Abundance and characteristics of microplastics in major urban lakes of Dhaka, Bangladesh

Abstract

Microplastics (MPs) are prevalent in nature due to the proliferation of plastic in the environment. However, the presence of microplastics in lakes is largely unknown in comparison to other aquatic bodies. This study was performed to evaluate the abundance and characteristics of MPs in water, sediment, and fish from three major urban lakes in Dhaka, Bangladesh, namely Dhanmondi, Gulshan, and Hatir Jheel lake. The highest concentrations of microplastics in surface water (36 items/L), sediment (67 items/kg), fish (17 items/individual), and the gastrointestinal tract (4.88 items/gm) were observed. Highest abundance of microplastic in an individual fish was observed in Oreochromis mossambicus from Dhanmondi Lake. The samples were visually examined using stereomicroscope and SEM, which revealed that films were the most prevalent kind of microplastics in both the water and the sediment samples, whereas pellets and foams predominated in the fish samples. Visual observation also revealed MPs dominated by <100 μm in size and transparent in color. According to the Fourier Transform Infrared (FTIR) analysis, the dominant polymers in the analyzed samples were high-density polyethylene, low-density polyethylene, ethylene vinyl acetate, polyvinyl chloride, polycarbonate, cellulose acetate, and polypropylene. MPs were relatively higher in the water and sediment samples of Gulshan Lake, and fish samples of Dhanmondi Lake. The results of this study indicate that microplastic contamination has occurred not only in the water and sediment but also in the inhabitant fishes of the lakes. However, it is discovered that the microplastic intake of fish was significantly related to body weight and length. The implication of the finding suggests that the presence of MPs in urban lakes has raised concerns about the potential human health impact.

Graphical abstract

Highlights

• •The presence of microplastics was evaluated in three major urban lakes of Dhaka, Bangladesh.
• •Microplastic abundance was relatively higher in water and sediment samples of Gulshan Lake.
• •The highest microplastic abundance in an individual fish was observed in Oreochromis mossambicus from Dhanmondi Lake.
• •Films were prevalent in water and sediment samples, whereas pellets and foams were dominant in fish samples.
• •The dominant polymers were Polyethylene (PE), Polycarbonate (PC), Polypropylene (PP), Polyvinylchloride (PVC).

1. Introduction

Plastic pollution is the accumulation of plastic objects and particles in the water, soil, and air on earth, and it is considered an emerging issue that might adversely affect humans, wildlife, and their habitats [1, 4, 9, 36]. Plastic pollutants are classified according to their size as macro plastics, microplastics, and nano plastics. Plastic is used extensively due to its various exceptional properties, such as low cost, outstanding corrosion resistance, lightweight, longevity, outstanding insulator properties, fast deformation and manufacturing[34]. Nevertheless, mismanagement of plastic waste and the long deterioration cycle of plastics have contributed to prevailing and global environmental concerns [50].

Microplastics are diminutive plastic particles with a diameter of less than 5 mm [18]. Microplastics are introduced into the environment through a variety of sources, and primary and secondary sources are considered to be the key sources [37]. Primary MPs are the micro-sized plastic particles that originate from spillage after the manufacturing or disposal of plastic and micro-cleansing particles of personal care products [26,46]. Secondary MPs are derived from broken fragments of larger plastic pieces by mechanical forces, solar radiation, thermo-oxidation, photolysis, and bio-degradation processes [16].

Up to 10% of the produced plastic finds its way into the aquatic ecosystem, where it persists and accumulates in the terrestrial environment [59,80]. The polymer density of plastic particles is considerably light, which makes them easily transportable by winds and currents and recirculates them between sediments and water [74]. Plastic debris with a density greater than 1.0 g/cm³ naturally sinks and settles in the sediment, while low-density debris floats [2]. Studies have shown that low-density debris can deposit in sediment when the polymer density increases as a result of biofilm formation, pollutants adhering, or when aquatic species, especially fish, ingest it [46, 73, 92]. The proliferation of these potentially hazardous particles in the environment has raised concerns about the biological effects of plastic ingestion and their possible health effects [31,64,82].

MPs are consumed by a diverse range of aquatic species, including fishes, which can have deleterious effects [25,68,70]. Fish may purposely or accidently ingest microplastics when searching for prey (e.g., plankton). Furthermore, it is possible that the MPs are already present within or attached to the prey of fishes. Sometimes fish deliberately consume MP since they mistakenly believe it to be their regular food because of its color or shape [17]. Since fish is one of the most vital food sources for human beings, the abundance and distribution of microplastics in fish bodies are commonly analyzed [22,60,67]. Microplastic accumulation in fish bodies from different ecosystems and habitats has been well documented after studying a range of fish species [5,12,19]. These comprehensive studies have indicated that high microplastic concentrations may have harmful effects on fish [17, 43, 62]. Fish size, weight and habitat could be related to MPs consumption by fish from the aquatic environment. Previous studies claimed that fish body length and weight are directly related to the MP intake of fish. Microplastic consumption by fish may be related to the proliferation of plastic in the environment rather than other factors [14,60]. Because of this, identification of the dominant factors that influence the MP consumption of fish is essential.

