan et al. (2020b) also reported a similar trend in relative abundance, with fibres being dominant over fragments and beads. The low quantity of sheets identified from the sediment owing to flow just beneath the water surface. That's why sheets are generally not deposited into sediments because of the low density of sheets, large surface area, and fast streamflow. Similarly, the absence of beads indicates negligible manufacturing and usage of industrial and cosmetic products in the catchment area.
Following the similar trend as observed in water and sediments, fibers were also dominant in fishes (88%) followed by sheets (7%), fragments (5%), and foams (0%). In the present study, the sheets ranked as the second most ingested form of MPs because they keep floating below the water's surface for being lighter in weight than fragments, which increases their chance of ingestion during fish feeding. Recently, Kuśmierek and Popiolek, (2020) described the similar trend of the highest proportion of fibres (99.8%) in the gastrointestinal tract of freshwater fish. Zhang et al. (2020 b) and Wang et al. (2020) also reported that the fibres with the highest concentration in native freshwater fish belonging to different trophic levels from Pearl River South China. The possible reason for the highest proportion of fibres in fish species may be its difficulty to excrete with faeces as fibres stick with intestinal walls firmly compared to other shape MPs. Further comparison among relative abundance of MPs shapes given in Table 2. Phytophagous species may ingest the fibres accidentally as these fibres may resemble algae and phytoplankton.
Table 2
Relative abundance of different MPs shapes reported by different studies
Medium
|
Fibers
|
Sheets
|
Fragments
|
Foam
|
Beads
|
Reference
|
Water
|
80%
|
2%
|
6%
|
12%
|
0%
|
Present study
|
|
25–50%
|
-
|
< 25%
|
-
|
-
|
Jian et al. (2020)
|
|
46.5%
|
-
|
22.9%
|
-
|
-
|
Scherer et al. (2020)
|
|
73.9%
|
-
|
13.8%
|
-
|
-
|
Zhang et al. (2019)
|
|
59%
|
5%
|
33%
|
2%
|
1%
|
Mason et al., (2016)
|
|
80%
|
2%
|
17%
|
1%
|
0%
|
Sutton et al., (2016)
|
|
71%
|
-
|
-
|
-
|
-
|
Baldwin et al., (2016),
|
|
59.28%
|
0.07%
|
37.14%
|
1.25%
|
2.26%
|
McCormick et al., (2014)
|
Sediments
|
92%
|
2%
|
6%
|
0%
|
0%
|
Present study
|
|
41–100%
|
-
|
-
|
-
|
-
|
Yin et al. (2020
|
|
60%
|
-
|
30%
|
-
|
-
|
Zhang et al. (2019)
|
|
24.86%
|
52.3%
|
22.74%
|
0%
|
0.1%
|
Wahyuningsih et al., (2018)
|
|
73%
|
2%
|
21%
|
4%
|
0%
|
Eshom-Arzadon, (2017)
|
|
22.3%
|
12%
|
47.7%
|
18%
|
0%
|
Wessel et al., (2016)
|
Fish
|
88%
|
7%
|
5%
|
0%
|
0%
|
Present study
|
|
99.8%
|
-
|
-
|
-
|
-
|
Kuśmierek and Popiolek, (2020)
|
In terms of defined size, the large sized MPs (300µm) were dominant in water samples (62%) followed by medium-size MPs (150 µm; 20%) and small size MPs (50 µm; 18%). This trend was comparable to the findings from Ravi River, Lahore-Pakistan, where larger MPs particles were dominant (Irfan et al.2020a). Zhang et al. (2019) also reported MPs in size range 330µm-1mm to be in the highest proportion (59.1%) compared to other size classes. The highest proportion of larger sizes MPs may be due to continuous activities by local inhabitants and tourists, such as using nets for fishing and the breakdown of plastic disposable being left at the bank of the river or directly thrown into the river. In fish samples, the overall relative abundance of MPs followed a similar trend as in the surface water, and prominent size MPs were dominant (43%), followed by small size MPs (29%) and medium-sized MPs (28%). MPs with sizes up to 1000 µm reported being highest (70.5%) in a study by Sun et al. (2020), whereas Wang et al. (2020) reported that 80% of the detected MPs belonged to the size range of 500–1000 µm. The relative proportion of ingested MPs sizes also varied according to the fish feeding habit. Large-sized MPs contributed 76% in carnivorous, 75% in filter-feeding, 59% in omnivorous, and 53% in herbivorous fish. Furthermore, the dominance of larger MPs particles and the lower proportion of smaller sizes particles in the gut contents may be due to fish inability to excrete along with faeces.
Larger size MPs (300µm) were also dominant in the sediment samples (44%), followed by medium and small-sized MPs in an equal proportion. A similar trend in MPs ratio reported from Veeranam Lake India, with large sized MPs being the most abundant (Srinivasalu et al. 2020) Zhang et al. (2019) reported the highest concentration (44.8%) of MPs with the size range of 330–1000 µm as compared to other sizes. Rao et al. (2019) also found larger MPs particles (200–1000 µm) to be the dominating (40.68%). The reason behind the highest proportion of large-sized MPs in sediments might be their high density causing them to offloaded on sediments as compared to smaller particles (Fig. 4). Another deciding factor may be the velocity of the water. In the case of Ravi River, the medium-size MPs (150–300 µm) reported being with the highest proportion in the sediment samples (Irfan et al. 2020a). The higher velocity of the Swat River slows down the bottom settling of large-sized MPs, while in the case of the Ravi River, it is higher due to lower speed.