Abundance of MPs in water, sediment and aquatic species
This study investigated microplastics (MPs) pollution in water, sediment, and aquatic species of Dhaleshwari River, Dhaka. In water samples, the highest concentration of MPs was found in the sampling station at W.3 (195 ± 18.95 MPs/L) and the lowest at W.5 (101 ± 31.45 MPs/L), with an average concentration was 143.1 ± 28.52 of MPs/L (Fig. 2A). The abundance of MPs in water samples of Dhaleshwari river was comperatively higher than the previous studies of MPs in Buriganga Rivers water 117 to 250 MPs/L2, Ganges to Meghna River in Bangladesh 50.9 ± 24.4 to 64.1 ± 26.3 MPs/L47, Xiangjiang River in China 7.32 ± 2.36 to 11.0 ± 3.08 MPs/L50. In sediment samples, the highest concentration of MPs was found in the sampling station at S.4 (46789 ± 3124.24 MPs/kg) and the lowest at S.8 (13456 ± 1123.44 MPs/kg), with an average concentration was 30153.8 ± 2313.62 of MPs/kg (Fig. 2B). The abundance of MPs in sediment samples of Dhaleshwari river was also found higher than the previous studies of MPs in Buriganga Rivers sediment 3500 to 8170 MPs/kg2, Ganges to Meghna River 2953.49 ± 1670.52 to 4014.66 ± 1717.59 MPs/kg47, Karnaphuli River 22.29 to 59.5 MPs/kg51, Turag River in Bangladesh (19.2 ± 2.44 MPs/kg5, Major tributaries of Yellow River in China (430.30, 145.09, 253.33 MPs/kg52, Xiangjiang River in China (150 ± 75.6 to 249 ± 182 MPs/kg50. This variation may be due to sampling location, sampling volume, geographical position, and detection method. The abundance of MPs in a different compartment of Dhaleshwari River varied, which could be the different sources of pollution, such as industrial waste dumping, discharges of industrial wastewater, fishing activities, disposal of urban waste, thrown of domestic wastewater, stormwater, and so on53,54. Pearson correlation was used to determine the relation between water and sediment contamination in Dhaleshwari river. A negative relationship was discovered (r = − 0.22), with MPs contamination in the water and sediment of the Dhaleshwari River (Fig. 4A).
Abundance of MPs in Water (A), and Sediment (B) of Dhaleshwari River. Error bar indicates mean ± SD. At the top of the pictures are MPs in sediment and water samples.
In aquatic species of Dhaleshwari river, the highest MPs was found in D. rusticus (57.82 ± 14.98 MPs/g; 2.91 ± 1.78 MPs/ind), followed by B. contaminata (38.53 ± 6.87 MPs/g; 5.45 ± 3.21 MPs/ind), Ranatra sp. (34.05 ± 5.39 MPs/g; 2 ± 2.89 MPs/ind), C. servilia (26.99 ± 7.88 MPs/g; 4.11 ± 3.21 MPs/ind), D. annulatus (16.44 ± 6.95 MPs/g; 7.97 ± 4.78 MPs/ind), and O. sabina (14.13 ± 4.52 MPs/g;1.98 ± 0.91MPs/ind) (Fig. 3).
Over 600 genera and 5,740 species of odonates have been reported globally. There are currently 179 species of Odonataknown to exist in Bangladesh, including 90 species of dragonflies and 89 species of damselflies55. Among them D. rusticus and D. annulatus are in similar genera (Diplonychus) and family (Belostomatidae); B. contaminata, C. servilia, O. sabina, and are from the Libellulidae family but different genous Brachythemis, Crocothemis, Orthetrum; and Ranatra sp. is from Ranatra genus and Nepidae family. The MPs distribution among genous was Brachythemis (38.53 ± 6.87 MPs/g; 5.45 ± 3.21 MPs/ind), Diplonychus (37.13 ± 10.96 MPs/g, 5.41 ± 3.28 MPs/ind ), Ranatra (34.05 ± 5.39 MPs/g; 2 ± 2.89 MPs/ind), Crocothemis (26.99 ± 7.88 MPs/g; 4.11 ± 3.21 MPs/ind), Orthetrum (14.13 ± 4.52 MPs/g;1.98 ± 0.91MPs/ind). In case of family the distribution of MPs pattern is Belostomatidae (38.53 ± 6.87 MPs/g; 5.45 ± 3.21 MPs/ind), Nepidae (34.05 ± 5.39 MPs/g; 2 ± 2.89 MPs/ind), Libellulidae (26.55 ± 6.42 MPs/g, 3.84 ± 2.44 MPs/ind.
