Assessment of microplastics and associated ecological risk in the Hirakud Reservoir, Odisha, India

The key role played by plastics in providing and sustaining the quality, comfort, and security of modern lifestyles by virtue of its lightweight, durability, and impressive cost-to-performance ratio, has resulted in a sharp ascent of plastic production to the extent of 367 million tons as of 2020 (PlasticsEurope 2020) making it an emerging global threat. Furthermore, microplastics defined as polymer particles with a diameter <5 mm (U.S. National Oceanic and Atmospheric Administration) have also become malignant to the environment and their unchecked increase has proved to be dangerous for ecological security because of their persistence, biomagnifying and bio-accumulative characteristics (Zhang et al. 2019). Intentionally manufactured primary microplastics, e.g. cosmetics, toothpastes, facial scrubs, and industrial abrasives can enter the aquatic bodies through industrial effluents that might contain pellets from pre-production spillage or during transport. Meanwhile, secondary microplastics degraded from larger plastics because of mechanical shear, thermo-oxidative degradation, hydrolysis, photodegradation, etc. can enter the aquatic bodies through discarded fishing gears, domestic effluents containing synthetic cloth strands, plastic food packets, and kitchenware rejects, stormwater runoff containing plastic litter from tourism, leisure activities, tire and road markings, sewage sludge reused in aquaculture, etc.

Extensive study on microplastics has become crucial because of its several deleterious effects on humans and the environment. Its small size expedites its ingestion (because of visual confusion, misidentification of microplastics with the potential prey, and hotspot overlapping their habitat) at each trophic level leading to biomagnification and bioaccumulation, affecting metabolism, growth, nutrient absorption, survival and causing damage to digestive tract, etc. It can also adversely impact animal behavior. Its inherent lipophilicity facilitates the formation of pathogenic biofilms and also enables it to adsorb persistent organic pollutants (POPs), heavy metals, antibiotics, and endocrine-disrupting compounds which can cause severe synergistic toxicity. Moreover, the long transport distance of microplastics along with their buoyancy and lipophilicity makes them a suitable vector that can affect the fate, bioavailability and mobility of the pollutants. If exotic bacteria or pathogens enter the freshwater ecosystem (usually as biofilms attached to microplastics), they can pose a threat of diseases against which organisms have no prior immunity (Campanale et al. 2020).

In the human health context, microorganisms carried by microplastics to the target tissues as vectors were protected from the immune system by them and caused pro-inflammatory responses, which led to tissue damage that might promote infection (Kirstein et al. 2016). The high surface area of microplastic also enables it to carry oxidizing species like reactive oxygen species. All these characteristics coupled with its chronic ubiquity have resulted in severe ecotoxicological effects such as induced oxidative stress in epithelial and brain cells, cytotoxicity, neurotoxicity, immunotoxicity, disruption of endocrine function, abnormal inflammatory response, and emaciation on humans and organisms alike. Microplastic intake causes chronic inflammation and irritation which might lead to cancer DNA because of DNA damage (Prata 2018). Furthermore, microplastic exposure can significantly alter the gut microbiome, thus resulting in abrupt propagation of opportunistic species, increased pro-inflammatory responses, and endotoxemia (West-Eberhard 2019). Furthermore, it has been established that the monomers of plastics like vinyl chloride and styrene are considered type 1A (IARC 2007), and type 2A carcinogens, respectively (IARC 2018). The additives present abundantly in them such as dyes, plasticizers, and stabilizers upon their leaching can also be potential carcinogens and mutagens causing further ecological damage upon their emission. Additives like flame retardants are assumed to interfere with fetal brain development, and can thus affect normal brain development in children. Apart from the presence of microplastics in various environmental compartments, food items like salt, fish, beer, etc., a recent study has also shown the presence of microplastics in human blood (Leslie et al. 2022).