Microplastic associated risks in the freshwater ecosystem have triggered serious environmental concerns [20,52]. Microplastics are ubiquitously present in marine ecosystems from beaches to the depth of the ocean, fishes to microorganisms, the proliferation of these particles have been observed in every part of our environment [49,71,80,82]. Microplastics have been found even in abiotic sea minerals like salt [58]. In recent years microplastics have also been found in various freshwater ecosystems, such as rivers and lakes [15,27,32].

At the same time, lakes are the providers of numerous significant ecological services, which include providing habitat for local species, producing food, protecting the area from floods, working as a water reservoir and regulating climate catastrophes [30,42]. Conservation of the lake ecosystem is a major concern in environmental management [66]. Some urban lakes are particularly intended to manage urban runoff and receive rainwater, which makes them more susceptible to anthropogenic pollution and environmental stress [55,86]. Increased anthropogenic activity indicates that microplastics will likely be more abundant in urban lakes [25,33,41]. According to some primary studies, microplastics can be found in water, sediment, and organisms of every natural lake [5,7,53,65]. But unfortunately, monitoring of plastic pollution in urban lakes and their inherent species is very rare. Urban lakes are smaller than most other freshwater bodies but receive a significant volume of surface runoff, which as a result turns these crucial parts of the urban ecosystems into reservoirs of MPs [23,63].

In this study, microplastics were investigated in the urban lakes of Dhaka, the capital city of Bangladesh. The study sites were Dhanmondi Lake, Gulshan Lake, and Hatir Jheel Lake. These lakes are located around the most populous and rapidly developing residential and industrial areas of Dhaka metropolitan city, where MP contamination due to anthropogenic sources is very widespread [24,75]. This study's specific objectives were to: a) determine the prevalence and characteristics (size, color, morphotype, polymer) of microplastic contamination in the water, sediments, and fish of major urban lakes in Dhaka; b) study the correlation between fish body weight/length and the abundance of MPs; and c) determine the characteristics of MPs through high-resolution microscopic observation and polymer identification.

2. Materials and methods

2.1. Studied urban lakes

Dhaka is the capital of Bangladesh and one of the most densely inhabited cities in the world, with a population density of 29,373/km² within a total area of 300 square kilometers [8]. It has a tropical climate where the monsoon season (May–September) brings nearly 80% of the annual average precipitation of 1854 mm [45]. In this study, three major lakes of Dhaka were investigated, and the sites were Dhanmondi Lake, Gulshan Lake, and Hatir Jheel Lake (Fig. 1), located in the most populous and rapidly developing residential areas of Dhaka metropolitan city. Dhanmondi Lake has a total area of 37.37 ha, is 3 km in length, 35–100 m in width, and has a maximum depth of 4.77 m [61]. It is only connected to a culvert, a sole outlet which helps to drain excess floodwater caused by heavy precipitation. Therefore, the water level remains nearly constant throughout the year. Gulshan Lake has a surface area of 58.86 ha and a mean depth of 2.50 m. Whereas, Hatir Jheel Lake covers a total area of 32.70 ha and has a mean depth of 2 m [72]. Gulshan and Hatir Jheel Lakes have inlets that connect them to some old river channel, therefore these lakes experience water level fluctuations throughout the monsoon. These locations were chosen because they receive direct domestic sewage, stormwater runoff, and industrial effluent due to intense anthropogenic activities surrounding that area.

Fig. 1.

Location of sampling areas (Dhanmondi Lake, Gulshan Lake, Hatir Jheel Lake) and different color dots represent sampling sites of each lake.

2.2. Sample collection

In September 2021, water and sediment samples were collected from three different locations of each lake. Water samples were collected from 1 m inside the shoreline of each lake and the exact location of each site was recorded using a global positioning system (GPS) device. From each site of a lake, 5 L water sample was collected in triplicate using a steel bucket within 0–10 cm from the surface. From each location, physical (Turbidity, Electrical conductivity, Salinity, Temperature) and chemical (Total Dissolved Solids, pH, Biological Oxygen Demand, Dissolve Oxygen) parameters of collected water samples were measured using multi-parameter (HANNA HI 9829) and DO was observed using DO meter (YSI PRO 20i).

Sediment samples were collected nearly 1 m deep inside the shoreline of each location, which is typically inundated throughout the year. From each site, approximately 1 kg of wet shoreline sediment was collected in triplicate using “Eijkelkamp Agrisearch Equipment” within 0–10 cm of the benthic layer. Sediment samples were kept in a clean, labeled aluminum foil bag before being brought to the laboratory and stored at −20 °C until analysis. Prior to sampling, all instruments and containers were washed with distilled water and sealed.