Among this species two groups of insects are well-known bioindicators of water quality. One group is dragonflies, and another is aquatic Heteroptera. Dragonflies spend one part of their life cycle (nymph/wrigglers) in water and another in terrestrial and aerial environments. The ditch jewel, Brachythemis contaminata, is a well-known bioindicator of pollution that is abundantly found in polluted ditches and drains. The high abundance of B. contaminataindicates elevated pollution levels56,57. Orthetrun sabinais has a wide distribution range, can live in diverse habitat types and is tolerant to disturbed habitats that experience pollution56,58. Crocothemis serviliais one of the common dragonflies frequently found in ponds, puddles, rivers, big wells, tanks, ditches and paddy fields59. Among the aquatic heteroptera, several species of gerrids have been reported as well-known bioindiocators of heavy metal pollution60. However, data on the potential of bioindicator of Diplonychus rusticus, D. annulatus and Ranatra sp. is completely lacking.
Abundance of MPs in Aquatic species of Dhaleshwari River. Error bar indicates mean ± SD.
This study’s findings reflected the first-ever assessment in terms of MPs found in aquatic species of Bangladesh. However, a recent study was conducted in Osun and Ogun Rivers, Nigeria by Akindele et al., (2020) reported that identified MPs pollution in three aquatic insects such as Siphlonurus sp. (62.36 ± 3.53 MPs/g), Lestes viridis (43.29 ± 43.29 MPs/g) and Chironomus sp.(291.76 ± 26.55 MPs/g)21. Another study was done on aquatic insects living in rice fields in Thailand by Maneechan & On Prommi, (2022)61. In this study, MPs were detected in the whole body, gastrointestinal tract (GIT), and body without the GIT of Pantala sp.(1.34 ± 1.11, 1.06 ± 0.77, and 0.73 ± 0.51 items/g, respectively)62. Variations in these results might be due to the nature of species, number of species, habitat of species, feeding habitat, and detection methods. The study findings showed that MPs were present in every single species. The presence of MPs in the water and sediment of the Daleshawari River could be the possible pathway of MPs in Aquatic species. In addition, The aquatic heteropteran bugs or water bugs of the genus Diplonychus Laporte, like D. annulatus (Fabricius) and D. rusticus (Fabricius) are well known to prey upon mosquito larvae63,64,65. They are also known to feed on chironomids, aquatic crustaceans, small fishes and amphibians. Another water bug, Ranatrasp. is also a sit-and-wait predator of small insects and crustaceans66. Moreover, Aquatic nymphal stage of B. contaminata is omnivorous and eats a variety of foods, including debris and chlorophyceae. They are also known to be effective predators of mosquito larvae, including those of Anopheles stephensi, Culex quinquefasciatus, and Aedes aegypti67,68. The aerial stage of C. serviliafeeds on mosquitoes, flies and other insects in mid-air69,70. This might be the source of MPs as they act as insect food sources. Aquatic animals that consume MPs can also facilitate the transfer of hydrophobic and persistent organic pollutants, such as dichlorodiphenyltrichloroethane, polychlorinated biphenyls, and dioxins, to higher levels in the food chain. This is because MPs act as carriers for these pollutants21. This multiplies the effects of MPs on the environment and humankind.
MPs pollution assessment in Dhaleshwari River
MPs pollution assessment according to shape, color, and size
MPs comparison according to shape.