As a result, microplastic research has expanded in recent years, reflecting the increased interest academia has vested in it, with most studies concentrating on microplastics' presence in marine ecosystems going as far as polar water of arctic and deep-sea sediments, even though more scientific attention is warranted by microplastic pollution from inland water since it acts as a major conduit of microplastics to ocean owing to the usage and disposal of majority of plastic on the land, studies on microplastics in reservoirs, which can act as its potential sink, are found to be obscure (Kooi et al. 2018). Investigation of microplastics in freshwater systems gained momentum around 2010, which led to the publishing of first data on the Rhine River (Campanale et al. 2020). In the Indian context, a review by Vaid et al. (2021) exhibited that 63% of the research conducted on microplastics was concentrated in marine environments, whereas only 14% of the study explored microplastics in freshwater systems, thus emphasizing the huge disparity. Moreover, it was also noted that up to 72% of the Indian investigation on microplastics focused on Southern India, thus leaving Eastern India where the current study is aimed with only 6% of deliberation.

The center of this study, the Hirakud Reservoir has been impounded for six decades thus making it a suitable sink for both primary and secondary microplastics. Located in Odisha, it is the longest earthen dam and has been recently declared as a Ramsar site (Ramsar List 2024). It provides water for irrigation and industrial activities in Bargarh, Balangir, Jharsuguda, Sambalpur and Subarnapur districts, and is the surface water source for drinking water supply for the entire district of Sambalpur. It also facilitates pisciculture practice for people. It implies that the discharge of microplastics into the reservoir subsequently follows diverse pathways leading to its entry into the human body through drinking water, consumption of fish, etc. These detrimental effects make it imperative to conduct an ecotoxicological risk assessment (ERA) to assess the risk posed by microplastics upon the organisms of the Hirakud Reservoir. Even though few ecological risk assessment models are explored, they could not conclusively determine microplastics' safety thresholds for protecting freshwater organisms, but they are crucial since they can be used to establish water quality criteria for freshwater systems that can be further used to determine risk level. ERA can be conducted using the species sensitivity distribution (SSD) method by the US EPA guideline (Environmental Protection Agency 1985), from which the PNEC (Predicted No-Effect Concentration) value will be determined. The measured environmental concentration divided by the PNEC value further enumerates the risk quotient (RQ) (Everaert et al. 2018) which implies negligible ecological risk at an RQ value less than 1 and considerable risk at an RQ value greater than 1 for the freshwater species in all the sampling sites of Hirakud Reservoir. Therefore, the present study intended to isolate, characterize and quantify the microplastics in water and sediments of the Hirakud Reservoir. The study further assesses the ecological risk associated with microplastics in the Hirakud Reservoir through risk modeling.

Built across the Mahanadi River, the Hirakud Dam is situated about 15 km from Sambalpur and 9 km from Burla in the state of Odisha, India. The Hirakud Dam is the longest earthen dam in the world confining the Hirakud Reservoir behind it which is 55 km long and has a full holding capacity of 743 km², with a shoreline of over 639 km. The storage capacity of the reservoir stands at 5.818 km³ with a gross capacity of 8.136 km³. Hirakud Dam has a catchment of 83,400 km² and is encompassed by 21 km of earthen dykes on both the right and left sides.

Manta trawl net (diameter 25 and 40 cm long with 10 μm mesh size), brass test sieves (mesh size of 5 mm, 3 mm, 1 mm, 300 μm and 53 μm) and Ekman dredge were the apparatus used. 5% formalin, 30% H₂O₂ and 60% sodium iodide solution were the chemicals used. Glass microfiber filter paper (GF/F, 25 mmØ, Whatman) and distilled water were used for filtration. Glass wares like glass jars, reagent bottles, beakers, conical flasks, glass rod and Petri dishes were used throughout the sampling and analysis.