In September 2021 and May 2022, fish samples from the individual lakes were collected within a depth of 1–10 m, throwing a net from a boat that sailed throughout the lakes. A total of 7 species (n = 90) from above mentioned lakes were collected and used for the investigation. From Dhanmondi Lake 5 species (50 individual), Channa marulius, Channa punctate, Chitala chitala, Oreochromis mossambicus, and Catla catla were collected. From Hatir Jheel Lake 3 species (30 individuals), Pangasius pangasius, Catla catla, and Channa striata, were collected. From Gulshan Lake only 1 species (10 individuals), Pangasius pangasius was collected. The number of species varied with the availability of fish species in the lakes. After collection, species were identified and photographed. For further analysis collected fishes were wrapped with aluminum foil and kept at −20 °C in the refrigerator.

2.3. Isolation of microplastics

The MPs were isolated from collected water samples (Figure S1) using the method of Su et al. (2016) [79]. At first water volume was recorded and filtered through a nylon membrane with a pore size of 20 μm and diameter of 47 mm (Ultrapor N66 Nylon, 6.6 membrane) and the speed was accelerated using a vacuum pump (L-79200, Cole Parmer). The substances on the membranes, including organic matters were then rinsed into a 250 ml glass flask with 100 ml of 30% H₂O₂ (v/v), and the flask was promptly covered with a glass dish. To digest organic matter, these flasks were transferred to an oven (Biolab BODR-305) and kept at 65°C, for not more than 72 h. Afterward, the resultant digestate were filtered through a nylon membrane with a pore size of 0.2 μm and diameter of 47 mm (Ultrapor N66 Nylon membrane). Subsequently, the residue along with the nylon membranes were transferred into glass petri dishes and air-dried overnight for further observation.

Sediments were pre-sieved using a 5 mm stainless steel mesh to isolate microplastic from sediment samples (Figure S2) because raw plastics were abundant. Particles larger than 5 mm were discarded [52]. Samples were then placed in aluminum foil and oven dried (Biolab BODR-305) at 65°C for 3 days. Then 300 gm of sediment was weighed at a precision balance (Radwag PS 4500/C/1). A solution of NaCl was prepared (360 gm/L) and it was mixed with the 300 gm sediment in a ratio of 1:2 (V/V) to isolate MPs through density separation. Then the mixture was stirred for 30 min in a magnetic stirrer (SciLogex MS-H-S) and allowed to settle overnight. After that, the solution was carefully decanted using a nylon membrane, isolating MPs, and leaving sediment in the beaker. Following that, the MPs in the membrane were backwashed from the membrane using Hydrogen Peroxide (H₂O₂) into a beaker for the H₂O₂ treatment and microplastic isolation was performed as described above for the water sample processing procedure.

The method of Li et al. (2018) [52] was used to isolate microplastics from fish species (Figure S3). To determine the occurrence of MP consumption in fish, from each lake 10 individuals per species were selected for the analysis. In the laboratory, the preserved fish species were thawed in a metal tray and rinsed with double distilled water. Each specimen's total length and body weight were measured. The specimens were dissected individually in a metal tray using dissecting scissors, blades, scalpels, and forceps. The gastrointestinal tract (GIT) was removed and weighed in a Petri dish. Following that, each sample was placed in a 500-ml glass beaker. Depending on the weight of the soft tissue in each beaker, 100–200 ml of 30% H₂O₂ (v/v) was added, and these beakers were kept in an oven at 65°C, for 72 h to digest the organic matter [52,79]. After digestion, the solution was clear, and no particles were visible. The solution was then filtered through a nylon membrane and the residue along with the membranes were transferred into glass petri dishes and air-dried overnight. A concentrated NaCl solution of 1.19 g/cm³ was prepared (360 gm/L) and the residue was washed into a beaker using this solution. Approximately 500 ml filtered NaCl solution was added to each beaker and kept overnight. The microplastics were separated through flotation method where only the overlying water was again filtered using nylon membrane. The filter was then placed in a petri dish and covered for further analysis.

2.4. Quality control

During the study, precautions were taken to minimize errors. To minimize contamination, 100% cotton laboratory coats were worn throughout the analysis. Prior to analysis, liquid solutions including tap water, distilled water, saturated NaCl solution and H₂O₂ (30% v/v) were filtered. All instruments and containers were rinsed with filtered tap water before and after use and stored covered until needed. All the samples along with the chemicals used for sample processing included procedural blanks. While handling the samples, airborne microplastic contamination was prevented. The procedural blanks contained relatively low (average of 0–2 microplastic items/filter) particles, where NaCl solution having the highest concentration of 2 items/filter. Cross contamination was eliminated during sample processing, transportation to the laboratory, storage, thawing and cleaning, dissection and removal of GIT, digestion, and microplastic detection. At the earliest convenience, the experimental procedures were completed.