A total of five different shapes, including fibers, fragments, pellets, films, and foams, were discovered in the aquatic species, water, and sediment samples of Dhaleshwari river (SF. 1 A). Fibers were the most dominant MPs in all the samples, including aquatic species (74%), water (65%), and sediment (55%) followed by fragments in sediment (23%), water (21%), aquatic species (12%), foams in sediment (15%), aquatic species (7%), water (6%), pellets in water (5%), aquatic species and sediment (4%), and films in aquatic species, water, and sediment (3%) (SF. 1 A). Fibers are expected MPs in freshwater environments of previous scientific1,5,21,71kinds of literature, which supported current study findings. The relatively long fibers in river streams can originate from a number of things, including urban debris, fishing gear, roping, clothing, and industrial wastewater discharges. The relatively long fibers in river streams can originate from a number of things, including urban debris, fishing gear, roping, clothing, and industrial wastewater discharges1,5. Tire wear is the direct cause of MPs exposure in the Dhaleshwari River test locations because they are surrounded by automobile roadways. According to Baensch-Baltruschat et al. (2020), tire wear produces 6.1 million tons of MPs annually, of which 75.87% are fibers72. Fragments and foam indicate discarded packing materials and shattered plastic debris. The pellet is primarily found in skincare products, including face cleansers47.
MPs comparison according to color.
A total of five different colors, including black, blue, red, green, and white, were identified in the aquatic species, water, and sediment samples of Dhaleshwari river (SF. 1B). Black was the most dominant MPs colors in all the samples, including aquatic species (41%), water (39%), and sediment (32%) followed by red in sediment (27%), water (21%), aquatic species (15%), blue in water and sediment (25%), aquatic species (23%), green in aquatic species (14%), sediment (10%), water (9%), and white in aquatic species (7%), water and sediment (6%) (SF. 1B). Studies73,74,75have found a high prevalence of black plastic in freshwater environments worldwide. However, some investigations come to different results. For instance, Gabriel & Bacosa, (2024) and Haque et al. (2023) found that blue was the most dominant color in freshwater river water and sediments17,76, and Khedre et al. (2023) found that blue was dominant in aquatic organisms18. Bangladeshi textiles, commercial bags, packed bags, and aquatic organisms are mostly made of translucent or brightly colored plastics that mimic the food that zooplankton and other aquatic animals eat. Sunlight also stains plastic materials17.
MPs comparison according to size.
Three MPs sizes were found: < 0.5 mm, 0.5–1.0 mm, and 1.0–5.0 mm groups in the aquatic species, water, and sediment samples of Dhaleshwari river (SF. 1 C). The < 0.5 mm in size of MPs were dominant in all the samples including aquatic species (72%), sediment (65%), and water (55%) followed by 0.5–1 mm in water (25%), sediment (21%), aquatic species (14%), and 1–5 mm in water (20%), aquatic species and sediment (14%) (SF. SF. C). A recent study by Haque et al., (2023) found that less than 0.5 mm of water (42.38%), and sediment (49.92%) dominated their analysis of the Buriganga River in Dhaka City, which is consistent with the current study findings. MPs sizes play a significant role in their ingestibility by aquatic creatures51. In this present study, the majority portion of MPs were less than 0.5 mm in size. Consequently, it is easier for aquatic species of Dhaleshwari river to eat because of its size. Since plastic elements deteriorate gradually in the presence of sunshine17, it suggests that the Dhaleshwari river has been polluted with plastic for a long time. MPs size plays a vital role in MPs consumption by aquatic insects. Small particles can easily falsify aquatic organisms, and they can easily consume it rather than large particles. For that reason, small particle rate is higher in aquatic insects. MPs accumulated inside their prey as well as within the surrounding environment among predatory species, indicating that the MPs capacity to select food is restricted by size.
Factor responsible for aquatic insect’s MPs consumption
Insect’s weight, height, feeding habit, and host reservoir may be responsible for the MPs consumption among aquatic insects. Pearson correlation was used to ascertain any relationship between MPs abundance in water, sediment, and aquatic species with their length and weight samples of the Dhaleshwari River followed by linear regression (Fig. 4).
MPs consumption relation with aquatic species length and weight
A negative relation (r = − 0.61) was observed between the MPs abundance and their existing height Fig. 4D. That indicates MPs consumption is decreasing with their length. On the other hand, positive relation (r = 0.03) has been observed from the weight and MPs abundance. That indicates MPs consumption is increasing with their weight. This may be due to the physical characterestics of the aquatic insects. For example, Orthetrum sabina, Ranatra Sp. Crocothemis servilia has higher body length or height, while in the case of weight, they are lowest. However, in the case of Brachythemis contaminata, Diplonychus rusticus and Diplonychus rusticus showed opposite chracterestics. Details description of aquatic insects’ height and weight are given in ST.1.