Hot oven (Thermo Fisher Scientific, Model number: 51028156), FTIR spectroscope (Nicolet 6700, Thermo Fisher, USA), Stereo microscope M205A with a magnification range of 40 × –100× (Leica, Germany), Vacuum filter (Sigma-Aldrich, Model: Z290467), shaker machine (Sigma-Aldrich, IKA KS 130 Control orbital shake, Model: Z341819, automatic stirrer (Sigma-Aldrich, IKA C-MAG MS, Model: Z671886), and weighing machine (Mettler Toledo, Model: ME204/A04), were the instruments used for the study.

Collection for both water and sediment samples took place for microplastic isolation as per Sighicelli et al. (2018) with minor modifications. Microplastic in the water surface was collected using a Manta trawl net which was tied and hauled beside a paddle boat at a speed of 5 km/h at water sampling site A, and beside a motorboat at a speed of 10 km/h at water sampling site B. Surface water sample at sampling site B, was volume reduced by passing it through a brass test sieve. The remnant residues on the test sieve were then washed with distilled water and poured into a glass jar, which was then preserved with a 5% formalin reagent bottle at 4 °C till further analysis. This process was replicated twice for each site. Sediment samples were also collected at the same sites using an Ekman dredge. Three replicates were randomly collected 10 m apart for each site and were pooled as one. Sediments thus obtained were then preserved at 4 °C in a glass jar, marked, and transferred to the laboratory for further analysis.

In three randomly selected sampling sites, field blank tests were carried out by pumping 10 L of distilled water, which was subsequently passed through a 53-μm stainless steel sieve. The replicates were set in triple. Since no microplastics were identified on the filter paper following characterization, it was concluded that contamination during the analysis process was almost negligible. Since, appropriate fiber-free sediments are difficult to obtain, running the procedural blank for the sediment analysis is challenging. Furthermore, to assess and avoid any airborne contamination, filter papers placed in properly rinsed Petri dishes were left exposed to air on board the paddle and motor boat during the sampling as well as during the laboratory analysis. Measures taken to prevent contamination during the laboratory analysis were followed as prescribed by Chae et al. (2015); Lusher et al. (2014); Nuelle et al. (2014). Preceding the laboratory analysis, the workstations were cleaned up with ethanol. The glass wares were properly rinsed with distilled water and covered with aluminum foil after every step. The brass sieves were also rinsed properly with distilled water prior to as well as after every step; after use, it was further wrapped in a layer of aluminum foil to avoid any contamination. Throughout the analysis it was mandatory to wear cotton laboratory coats and nitrile gloves, to avoid any external contamination. Pre-filtered water was passed through clean GF/F filter paper under vacuum to analyze the scope of any contamination during the process of vacuum filtration. The filtrate obtained after sample filtration was meticulously placed in the Petri dishes and covered in aluminum foil. The workstation for the stereo microscope was thoroughly cleansed ahead of the analysis of microplastic samples in the Petri dish acquired after filtration.

The samples collected from surface water underwent treatment with wet peroxide (30% H₂O₂) in covered glass reagent bottles and benched in the dark for 72 h to dissolve the natural organic matter present in it. This was followed by dilution with distilled water and filtration under vacuum through a gridded 0.7 μm glass microfiber filter paper (GF/F, 25 mmØ, Whatman). The filter papers containing the isolated microplastic samples were then placed on a glass Petri dish which was then covered and dried in the oven at around 50 °C for around 1–2 h followed by microscope examination (Wang et al. 2017).

Two-step density separation method was utilized with minor modifications to extract microplastics from sediments (Nuelle et al. 2014; Di & Wang 2018). It commenced with the addition of saturated sodium chloride solution (26%) to wet sediments in a glass beaker in a ratio of 2:1. The mixture was continuously stirred for 10 min using a clean glass rod and then benched for an hour to allow stratification. The supernatant obtained after decantation was poured through a 53-μm brass test sieve which intercepted the microplastics. The filtrate thus obtained was washed with distilled water into a beaker and subsequently covered with tin foil. This process was executed thrice for every sample for the reduction of sample mass for the next step which utilizes the costlier and non eco-friendly sodium iodide.