2.5. Visual observation of microplastics

The presence of potential microplastics in the filtered samples were first visually analyzed using a stereomicroscope (Cobra Micro Zoom MZ1000, Micros Austria). Visual assessments have been used to quantify probable microplastics based on their physical properties. Microplastics have identical features such as burnished surfaces, vivid colors, and sharp and geometrical shapes. Artificial fibers are described to have vibrant color and luster, which are quite rampant because of anthropogenic activities [37].

Under the stereomicroscope, each sample was examined thrice at 40Х (up to >50 μm), and all photographs were captured using a microscopic camera (Olympus-DP22; model U-TV0.5XC- 3, SN-5M01493). The items were visually identified and counted based on their morphotype (Fig. 2) as either fiber (uniform plastic strands/fibrous); fragments (hard/jagged-edged); pellets (hard, rounded particles); films (2-dimensional plastic films) or foam (Styrofoam material) and classified based on their size and color of microplastics. The size (width/length) was measured using the millimeter scale on the microscope. Identified MPs were classified into 7 groups according to their size, i.e., <100 μm, 101–200 μm, 201–400 μm, 401–600 μm, 601–800 μm, 801–1000 μm and, >1000 μm. Color composition of the identified MPs was not homogenous; eight distinct colors of MPs were observed which were transparent, black, red, yellow, white, blue, green, and golden.

Fig. 2.

Visual identification of microplastic particles under stereomicroscope (40X); grouped into five categories: (a) Fiber (uniform plastic strands); (b) Fiber (fibrous); (c) Fragments (hard); (d) Fragments (jagged-edged); (e) Pellets (hard, rounded particles); (f) Films (2-dimensional plastic films) (g) Foam (Styrofoam material). The black bar on the lower left corner of each picture indicates a 200 μm scale.

After that, scanning electron microscope (SEM) (EVO18, Carl Zeiss AG, UK) was used, which helped to produce high-resolution images of the MPs surface and provide information on the precise size, morphological surface structure and topography. It enabled further morphological classification and quantification of size of MPs, but it doesn’t show color. SEM scans the sample with an incident electron beam to obtain high-resolution surface pictures of the particle, showing depth and finer details like factors contributing to aging and weathering. The electrons in the beam contact with the sample, causing backscatter and generating many signals that are used to produce a picture of the sample. SEM was done at 1KX and 5KX magnification in an instrument (EVO18, Carl Zeiss AG, UK). With the help of an ion-sputtering device, a thin coating of evaporated gold (Au) was applied on the selected microplastics that were adhered to double-sided adhesive tape. It was performed on representative subsamples (20%) of identified particles, and samples were selected from all morphology and size from each media. It helped to identify physical features, surface properties and further permits chemical composition analysis by using FTIR, which helps to identify plastic polymers.

2.6. Characterization and chemical group identification

From each morphotype, a representative number of microplastics (10%) were selected and from them >94% particles were identified as plastic using FTIR (IRAffinity-1, A213748-SHIMADZU) which is ideal for high-precision analysis. It is a cost-effective and easy approach to deciding the polymer form of microplastics [11]. Spectra from 4000 cm⁻¹–450 cm⁻¹ were obtained using Attenuated Total Reflection (ATR). The spectral resolution was set at 8 cm⁻¹, data interval was set at 1 cm⁻¹ and the resolution was 4 cm⁻¹. For each measurement, the collection time was 10s and 40 co-scans. For the detection of the structure and origin of microplastics, plastic polymers have extremely complex IR spectra of distinct band patterns [44]. The spectrums were identified using an open access online spectral library called SpectraBase from John Wiley & Sons, Inc. With a match rate ≥70%. For further confirmation, the spectrums were compared with the reference spectrum [44] to evaluate the authenticity of polymers and chemical groups. The identified polymer types were HDPE (High density polyethylene), LDPE (Low-density polyethylene), EVA (Ethylene vinyl acetate), PVC (Polyvinyl chloride), PC (Polycarbonate), CA (Cellulose acetate), and PP (Polypropylene).

3. Results

3.1. Distribution of microplastic in water and sediment samples

According to the data based on (Table S1) the water quality of urban lakes is not suitable for aquatic organisms as it can impede fish growth and reproduction ability. It was observed in the study that the condition of Gulshan Lake was worse than in other lakes. EC and Turbidity were higher and DO was lower than the permissible limit set by WHO in 1993. As a result, it was discovered that fish were not naturally present in Gulshan Lake; rather, the only available fish species, Pangasius pangasius, was artificially introduced, and the sizes and weights of the fish were comparatively less than in other lake [76]. Even dead fish was observed floating on the lake water.