MPs intake in relation to the feeding behavior of aquatic insects
One of the key elements influencing MPs consumption is feeding patterns. Two types of feeding habits are studied in this study. One type is aerial, and the other is predatory. The mean abundance of MPs in predatory and aerial species were 36.10 ± 5.14 and 26.55 ± 1.71 MPs/g, respectively (SF. SF.D). Besides, the abundance of MPs in predatory and aerial species was not statistically significant (P = 0.529, p> 0.05). The results show that species can consume and store MPs in their bodies at varying rates depending on their feeding strategy. A recent study was done in Egypt18 found that predatory aquatic insects (Aeshna sp.: 37 MPs/g) accumulated significant levels of MPs, which is consistent with current study findings. Predatory aquatic species contained more MPs than aerial aquatic species in this investigation. In their natural habitat, predatory species typically capture zooplankton and phytoplankton and feed on organic debris on the sediment surface. Given sediment’s role as a repository for pollutants in the freshwater system, settled MPs can likely be readily ingested. In contrast, aerial species usually inhabit bodies of water and are greatly affected by MPs on the water’s surface.
MPs consumption relation with insect host reservoir abundance
This insect spends their total life in the water and sediment of Dhaleshwari river. For that reason, its contamination is one of the major factors for MPs consumption in aquatic insect’s body. A positive relation has been observed with the MPs abundance in aquatic insects with sediment (r = 0.71) and water (r = 0.16) MPs contamination in Dhaleshwari river (Fig. 4C, D). That indicates MPs consumption increased with the contamination of the host reservoir.
Linear regression of MPs abundance in water and sediment and aquatic species with length and weight.
FTIR analysis
FTIR analysis had been performed on a representative number of samples. Of them, 95% of the samples had plastic polymer identified in them. Eight different types of polymers were found in the water, sediment, and aquatic species of Dhaleshwari river using FTIR analysis (Fig. 5). These polymers include HDPE, Low Density Polyethylene (LDPE), PVC, Latex, EVA, PP, PS, and Nylon (Fig. 5). The most prevalent of them in aquatic insects is HDPE, which is followed in distribution pattern by PS (13%), PP (15%), PVC (20%), EVA (24%), and HDPE (28%). In the case of the sediment sample, the most prevalent polymer type was also HDPE, which was distributed as follows: HDPE (31%), PVC (21%), EVA (18%), Nylon (14.5%), PS (6.5), PP (4.5) and LDPE (3.5%). LDPE were the most prevalent polymers in the water sample, and their distribution pattern was distributed as follows: LDPE (38%), PVC (27%) Latex (17%), PS (11.5%), PP (6.5%).
Details description of polymer peak and identical characteristics are provided in (ST 2). Accurately predicting the most common polymer groups in MPs from a limited sample size investigation is difficult, though. Therefore, there’s a good likelihood that more polymers will be found in the Dhaleshwari River’s water, sediment, and aquatic insect species.
Polymer characteristics of detected MPs by Fourier Transform Infrared Spectroscopy.
All of the polymer impacts on aquatic insects have not been studied yet. Only a few preliminary impacts of this type of polymer have been reported77,78,79,80. According to Lei et al. (2018), PVC and PP show significant effects on mortality, reduction of body length, inhibition in reproduction, and decreasing intestinal Ca levels in Caenorhabditis elegans77. PS shows negative impacts on Dreissena polymorpha, Daphnia magna, Gammarus pulex, Hyalella Azteca, Asellus aquaticus, Sphaerium corneum, Lumbriculus variegatus, Tubifex sppin case of mortality, growth, feeding rate, cellular stress, oxidative damage, neuro genotoxicity81,82,83,84.
MPs surface analysis by SEM
SEM is frequently used to describe or identify MPs because it produces a high-resolution image that amply reveals its surface. A representative number (15%) of MPs-like particles were taken to conduct SEM analysis. Figure 6 shows SEM for representative MPs. However, it can be quite challenging to recognize the MPs surface. The SEM results showed that the MP fiber contained flake, crack, linear fracture, crescentic fracture, protrusions, and scratches (Fig. 6)17,44,85. Furthermore, the SEM image clearly shows that the MPs have adequately changed into nano plastic, which presents another challenge for us. It will undoubtedly have a disastrous impact on the ecosystem.
MPs surface inspection by Scanning Electron Microscopy (SEM).