The residual sediments after the first extraction were collected and transferred to a 500-mL conical flask to which 60% sodium iodide solution was added to three-fourths of the flask. The obtained mixture was then continuously stirred for 2 min at 200 rpm followed by benching for 10 min. The resultant supernatant was poured through a 53 μm brass test sieve thus intercepting the microplastics which were subsequently washed into a glass beaker using distilled water. Each sample went through refilling, shaking, precipitation and decantation twice. Finally, the suspensions obtained from this two-step extraction were combined together and underwent wet peroxide oxidation (30% H₂O₂) for the complete digestion of the natural organics present in them. The process followed after wet peroxide oxidation remained the same as the microplastic extraction process from the water samples.

A stereomicroscope was utilized to inspect and count the microplastic residues on the dried filter papers. The morphological characteristics of microplastics viz. size, shape, and color were analyzed for every individual sample, to trace the origin, source, and residence time of the microplastics. On the basis of their shape, microplastics were categorized into fiber, fragment, film, foam and pellet. Criteria taken into consideration for precise identification of microplastics were: (1) absence of organic structures in the microplastic particle under observation; (2) equally thick and flexible fibers; (3) difficulty to sever with tweezers; (4) microplastics if colored, must be homogenous in color (Norén 2007). All microplastic particles were classified according to their sizes into four groups: group 1 (53–300 μm), group 2 (300–1,000 μm), group 3 (1–3 mm), and group 4 (3–5 mm), which was carried out by sieving the microplastics identified during microscopic analysis with brass test sieves having mesh size of 5 mm, 3 mm, 1 mm, 300 μm, and 53 μm. It has been observed that small-sized microplastics have more prospect of getting ingested. Microplastics' abundance for every category was calculated as the number of particles per liter of water (n/L) and particles per kilogram of sediment (n/kg), respectively.

After microscopic analysis, suspected microplastics were randomly selected for Fourier-Transformed Infrared spectroscopic analysis (FTIR; Nicolet 6700, Thermo Fisher, USA) with an attenuated total reflection device (ATR, Thermo Fisher Scientific, USA) in order to validate their chemical composition. The spectrum range for the FTIR analysis was set to 550–4,000 cm⁻¹ and the resultant polymer spectra were compared with the standard spectral database of the polymers reference provided by Thermo Fisher Scientific (HR Nicolet Sampler Library) to verify microplastic polymers. Characterization of the polymer composition is useful to determine the origin source as well as the input pathways of the microplastics.

Ecological risk assessment involves determination of safe limit of microplastics in the freshwater system which employs the statistical extrapolation method called SSD. SSD Toolbox software was used to generate the SSD curve (Etterson, U.S. EPA, 2020). SSD utilizes NOEC and LOEC values for varied taxonomic groups, which are extrapolated to draw a distribution that is used to derive a PNEC. SSD employs the assumption that the results of single-species toxicity tests utilized for the said extrapolation are representative of all species. The complete procedure entailed the following steps: (1) Extraction of toxicity data sets like NOEC and LOEC as per US EPA guidelines (Environmental Protection Agency 1985), (2) Derivation of SSD curve by log transforming the NOEC values and fitting them into a log-normal distribution, (3) HC₅ value which can be defined as hazardous concentration at which it is assumed that 5% of all species will be definitely affected by a toxicant (microplastic in this case) was estimated using the resultant SSD curve, (4) This HC₅ value was then further divided by a relevant assessment factor (it is also known as uncertainty factor since it manages the variability rising from multiple sources of uncertainties during the assessment of ecotoxicological effects) to derive the PNEC value, (5) Derivation of RQ, where the RQ = MEC/PNEC (Everaert et al. 2018). MEC is the measured environmental concentration of microplastics in each sampling site. The RQ results were expounded as RQ < 1 suggests inconsequential risk for the ecosystem, whereas RQ > 1 indicates steep potential risk.