The presence of microplastic debris was identified in all the water and sediment samples (Table 2) of urban lakes. In all, 88 particles in water samples, 144 particles in sediment samples were identified. The microplastic abundance (Fig. 3) in water samples varied from 8 to 36 items/L. The highest abundance of microplastic was observed in Gulshan Lake 36 items/L, followed by Hatir Jheel Lake 33 items/L, and a comparatively low abundance of microplastic was observed in Dhanmondi Lake (19 items/L). In sediment samples (Fig. 3), abundance varied from (11–67 items/kg). The abundance of microplastic was highest in Gulshan Lake 67 items/kg, followed by Hatir Jheel Lake 48 items/kg, and a comparatively low abundance of microplastic was observed in Dhanmondi Lake (29 items/kg) because it is situated in a residential area and no domestic sewage flows directly into the lake via sewer outfalls [75]. This results in a low discharge of microplastics into the receiving water over a brief period. Whereas Hatir Jheel and Gulshan Lake are surrounded by industrial, commercial, and residential areas, and the influx of domestic sewage, industrial waste, and stormwater runoff is a significant source of microplastic particles. It can be assumed that excessive human activities near the Gulshan Lake area result in a higher quantity of microplastic in the lake.

Table 2.

Abundance of MPs in fish samples from each lake.

Site	Name	Local Name	Scientific Name	Number of Individuals	Average Items/individual	Average Items/gm (GIT)	Dominant morphotype
Dhanmondi Lake	Great snakehead	Gajar	Channa marulius	10	3.5	0.31	Foam
	Spotted snakehead	Taki	Channa punctuate	10	1.5	0.60	Film
	Indian featherback	Chital	Chitala	10	5.3	2.30	Micro-pellet
	Mozambique tilapia	Tilapia	Oreochromis mossambicus	10	8.2	4.88	Micro-pellet
	Catla	Catla	Catla catla	10	1.1	0.11	Foam
Gulshan Lake	Pangas catfish	Pangas	Pangasius pangasius	10	2.1	1.2	Fragment
Hatir Jheel lake	Pangas catfish	Pangas	Pangasius pangasius	10	1.6	0.13	Micro-pellet
	Snakehead murrel	Shol	Channa striata	10	3.6	1.23	Foam
	Catla	Catla	Catla catla	10	5.7	1.40	Micro-pellet

Fig. 3.

Abundance of detected microplastics in (a) surface water, (b) sediment, (c) fish gut, (d) individual fish samples and (e) distribution by the percentage of microplastics in samples from all sites.

The dominant morphotypes were identified for all environmental samples. In water samples, the prominent types of microplastic were film (0.28), followed by pellets (0.25), fiber (0.23), fragment (0.21), and foam (0.01). In sediment samples, fiber and films were highly abundant, accounted for about 0.35 and 0.34. There were also fragment (0.27), pellets (0.02) and foam (0.01). The dominant morphotypes were fiber and film, found almost in every sample. The samples contained two types of fibers: fibrous and uniform plastic strands and fragments of hard and jagged edges. Between them, fibrous fiber and hard fragments were the most prominent.

3.2. Distribution of microplastic in fish samples

Microplastic abundance (Fig. 3) was observed in 90 fish samples representing 7 species (Table 1), and 322 microplastic particles were found in (n = 82) 93.3% of the fish samples after digestion. Based on the data from (Table 2), the highest abundance of microplastic observed in an individual fish was in Oreochromis mossambicus from Dhanmondi Lake, showing average presence of 8.2 items/individual, 4.88 items/gm in the GIT, followed by Catla catla from Hatir Jheel Lake containing 5.7 items/individual, 1.40 items/gm in the GIT. A comparatively low abundance of microplastic was observed in Catla Catla in Dhanmondi Lake and Pangasius pangasius in Gulshan Lake where in average 1.1 items/individual, 0.11 items/gm and 2.1 items/individual, 1.2 items/gm in the GIT were present, respectively. The percentage of dominant type was identified for all the fish samples. A prominent type of microplastic in fish samples were pellets 0.29, followed by fiber 0.27, film 0.24, fragment 0.17, and foam 0.13. Plastic films were prevailing in water and sediment samples, whereas pellets were dominant in fish samples.

Table 1.

Abundance of microplastic in abiotic and biotic component of urban lakes.