Bioaccumulation factor (BAF) analysis
MPs bioaccumulation in aquatic insects refers to the accumulation of MPs from water and sediment of the host river. According to the bioaccumulation factor from water and sediment to aquation insects, the total species of insects showed hyper accumulation from water. In the case of sediment, 50% of species were hyperaccumulators. The other 50% also shows accumulation from sediment but not hyperaccumulator (Table 1).
Ecological risk assessment
Contamination factor (Ci), Nemerow pollution index (NPI), Pollution Load Index (PLI), and Pollution Zone Index (PLIzone) are the most usable tools for ecological risk assessment. All of these parameters for both sediment and water have been analyzed. According to the contamination factor (Ci), all of the sediment and water samples indicate moderate ecological threats to us (ST.3). NPI values lower than 2 indicate lower bioavailability86. This study found that the sediment and water of the Dhaleshwari river pose a moderate threat as the value of NPI was close to value 2. In the case of the Pollution Load Index (PLI), it poses a threat to category 1 (ST. 3). The Polymer Hazard Index (PHI) is created from the existing polymer hazard score87. PHI value of sediment and water of Dhaleshwari river revealed that Bansghi river poses risk category 5 (ST.3). That indicating extreme danger towards us. All of the risk assessment tools show higher or potential danger towards us, which may be because the ecosystem of the Dhaleshwari river is highly contaminated with MPs.
Ecological implipication
This study detected MPs in six species of aquatic insects. That raises concern about the impacts of MPs in aquatic ecosystems. This MPs has several direct effects on aquatic insects and indirect effects on their environment. MPs in aquatic insects reduce the weight of aquatic insects by hindering digestive tracts and nutrient absorption88. Rojo et al., (2020) reported detritivore mortality and reduced leaf litter decomposition due to MPs contamination89. Fudlosid et al. (2022) also reported the reducing growth effect of MPs contamination in Gryllodes Sigillatus.
species90. MPs have also shown effects on aquatic insect’s Intestinal permeability91, Locomotion behaviour91, Reproduction (PE)79, Emergence rate92, Assimilation efficiency84.
MPs can also coat in water and reduce suitable habitats or smother insect eggs for certain species88. It also mostly affects in the aquatic food chain by entering into the insect body and being transferred to predators further up the food chain, eventually fish, birds, and even humans17,93,94. As MPs pollution had negative impacts on the growth and population of aquatic insects78,82. It may cause cascading effects or disrupt the entire aquatic ecosystem.
Management plan for reducing microplastic pollution
Bangladesh is a riverine country. It lacks of proper drainage system and wastewater treatment facilities in the urban areas. All of the domestic wastewater goes into the improper drainage system, along with industrial wastewater and landfill leachate. Finally, this water is introduced into the river through several canals. In Bangladesh, the destination of untreated wastewater is either ponds or rivers95,96. That makes the rivers of Bangladesh a major reservoir of any kinds of contaminants and makes it difficult to manage. In addition, as a downstream deltaic country, Bangladesh shares 54 transboundary rivers from neighbouring countries like India and Myanmar, which makes the management more complex. However, in Bangladesh, the DoE (Department of Environment) under the Ministry of Environment, Forest and Climate Change is responsible for regulating any point sources of pollution that ultimately lead to any management initiatives. They set guidelines and standards for any kind of wastewater discharge97. Unfortunately, in Bangladesh’s guidelines, still there is no standard for microplastic pollution. To manage this microplastic, Bangladesh first has to reduce its production and disposal. Recently, the Government of Bangladesh has taken steps to phase out the single-use plastics from the country by imposing restrictions on the production value chain as well as at the consumer end98,99. However, proper implementation of this restriction may not be possible without creating and introducing alternatives to single-use plastic. At first, alternatives to plastic products must be introduced and popularized. Point sources of plastic and microplastic pollution must be identified and monitored. Drainage water from the drainage system must be treated before being introduced into the river. A standard for microplastic can be fixed for wastewater and household water discharge into the drainage system. An independent advisory and monitoring committee must be formed to reduce plastic pollution. Through public campaigns and advertisements, people must be aware of the destructive effects of microplastics in our food chain and ecosystem. This management plan must be improved continuously through the PDCA (Plan-Do-Check-Act) cycle.