The high abundance of microplastics found in the present study can be ascribed to anthropogenic activities like extensive aquaculture and tourism. Fishing gears, nets, ropes, lines, etc. eventually get discarded in and around the reservoir and decompose over time, further, the littering of single-use plastic bottles, disposable cups, plates, food packets etc. by tourists along with the absence of proper solid waste management leads to the fragmentation of plastics into small pieces which when paired with rainfall results in the disposal of microplastics into the reservoir thus contributing toward their accumulation in the Hirakud Reservoir, wherein further degradation can take place. Moreover, the small size of the mesh (53 μm) used in the study could have augmented the microplastic concentration collected in the samples.

Microplastics' abundance in sediments was observed to be higher than surface water samples. It could be ascribed to the six decades of impoundment of the Hirakud Reservoir rendering it to be a potential sink for microplastics; giving ample residence time to microplastics for sinking due to further degradation and/ or biofouling along with other hydrodynamic factors. This has been observed in previous studies of microplastic contamination in Chinese lakes viz. Taihu Lake (Su et al. 2016), Poyang Lake (Yuan et al. 2019) and Rawal Lake, Pakistan (Irfan et al. 2020). The increased stability of the microplastics detected in the sediments and their slowed transport in contrast with the flowing water can also contribute to this disparity of abundance (Su et al. 2016).

Assessing the microplastics on the basis of their shapes can help in determining the source of the microplastics in the environment. The main sources of fibers (which are long and thin in appearance) are fishing nets and ropes, textiles, domestic effluents etc. which has recently been corroborated by a study conducted in Lukut estuary, Port Dickinson (Zainuddin et al. 2022) and has also been established in previous studies (Di & Wang 2018; Xiong et al. 2018; Zhang et al. 2019; Hu et al. 2020; Zaki et al. 2021). Literature also suggest that apart from fishing tools and textiles, the presence of fibers can be attributed to atmospheric transport, precipitation and high tourism activities (Dris et al. 2016; Wang et al. 2021; Zainuddin et al. 2022).

The source of microplastic films can be attributed to the fragmentation of disposable plastic bags, cling films, plastic wrappings etc. which have undergone further degradation in the environment as validated in a study of coastal mangrove ecosystems of Singapore (Nor & Obbard 2014). Wherein fragments could be derived from thicker macro plastic litter such as plastic bottles, plastic containers, cosmetics, cleaning media and food packaging that are discarded into the reservoir environment as observed in various studies (Zhang et al. 2015; Goswami et al. 2020). According to Irfan et al. (2020), the pervasiveness of fibers and fragments in both sediments and surface water indicated that the microplastics detected were mostly secondary in nature and were a result of the mismanagement of plastic waste. The sources of foams are expanded polystyrene products and pellets are usually derived from cosmetics.

According to their size microplastics investigated in this study were delineated into the following four categories, 53–300 μm, 300–1,000 μm, 1–3 mm, and 3–5 mm.53–300 μm accounted for 53.24% of total microplastics in surface water samples followed by 300–1,000 μm (25.18%), 1–3 mm (13.42%), and 3–5 mm (8.16%) respectively.

Several studies corroborate the result in the study, wherein it has been noted that microplastic proportion declines with increment in the size class. The result found in the present study was in consonance with the observed size distributions established in preceding studies as presented in Table 5. The proportion of small-sized microplastics could be attributed to the fragmentation, degradation, or biotic processing of large plastics. Moreover, the utilization of a 53 μm steel sieve facilitated the detection and investigation of microplastics resulting in higher values in the present study.

It has been observed that with varying size ranges effects of microplastics on the organisms present in the water also vary (Zhang et al. 2019). The prevalence of smaller microplastics poses considerable risk to the organisms since their larger surface area combined with the lipophilic nature of the plastic can augment its chemical reactivity (Adegoke et al. 2023), enabling them to adsorb contaminants like POPs, and heavy metals, antibiotics or endocrine-disrupting compounds which can cause severe synergistic toxicity.