Microplastic abundance in abiotic component of lakes
Water Samples
Type Location	Fiber (fibrous)	Fiber (plastic strands)	Fragments	Films	Pellets	Foams	Total MPs
Dhanmondi Lake Water	9	0	4	1	4	1	19
Gulshan Lake Water	8	0	6	18	4	0	36
Hatir Jheel Water	4	0	9	6	14	0	33
Sediment Samples
Type Location	Fiber (fibrous)	Fiber (plastic strands)	Fragments	Films	Pellets	Foams	Total MPs
Dhanmondi Lake soil	7	0	13	6	3	0	29
Gulshan Lake soil	13	4	21	28	0	1	67
Hatir Jheel soil	18	8	6	15	0	1	48
Microplastic abundance in biotic component of Lakes
Dhanmondi Lake Fish Samples
Scientific Name	Weight (gm)	Length (c.m)	Soft Tissue Weight(gm)	MPs/species	MPs/GIT	Total MPs
Channa marulius	486 ± 16.87	38 ± 7.5	22.08 ± 8.04	0–7	0–0.31	192
Channa punctate	49 ± 3.4	16.4 ± 4.38	1.67 ± 0.7	0–4	0–0.60
Chitala	63 ± 6.24	21.1 ± 5.23	4.77 ± 1.9	2–11	0.9–2.30
Oreochromis mossambicus	115 ± 19.39	18.1 ± 2.65	3.48 ± 0.7	4–17	1.6–4.88
Catla catla	297 ± 48.31	27.3 ± 4.37	19.38 ± 3.53	0–2	0–0.11
Gulshan Lake Fish Samples
Sample Name	Weight (gm)	Length (c.m)	Soft Tissue Weight (gm)	MPs/species	MPs/GIT	Total MPs
Pangasius pangasius	34 ± 9.4	16.7 ± 5.2	2.58 ± 1.76	0–4	0–1.2	21
Hatir Jheel Fish Samples
Sample Name	Weight (gm)	Length (c.m)	Soft Tissue Weight (gm)	MPs/species	MPs/GIT	Total MPs
Pangasius pangasius	186 ± 32	25.4 ± 7.8	14.7 ± 2.14	0–3	0–0.13	109
Channa striata	141 ± 16.04	25.3 ± 3.9	5.69 ± 0.97	1–7	0.17–1.23
Catla catla	214 ± 59.73	25.1 ± 1.34	11.97 ± 1.07	0–15	0–1.40

3.3. Size and color distribution of MPs

Microplastics of different sizes and colors were identified in all three media of the lakes (Fig. 4). The size of the microplastics ranged from less than 100 μm to 5 mm. A total of 29.53% of the MPs in water samples were less than 100 μm, compared to 101–200 μm (20.45%), 201–400 μm (22.73%), 401 μm–600 m (6.58%), 601 μm–800 m (4.55%), 801 μm–1000 μm (9.09%), and >1000 μm (6.82%). In sediment samples, the smallest size <100 μm comprised the highest proportion (34.03%), followed by 101–200 μm (26.39%), 201–400 μm (11.11%), 401–600 μm (7.64%), 601–800 μm (4.17%), 801–1000 μm (6.25%), and >1000 μm (10.42%). Fish samples displayed a similar result as MPs <100 μm were highest in proportion (28.88%), followed by 101–200 μm (14.29%), 201–400 μm (11.49%), 401–600 μm (5.90%), 601–800 μm (8.39%), 801–1000 μm (10.56%), and >1000 μm (20.50%). Smaller microplastics pose a greater risk than larger particles because they are more difficult to remove from the environment using traditional methods. Furthermore, the smaller the size, the more readily available they are and the more easily they can penetrate living cells [92].

Fig. 4.

Size and color composition of detected microplastics in water (a, b), sediment (c, d), and fish (e, f) of urban lakes.

The color composition of the identified MPs was not homogenous. Five distinct colors of MPs were observed in water samples, where half of the MPs were transparent (50%), and the rest of the MPs were black (20.45%), white (9.09%), blue (13.64%), and red (6.82%). Transparent (39.58%), black (20.14%), blue (7.64%), red (13.19%), yellow (2.78%), white (12.50%), green (2.78%), and golden (1.69%) were found in sediment samples. In fish samples, MPs of 7 distinct colors were observed; among them, transparent MPs were most abundant (32.92%), followed by black (19.25%), white (11.8%), blue (10.56%), red (16.46%), yellow (6.52%), and green (2.48%). The extensive use of transparent plastic in textile fabrics, shopping bags, packing bags, plastic bottles, etc. in Bangladesh could be linked to the wider availability of transparent colors in all media [75].

3.4. MP identification and screening under SEM

SEM images of microplastic surfaces exhibited distinctive cracks and textures that are thought to be created due to prolonged environmental exposure. Although it is challenging to identify the sources of the MPs, the history of aging, deterioration, and fragmentation of larger particles in the environment can be known from the surface texture. MPs in sediment (Fig. 5c) exhibited signs of adherent particles, oxidation flakes, fractures, pits, grooves, and scratches on the surface, which signified mechanical weathering, oxidative breakdown, and aging. In contrast, MPs from water and fish showed comparatively fewer signs of degradation. MPs of water showed signs of cracks, flakes, fractures, pits, and adhering particles, which indicated aging, mechanical weathering, and oxidative breakdown (Fig. 5a). Whereas MPs of fish samples showed grooves, fractures, pits, scratches, and adhering particles, which indicated mechanical weathering (Fig. 5b).

Fig. 5.

Microplastic surface morphology under a Scanning Electron Microscope (SEM). SEM images of a) fiber (water) under 5KX magnification, b) fiber (fish) under 1KX magnification, c) fragment (sediment) under 5KX magnification.