The dominance of transparent microplastics in the present study is further established by similar findings in several studies viz. urban surface waters of Wuhan (24.7%) (Wang et al. 2017), Three Georges River (21.4–81.6%) (Di & Wang 2018), Dongting Lake (28.7%) and Hong Lake (22.1%) (Wang et al. 2018), Heima River (Xiong et al. 2018), (56%) Ofanto River (35%) (Campanale et al. 2020), Maozhou River sediments (38%) (Wu et al. 2020), Dafangying Reservoir sediments (49.22%) (Liu et al. 2021), etc. A study on beach sediments of Odisha by Patchaiyappan et al. (2021) showed white microplastics (36.57%) to be dominant followed by transparent microplastics (34%).

The prevalence of transparent microplastics followed by white in both the water and sediment samples in both the surface water samples and sediment samples can be attributed to the fading of microplastics because of photodegradation, weathering (Liu et al. 2021) and other degradation processes, caused over a long residence time period. Its origin can be traced to disposable plastic bottles, cups and bags, food packets, fishing lines, ropes, etc. Whereas, the presence of colored microplastics can be associated with the varied applications of dyes in plastic goods like food packets, etc. to increase their appeal to consumers.

Chemical characterization of polymers with ATR-FTIR indicated the presence of polypropylene, polystyrene, high-density polyethylene (HDPE), and low-density polyethylene (LDPE). Categorization of plastics into different types of polymers was done by identifying the peaks of the spectra of major functional groups in the HR Nicolet Sampler Library and matching it with the IR spectra obtained from the probable microplastics samples. The peak assignments of functional groups to identify polypropylene are 2956, 2875 (CH₃ Asymmetric and symmetric stretches); 2921, 2840 (CH₂ Asymmetric and symmetric stretches); and 1377 (CH₃ Umbrella mode). LDPE absorption bands were categorized by using the peak of the spectra as 2917 (CH₂ Asymmetric C-H stretch), 2852 (CH₂ Symmetric C-H stretch), 1377 (CH₃ Umbrella mode) and 718 (CH₂ Rock). Whereas, for HDPE peaks of the spectra used are 2919 (CH₂ Asymmetric C-H stretch); 2850 (CH₂ Symmetric C-H stretch); 730, 720 (Split CH₂ rock). Peak assignments of IR spectra to identify the polystyrene are 3081, 3059, 3025 (Aromatic C-H stretches); 2923, 2850 (CH₂ Asymmetric and symmetric stretches); 1600, 1492 (Aromatic ring modes); 756 (Aromatic out-of-plane C-H bend), 698 (Aromatic ring bend). Polypropylene was found to be dominant (38%), followed by HDPE (26%), LDPE (22%) and polystyrene (14%).

The dominance of polypropylene was attributed to the vastly used and often discarded fishing nets and ropes. Mishandled and littered food packages, and wrappers from sweets and snacks are also major contributors of polypropylene in the Hirakud Reservoir which attracts high footfalls throughout the year. An abundance of high and low-density polypropylene can be credited to food packages, plastic bottles & caps, straws and single-use plastic bags and juice containers, etc. Polystyrene's presence was associated with disposable plastic containers, plastic cups, floats and microbeads in personal care products.

The dominance of polypropylene polymer and polyethylene (HDPE and LDPE) in this study is supported by similar findings in studies conducted in Lake Garda, (Imhof et al. 2013), in three urban estuaries, China (Zhao et al. 2015), remote lakes of Tibet plateau (Zhang et al. 2016), Three Georges River, China (Di & Wang 2018), beach sediments of Odisha by (Patchaiyappan et al. 2021), Heima River (Xiong et al. 2018), Dongting Lake and Hong Lake (Wang et al. 2018), Red Hills Lake of Chennai city, Tamil Nadu (Gopinath et al. 2020). In a study conducted in Yongjiang River, China (Zhang et al. 2019) polyethylene terephthalate (28.3%) and nylon (15.0%) used in carpets, clothes, etc. were found to be dominant in microplastics due to their population-dense location as well as steady discharge of effluents from wastewater treatment plants.

The chemical type of microplastic plays a significant role in the risk it carries since the transport of additives like plasticizers, heat stabilizers, UV stabilizers, etc. that are added to plastic can cause ecological damage upon their leaching and emission, as they have low molecular weight and are present in large amounts as residual monomers which are usually not bound to the polymer matrix. For example, styrene used in polystyrene production has been categorized as a probable carcinogen to humans (type 2A) (IARC 2018).

The SSD curve was derived using the existing toxicity test literature of freshwater species as presented in Table 6. The sample size of eight NOEC values covering six taxonomic groups met the requirement of SSD curve derivation. The extracted toxicity data set was passed through the Anderson–Darling Test for normality after which it was log-transformed and fitted into a log-normal distribution. Since the NOEC values were normally distributed and thus parameterized by mean and standard deviation, those values were further utilized to derive the HC₅ value through the equation: log (HC₅) = μ – K₅. σ, where K₅ = 1.6449 (Aldenberg & Jaworska 2000).

The RQ method has been employed in studies conducted in Yongjiang River, China (Zhang et al. 2019), the coastal area of Port Dickson, and the estuarine area of Lukut (Zainuddin et al. 2022) for microplastics risk assessment, wherein the risk was found to be negligible as presented in this study.

Microplastics were identified in every sampling site of the Hirakud Reservoir. The microplastics' abundance in surface water ranged from 82 to 89 particles/L, whereas the abundance of microplastics observed in sediment samples varied from 159 to 163 particles/kg. Factors like biofouling, sinking due to fragmentation, and ample residence time along with other hydrodynamic factors, can be attributed to higher microplastic concentration in the sediments of Hirakud Reservoir. The study showed that the dominant shape of microplastics in Hirakud Reservoir was fibers, which accounted for 46.21 and 44.86% of surface water samples and sediment samples, respectively. The observed result was found cohesive with the dominant aquaculture activities in the reservoir. Significant contributions from water sports and tourism can be attributed to the prevalence of foam, film, and fragment-shaped microplastics. Small-sized microplastics (53–300 μm) prevailed in both surface water (53.24%) and sediment samples (41.78%). The microplastics were dominantly transparent followed by white, because of fading due to UV light and weathering. The study is also pioneering research encompassing ecological risk assessment of microplastics in a reservoir of Odisha, using chronic toxicity data collected from the literature. Based on the SSD method, the PNEC of microplastics in freshwaters was determined to be 3,954 particles/m³, and the RQ was estimated to be in the range of 0.02073–0.04122 which implied negligible ecological risk of microplastics to freshwater species in all the sampling sites of Hirakud Reservoir.

The evaluation of the microplastics present in the biota (such as phytoplankton and zooplankton) and the influence of hydrodynamics on them is required for enhanced comprehension of their distribution throughout the ecosystem.

The present encompasses ecological risk assessment of microplastics in a reservoir of Odisha using limited chronic toxicity data available in the literature and the PNEC value obtained has little uncertainty. Thus, extensive chronic toxicity tests are required taking the different microplastic sizes and concentrations into account to ensure precision while utilizing the SSD method for ecological risk assessment. Further study entails their flow pathway analysis along with the study of its effect on community health. It is essential to delineate the influence of all the factors responsible for the introduction and assimilation of microplastics in aquatic organisms and humans and the resultant toxicological consequences for establishing a precise ecological risk assessment model.

The authors would like to thank the Odisha State Higher Education Council, Govt. of Odisha for providing funding under the Mukhyamantri Research Innovation Plan. We are thankful to the Central Instrumentation Facility at the Department of Biotechnology and Bioinformatics, Sambalpur University for providing analytical support.

K.P. performed experiment, data analysis, and manuscript preparation. I.B. designed experiment, analyzed data, supervision, and fund acquisition.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.