3.5. Factors influencing MPs uptake by fish sample

Several studies indicated higher microplastic abundance in fish of higher weight/length [39, 47] whereas some indicated it is dependent to the intensity of plastic pollution in the surrounding environment [14,60]. One of the objectives of this study was to observe if MP intake differs with body weight/length of a fish species using linear regression. There is a significant possibility of MPs ingestion among large and heavy fishes as they need high energy resulting in high food intake [37]. In this study it was found that, microplastic abundance was linearly related to fish body weight/length (R² = 0.005–0.016, p < 0.05) (Fig. 6). The pollution level in Dhanmondi Lake was noticeably lower, but it was found that fish in this lake consumed more microplastic as they were larger in size. As a result of the above observation, it can be assumed that the abundance of MPs in fish is proportional to body length or weight, rather than intensity of plastic pollution in the surrounding environment.

Fig. 6.

Correlation of MPs abundance with weight and length of fish species.

3.6. Identification of potential MPs using FTIR technique

From each media a representative number of microplastics were selected for FTIR verifications out of 524 particles. Particles were carefully cleaned and dried before being analyzed to remove organic debris. A total 39 particles were selected for verification and failed to verify particles were considered as non-plastic material to ensure validity of result. From 39 particles, 37 were confirmed as plastic while 2 considered non-plastic material. The primary polymer types detected based on characteristic peak from FTIR analysis (Figure S3) are HDPE, LDPE, PP, CA, EVA, PC, and PVC [44, 48, 57]. The compositions of these polymers are as follows: HDPE (14 particles), LDPE (7 particles), PP (4 particles), CA (3 particles), EVA (3 particles), PVC (2 particles), PC (4 particles). The IR spectrums based on adsorption band (cm⁻¹) of the corresponding particles are shown in Fig. 7 and the chemical composition of each polymer is shown in Table (S2).

Fig. 7.

IR spectrum of the most prevalent type of MPs found in Urban Lakes. The representative particles showing significant adsorption bands of a) HDPE, b) LDPE, c) PVC, d) PC, e) PP, f) CA, and g) EVA.

In water samples, from 88 items a representative number (10%) of particles (n = 10) were selected and confirmed as plastic particles with FTIR analysis. The detected polymer types were HDPE, LDPE, PVC, PP, and PC. The dominant type was HDPE accounting for 40%, followed by LDPE 30%, PP 10%, PVC 10%, PC 10%.

In sediment samples, from 144 items a representative number (10%) of particles (n = 14) were selected and confirmed as plastic particles with FTIR. The detected polymer types were HDPE, CA, PC, PP and 1 remained unidentified. The most abundant polymer was HDPE, which accounted for 42.85%, followed by CA 21.42%, LDPE 14.28%, PC 14.28%, and PP 7.14%.

In fish samples, from 322 items a representative number (5%) of particles (n = 15) were selected and confirmed as plastic particles with FTIR. The polymer types were HDPE, LDPE, PVC, PC, PP, EVA and 1 remain unidentified. The dominant polymer was HDPE accounted for 40% followed by EVA 20%, PP 13.33%, LDPE 13.33%, PVC 6.67%, and PC 6.67%.

4. Discussion

4.1. Microplastic pollution level in urban lakes

The quantity of microplastic in the surface waters of Gulshan Lake (36 items/L), Hatir Jheel Lake (33 items/L), and Dhanmondi Lake (19 items/L) were considerably high when compared to global microplastic pollution in limnic water. Microplastic abundances in lake water in Asia range from 0.008 to 34 items/L in China, 2.5 items/L in India, and 0.7–9 items/L in Saudi Arabia [6,35,54,77,79,88].

In this study, the microplastic range in sediment varied from 29-67 items/kg, which is relatively moderate compared to other studies. Relevant research from China, India, and Mongolia shows that there is a large quantity of microplastic pollution in Asia, with values ranging from 11-1150 items/kg, 96–496 particles/m², and 14–24 items/kg, respectively [6,35,52,69,77,88,91]. In Europe, Lake Bolsena (Italy), and Lake Edgbaston (UK) exhibited a high amount of MPs in the sediment of 80–144, 200–300, and 319, and in Lake Tisza-to (Hungary), 0.77–0.92 items/kg of microplastics were found [10, 32, 83]. Lake Michigan, Lake Ontario, Lake Erie, Lake Ziway, and Lake Erie contain, respectively, 32.9–6229 items/kg, 8.3–1070 items/kg, and 0–391 items/kg [13,21,28,51,56].

In comparison to 0–18 particles/fish in Poyang Lake, microplastic was more common in the fish species of the lakes in Dhaka, ranging from 0 to 17 items/individual [89]. But the abundance was higher than that of freshwater fishes in the river in Bangladesh, which contain 0.5–9 items/individuals, and in Qinghai Lake (China), which contains 5.4–3.6 MPs/fish [60,88]. The significantly higher level of microplastic contamination in certain fish species may be connected to the high levels of microplastic pollution in the surrounding environment [78].

4.2. Source of microplastic pollution in lakes

Bangladesh is among the top 10 plastic polluting countries, despite the fact that it implemented the first entire ban on plastic bags in the world; they are still extensively used and pervasive in the environment due to the high prevalence of single-use plastics, the mismanagement of plastic waste, and improper waste handling [38]. Estimation shows that around 8000 billion micro-beads are released every month into the encompassing water bodies and wastelands in and around Dhaka city [29, 41]. The result of this study suggests that water, sediment, and fish of three major urban lakes are contaminated with microplastic. These lakes are located across the most developed regions in Dhaka city and confront heavy loads of pollution from the drainage, solid waste disposal, and discharge of untreated domestic and commercial wastewater. These lakes are popular recreational sites, but trash is dumped directly into the lakes due to a lack of civic awareness among the visitors. It is reasonable to observe a greater amount of microplastics in urban lakes with intensive anthropogenic activities [3,30].

In this study, fibers, pellets, and films were mostly observed in surface water, and fibers are the most common type of microplastic in Asian and American limnic waters, accounting for 64.5% and 56.2%, respectively [88]. However, pellets (75.7%) predominate in Africa, while fragments (68.8%) prevail in European lake water. The majority of microplastics found in lake sediments are pellets, film, and fiber which are quite similar to most of the Asian lakes where fibers comprise (93.5%), whereas the majority of microplastics found in lake sediments across Africa are pellets (70.3%). But the results are different from the United States, where fibers (37.0%), fragments (31.8%), and foams (299.9%) account for nearly identical amounts [40, 51, 79]. Geo bags have been used by the lakes to reduce soil erosion, and the degradation of these bags could be a potential source of fiber and films. In this study, the foam was hardly found in water bodies as fiber and pellets are used as raw materials by the local plastics processing factories instead of foams [52].

The surface of MPs exhibited different signs of weathering and aging. The flakes might be the consequence of UV-induced biological deterioration and oxidative weathering, while mechanical aging could create cracks, grooves, and pits in microplastics [54,81,85]. In general, oxidative breakdown of the polymer happens near the water's surface, and mechanical deterioration may occur when the polymers are stimulated by wind or storm waves [90]. Grooves are long, deep cuts in particles that result from dragging upon hard materials like rock, granules, and metal. Adhering particles can cause mechanical deterioration by forming pits or becoming stuck within fractures, which splits the particles apart and makes them smaller in size. The longer MPs are exposed to the environment, the more likely they are to acquire sticking particles on their surface [84]. As sediment is the ultimate sink for particles, they stay longer there and go through more weathering and aging than in other media. In water, particles face friction, wind and wave action and are exposed to UV light, which is the cause of weathering. When fish ingest microplastic from the environment, due to gill movement and digestion process, ingested MPs faces mechanical weathering.

The study revealed that most of the microplastics that were identified in the fish, sediment, and water samples were made mostly of PE and PP. This is not surprising considering the widespread usage of plastic bottles, bags, and containers made of PE and packaging materials and textile materials made of PP that often end up in aquatic ecosystems [87]. Globally, PP, PE, and PET were the most common materials, which is consistent with global plastic production.

5. Conclusions

The study provides valuable insight into the prospect of urban lakes becoming a significant reservoir of microplastics. This research is one of the first to deal with microplastic pollution in the urban lakes of Dhaka, Bangladesh. Evidence of microplastic pollution was found not only in the water and sediment but also in the fish of the lakes. Microplastic contamination is prevalent in water and sediment, especially in the form of films, fibers, and fragments, and is probably caused by household, industrial, transportation, and construction activities as well as urban runoff, storm sewers, wastewater disposal. Gulshan and Hatir Jheel Lakes might act as a sink for microplastics deposition in the lake basin as there is no water outflow from them. Microplastics in the lake can weather down into smaller particles and be consumed by resident fish, which suggest that microplastics may go up the food chain to higher aquatic or terrestrial trophic levels. The findings of this study can be used as future environmental monitoring indicators of anthropogenic impacts, which may help resolve problems impacting human and environmental health. Future studies should be conducted to assess the fate of microplastics and associated risks in order to develop further strategies to reduce contamination.

Author contributions statement

Fariha Tahsin Mercy: Conceptualization, Methodology, Data collection, Material preparation, Formal analysis, Writing- Original draft preparation. A.K.M. Rashidul Alam: Conceptualization, Visualization, Draft Preparation, Reviewing and Editing, Supervision. Md. Ahedul Akbor: Resources, Supervision.

Declaration of competing interest

The authors state no financial or other conflicts of interest that may have impacted the work presented in this study.

Data availability statement

The authors confirm that data related to the findings of the study is available within the article or supporting material.

Appendix A. Supplementary data

The following is the supplementary data related to this article: