Introduction

Microplastic contamination has been recorded all over the world, from the poles to the equator, and from the ocean’s surface to the deepest abyss (Blettler et al. 2018; Eerkes-Medrano and Thompson 2018; Li et al. 2018; Mendoza and Balcer 2019; Peng et al. 2018a, b; Rochman et al. 2015; Tursi et al. 2022). Microplastics are plastic particles ranging in size from 0.05 to 5 mm (Arthur et al. 2009). They are transferred from the terrestrial ecosystems via biotic and abiotic mechanisms, or they reach the aquatic environment directly through things that contain microplastics (Li et al. 2018; Ory et al. 2017). After that, microplastics are further classified as primary microplastics (plastic beads used in air blasting and cosmetic items) or secondary microplastics (plastic fragments made from bigger plastic particles) (Sharma et al. 2023). According to Burns et al. (2018), MPs may be categorized into several form categories with names like fragments, fibers, films, foam, and beads. MPs’ physical features, including density, shape, and size, may affect how they move across various environments and how they disperse (Hartmann et al. 2019). Stormwater runoff, industrial waste, and household rubbish are just a few of the pathways via which microplastics may enter an aquatic ecosystem (Horton and Dixon 2018; Nel and Froneman 2018). Based on Li et al. (2018), the mean abundance of MPs in freshwater systems varied from almost none to several million pieces per cubic meter. Since Gregory, (1977) discovered the first discovery of microplastics off the coast of New Zealand, the microplastic study has grown exponentially around the world.

Most of the research has classified each sampling site according to the predominant human activities that surround it to analyze the source of microplastics qualitatively (Lin et al. 2022). Furthermore, wastewater treatment plants (WWTPs) are well recognized as point sources of microplastics because huge volumes of microplastic-containing effluents are continually discharged (Grbić et al. 2020), even though the majority of the microplastics are removed from the influents (Talvitie et al. 2017). As a result, WWTPs are frequently associated with significant levels of microplastic contamination. Recent research, for example, discovered greater microplastic concentrations in WWTP effluents compared to those in a reference location (Magnusson and Norén 2014).

Organisms consume MPs unwillingly by eating prey that already includes MPs or incidentally by mistaking them for natural food (such as plankton) (Boerger et al. 2010; Davison and Asch 2011). MPs can change the biology of tissues when they are swallowed and accumulate in them. Microplastic exposure can have a variety of physiological effects on reproductive success (Ahrendt et al. 2020), metabolic changes (Hamed et al. 2019), behavior (Rochman et al. 2013), growth (Ahrendt et al. 2020), and histopathology (Hamed et al. 2020). Microplastics have also been demonstrated to collect trace metals and infections (Khdre et al. 2023; McCormick et al. 2014; Nakashima et al. 2012). As a result, the presence of MPs in aquatic ecosystems is thought to act as a conduit for the transfer of diseases, metals, and persistent organic pollutants to higher trophic chains (Gholizadeh and Patimar 2018; McCormick et al. 2014). Aquatic insects have important ecological functions in both the land and water environments as main consumers, detritivores, pollinators, and predators (Khedre et al. 2023a, b). Consequently, aquatic insects play a crucial role in preserving the ecological cycles. Due to their functions as disease vectors or bioindicators, several groups had their ecology well researched. Despite their inconsistent contribution to global biodiversity, freshwater aquatic insects have often not been a focus of diversification (Idris et al. 2020). Aquatic insects such as caddisflies, mayflies, stoneflies, beetles, dragonflies, true flies, and certain moths are among the groups of insects represented in streams by their larvae. These aquatic insects serve an important function in connecting the freshwater food chain between producers and higher consumers (Mohd Rasdi et al. 2012).

It is well-known that water mites are abundant parasites in aquatic ecosystems, nearly all insect orders with aquatic stages are considered as potential hosts for at least one water mite species (Smith and Oliver 1986). The life cycle of water mites consists of six stages (Smith 1988): the egg, larvae, protonymph, deutonymph, tritonymph, and adult (mostly predators) (Zhavoronkova 2006). Water mite parasitism is necessary for larval feeding and completing its development (Wohltmann et al. 2001). Because the water mites drain nutrients from their hosts, they affect the host condition and consequently host fitness as well (Paul 2022). So far, reduced mating success (Rehfeldt 1995) and reduced reproductive output (Rolff 1999) of aquatic insect hosts have been reported as direct consequences of mite inspiration. Few studies have been conducted to test the direct and indirect effects of parasites on population dynamics in freshwater ecosystems (Ebert et al. 2000) and the role of predators in shaping the behavior and life history of aquatic organisms (Eklöv, 2000).

As an alternative, several research studies have collected environmental microplastics using a one-time sampling technique, which showed just a snapshot of microplastic contamination at that time point (Naji et al. 2017; Wagner and Lambert 2018; Khedre et al. 2023c, d) and may give incorrect data on microplastic abundance. Studies on the temporal and geographical distributions of microplastics in the environment are available (Fan et al. 2022; Xia et al. 2021), but there are not many that specifically address microplastics detected in living organisms.

Two of the most contaminated wastewater sites in the Sohag Governorate were chosen for this investigation. Every day, a considerable volume of wastewater is dumped into these areas. Furthermore, wastewater can be used to irrigate land utilized for agriculture, leading MPs to be transported to agricultural land’s surface, which is a big concern. The current study aims to (i) assess the microplastics levels of the wastewater using a multicompartment approach for surface water, aquatic insects, and water mites throughout the four seasonal of the year to determine possible microplastics risks, where this study presents new information about MPs in three different edible aquatic fauna family: Coroxide, Notonectidae, Hydrachnidiae (water mites) collected from two wastewater sites; (ii) answer the following questions: a) what is the relationship between the microplastic loads in aquatic fauna and the different aquatic systems that differ in microplastic pollution?, b) what is the effect of the water mites’ parasitism on the number of MPs ingested by aquatic insects? and c) Finally, what is the ability of the aquatic insect and mite species to be used as qualitative and quantitative biomonitoring for microplastics in freshwater? (iii) identify shapes, colors, size, and polymeric characteristics of MPs extracted from water and aquatic fauna; and (IV) assess the potential risks of MPs through multiple indices. Furthermore, our findings were intended to provide information regarding the seasonal variation of MPs in the wastewater, hence increasing understanding of the seasonal effect on MP pollution and risk levels in freshwater ecosystems. Consequently, our findings will help policymakers and the government, in partnership with international organizations, implement suitable management strategies to minimize the waste of plastic.

Materials and methods

Sampling sites

The sampling area is in Sohag City, which is located in Upper Egypt, in the middle of the Nile Valley (about 125 km long) (Fig. 1A, B). Two wastewater disposal plants have been established in Sohag Governorate. One on the western plateau, designated the West wastewater treatment plant (S1), and the other in the east plateau, named the East wastewater treatment plant (S2), were chosen for sampling in the four seasons of 2022 (Fig. 1C). The two sites’ characteristics are summarized in Table 1.

Fig. 1
figure 1

Egypt map (A) showing Sohag Governorate (B) and Google Earth photo showing the collecting site (C)

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Table 1 The main characteristics of the two wastewater sampling sites
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Sample collection

Surface water and Aquatic fauna (aquatic insects and mites) samples were collected from the two wastewater sites (S1 and S2) across four seasons in 2022. 75 L of surface water samples were taken from five randomly chosen points at the same times and points as the aquatic fauna samples, using the grab sampling approach as outlined by McNeish et al. (2018), which was then kept in 5-L clean glass containers for laboratory examination. Using a pond net (200 μm mesh size; 0.30 m × 0.30 m opening), aquatic species were collected. Following collecting, fieldworkers performed sorting and preliminary taxonomical identification.

Separation and identification of aquatic fauna

Using a dissecting binocular and a light microscope in the laboratory, the gathered samples were divided into groups based on their taxonomical groupings before being preserved once more in 70% ethanol. When practicable, taxonomical identification was carried out down to the genus level, in accordance with Cook (1974) and Hirashima et al. (1989).

Extraction of microplastics from surface water and aquatic fauna

Each 5-L water sample received 150 ml of 10% KOH to help break down the organic debris overnight, in accordance with the Dahms et al. (2020) procedure. Then, water samples were filtered using cellulose nitrate filter paper (0.45 µm pore) in a vacuum filtration. To be sure there were no MPs remaining after each wash, the containers were rinsed five times with filtered distilled water.

Using Naji et al. (2018) methodology, the MPs in the aquatic insects and mites were separated, where five samples, each with 10 individuals, were prepared from each taxon. Before digestion, the wet weight of each individual and sample (in milligrams) was determined. Individuals from each sample were placed in a glass beaker and digested by adding 20 ml of 30% H2O2 (strong oxidizing acids to break down biological tissues). Each digested sample was filtered via 0.45 µm filter paper.

Microplastic identification and characterization.

The number, form, and color of the MPs were identified using a dissecting microscope equipped with a digital camera (Carl Zeiss Suzhou Co.). Additionally, the MPs’ dimensions (length and width) were measured using the ImageJ software. The chemical composition of the MPs was determined using (ATR-FTIR, Alpha Bruker Platinum, 1–211-6353). MPs were classified based on physical properties like color, shape, and size, and quantified using FTIR. The data were processed using the OPUS programme (Bruker Optics GmbH). The polymer type was determined by comparing the acquired spectra to known reference spectra (Primpke et al. 2018).

Experiment quality assurance

All samples were stored in sealed vials or Petri dishes to avoid contamination. Wearing cotton laboratory coats and gloves, the experimenters did not utilize any plastic products. Every container was cleaned using Milli-Q water before use. Every solution used in the investigation was run through three filters before use. Three procedural blanks were run to ensure there was no contamination from the laboratory setting. Throughout the course of the inquiry, the blanks did not contain any microplastic particles. For a day, three Petri dishes were placed close to the workstation to gather airborne particles and determine the level of airborne contamination. Because we only took one sample of cotton fiber, procedural contamination had little effect on the investigation’s findings.

Risk assessment of microplastics

The pollution load index (PLI), which was used to calculate the risk of microplastic contamination (Kasamesiri et al. 2023), was created using the following formulae (Wang et al. 2020).

$$CFj = {\raise0.7ex\hbox{${Ci}$} \!\mathord{\left/ {\vphantom {{Ci} {Co}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${Co}$}}$$
$$PLIj = \sqrt {CFj}$$

where Co is the background MP concentration and Ci is the MP concentration at sample site j. The global records for water (0.28 particles/m3, 0.00028 particles/L) were used as the basis for the MP reference values (Guo et al. 2021).

The polymer risk index (H), as determined by Li et al. (2020), is as follows:

$$H = \mathop \sum \limits_{n = 1}^{n} PnSn$$

where Sn is the polymer hazard score determined by Lithner et al. (2011), Pn is the percentage of each polymer type at each sample location. H was classified into four levels by Lithner et al. (2011) and Guo et al. (2021):

The ecological and toxicological effects of MPs have been evaluated using RI (Peng et al. 2018a; Ranjani et al. 2021).

$$Tj = {\raise0.7ex\hbox{$H$} \!\mathord{\left/ {\vphantom {H {Ci}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${Ci}$}}$$
$$RI = TjxCFj$$

Tj stands for MPs’ toxicity coefficient. Five RI contamination limits were determined by Guo et al. (2021).

Statistical analysis

Descriptive statistics (means and standard deviations) were used to characterize the main characteristics of MP concentrations in water and aquatic fauna collected at various seasons and sites. Statistical analysis was used to assess the basic assumptions about seasonal fluctuation in the number, shape, color, and polymer type of MPs in water and aquatic fauna. Season, location, taxon, and size are all independent factors. The data were analyzed using Shapiro–Wilk and Levene’s tests to assess normality, homogeneity, and equal variance. To analyses normally and evenly distributed data, one-way ANOVA was employed. For skewed and unequally distributed data, the nonparametric Mann–Whitney U test was utilized. P values < 0.05 were deemed significant differences. The data were analyzed using IBM SPSS (version 22, IBM Corp., Armonk, NY, USA).

Results

Seasonal abundance and characterization of microplastics in surface water

Data presented in Fig. 2 show the water MP abundance at the two wastewater sites throughout the different seasons. The mean number of MPs throughout the system was 2.55 ± 1.1 particles/L. S2 had a higher abundance of MPs (3.1 ± 0.9 particles/L) throughout the investigated period. The mean highest values of MPs were recorded in the winter season (3.27 ± 0.06 and 4.09 ± 1.1 particles/L in S1 and S2, respectively). High significant differences were obtained among the different seasons in each site (F = 7.4, P = 0.004). Moreover, MP abundance was significantly higher in S2 than in S1 over all seasons (F = 9.3, P = 0.019).

Fig. 2
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Mean seasonal abundance of MPs in the surface water of the two sites of wastewater

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The microplastic shapes detected in the surface water of the two wastewater sites were fibers and fragments only (Fig. 3). Annually, fibers were the shape of the most prevalent particles observed in S1 and S2 accounting for 88% and 84%, respectively (Fig. 4A). Statistically, no significant differences in the abundance of fibers and fragments were recorded between S1 and S2 throughout the investigated year (F = 4.2, P = 0.086).

Fig. 3
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Micro-FTIR spectra of representative microplastic polymers extracted from the different samples (PES = polyester, PE = polyethylene, and PP = polypropylene). The photographs indicate the MPs particles that were identified

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Fig. 4
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Percentage of the different microplastic shapes (A), sizes (B) with µm, colors (C), and chemical composition (D) collected from the surface water of the two wastewater sites

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The sizes of MP particles (fibers and fragments) obtained from these sites did not exceed 3000 µm. The percentage of seasonal variations in the different MP size classes showed fluctuations during the study period (Fig. 4B). The most abundant MP size classes were in the range of 1501- 2000 µm (27%) and 2001-2500 µm (26%) in S1. However, in S2 the frequent sizes were in the range of 1001-1500 µm (29%) followed by 1501–2000 µm (21.5%). Statistical analysis showed a significantly higher abundance of the smaller MP size classes (180-2000 µm) than the greater (2001–3000 µm) throughout the year (F = 4.7, P = 0.02). Also, significant differences in the abundance of MP size classes between the two sites were observed (F = 6.1, P = 0.009).

Fibers and fragments were introduced in a wide spectrum of colors, including red, blue, black, green, violet, orange, yellow, and white. The frequency distribution of MP colors in the two sampling sites in the different seasons was illustrated in Fig. 4C. Most of the fibers and fragments appeared blue (38%). Red and black were the second most abundant colors of the total MP particles and represented 24% and 22%, respectively. The annual proportion of blue color was significantly higher in S2 than that in S1 (F = 5 P = 0.016). However, the red color was significantly more abundant in S1 than in S2 (F = 5.3, P = 0.015).

Three types of polymers were identified by FTIR spectroscopy: polyester (PES), polyethylene (PE), and polypropylene (PP) (Fig. 3). Polymer concentrations observed in the water samples varied considerably among the different seasons and between the two sites (Fig. 4D), where the microplastic polymers in the winter, spring, summer, and autumn seasons were predominantly PES (75%,94%, 92%, and 90%, respectively), followed by PP and PE in winter and spring, and PE and PP in summer and autumn. A significantly higher abundance of PES was found in surface water during the year when compared with other polymers (F = 11.2, P = 0.001). PP and PE were significantly more abundant in winter (F = 6.82, P = 0.041) when compared with the other seasons. According to the two sites of wastewater, the most abundant polymers of MPs throughout the year were PES (88% and 84%, respectively), followed by PE and PP in S1 (7% and 5%, respectively) and PP and PE in S2 (10% and 6%, respectively). No significant differences in the abundance of PES were found among the two sites (F = 3.21, P = 0.09) throughout the year. However, significantly the PP was more abundant in S2 than in S1 (F = 6.4, P = 0.009).

Microplastics load indices

The PLI values of MP pollution during various seasons are displayed in Table 1. As can be observed in Table 2, the two wastewater sites’ calculated PLI values over various seasons were extremely high, indicating a very high pollution discharge level. The greatest PLI values were also recorded in the winter. Additionally, in every season, PLI values were greater in S2 than in S1. Table 2 lists the H values for MP contamination at the two wastewater sites over various seasons. In Fig. 4D, the percentages of the identified polymer used to calculate the H are shown. Approximately equal percentages of each polymer were found in each of the four seasons. As a result, obtained H values were nearly comparable between seasons. A medium danger to the environment is indicated by the H values of the polymers in various seasons, which would fall under category III. This is due to the polymers’ toxic properties. In the two wastewater sites, the highest H of MPs was identified during the autumn at S1 and in winter at S2. Additionally, the RI index of microplastic at the two sites indicated the very highest degree of danger (degree V).

Table 2 Seasonal variations of microplastics impact indices in the surface water of the two wastewater sites
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Size of the collected fauna taxa

Based on the mean body wet weight (w.w.) of the different taxa collected from the two wastewater sites, the smallest mean size was obtained for water mites (Hydrachnidiae) (0.27 ± 0.033 mg), followed by Coroxidae (5.7 ± 0.8 mg), and Notonectidae (6.8 ± 0.6 mg). Statistically, significant differences in the size of the individuals among the three taxa were recorded (F = 8.4, P = 0.007).

Seasonal abundance and characterization of MPs in the different fauna taxa at the two wastewater sites

Corixidae

A total of 139 MP particles were recorded from 400 adults across the two sites. The number of MP particles in S1 and S2 was 60 and 79, respectively. Summer had the highest number of MPs per individual in both S1 and S2 (0.41 ± 0.18 and 0.63 ± 0.07 particles/ind, respectively) followed by winter (0.34 ± 0.012 and 0.47 ± 0.07 particles/ind, respectively). Spring had the lowest number in S1 (0.16 ± 0.01 particles/ind), however, in S2 the lowest number was recorded in autumn (0.17 ± 0.03 particles/ind) (Fig. 5A, B). Significant seasonal differences in MP loads per individual were observed in both sites (F = 13.2, P = 0.001). Moreover, the MP load per individual was significantly higher in the S2 than in the S1 in the four seasons (F = 16.7, P = 0.0013).

Fig. 5
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Mean seasonal abundance of MPs per individual of Corixidae and Notonectidae in S1 (A), and Corixidae, Notonectidae, and Hydrachnidiae in S2 (B) collected from the two sites of wastewater

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All MPs ingested by Corixidae adults were fibers (100%) (Fig. 6A). Fiber’s lengths ranged from 382 to 961 µm with an average of 655 ± 234 µm. The microplastics in the size range of 501-1000 µm accounted for the highest percentage (78%) (Fig. 6B) and were mostly blue (65%) followed by red (16.5%) and black (12%) (Fig. 6C). The only polymer in Corixidae adults was polyester (Fig. 6D). The proportion of contaminated adults with MPs was 75% and 85% in S1 and S2 of all samples collected throughout the study period, respectively.

Fig. 6
figure 6figure 6

Percentage of the different microplastic shapes (A), sizes (B) with µm, colors (C), and chemical composition (D) collected from different taxa of the two wastewater sites

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Notonectidae

From 400 adults a total of 263 MP particles were extracted across two sites. In S1 and S2, the number of MP particles was 123 and 140, respectively. Where, the highest number of MPs per individual in S1 and S2 was observed in summer (0.61 ± 0.08 and 0.82 ± 0.1 particles/ind, respectively) followed by winter (0.511 ± 0.08 and 0.77 ± 0.1 particles/ind, respectively) and the lowest number was recorded in autumn (0.39 ± 0.16 and 0.54 ± 0.09 particles/ind, respectively) (Fig. 5A, B). Significant seasonal differences in MP loads per individual were observed at both sites (F = 11.45, P = 0.0051). Moreover, the MP load per individual was significantly higher in the S2 than those in the S1 among the four seasons (F = 13, P = 0.01).

Most MPs ingested by Notonectidae adults were fibers and ranged from 85 to 88% of all ingested MP particles in the two wastewater sites, and the fragments represented the remaining proportion (Fig. 6A). According to the length, fibers ranged from 614 to 1105 µm with an average of 797 ± 184 µm; while, fragments were between 107 and 167 µm with an average of 137 ± 46 µm. Overall, the microplastics in the size range of 501-1000 µm accounted for the highest percentage (65.5%) (Fig. 6B) and were mostly blue (60.5%), red (20%), and black (17%) (Fig. 6C). The dominant polymer in the adults was polyester (86.5%), followed by polypropylene (7.5%) and polyethylene (6%) (Fig. 6D). The percentage of adults contaminated with MPs was 75% and 85% in S1 and S2 of all samples collected throughout the study period, respectively.

Hydrachnidiae

Adults of the family Hydrachnidiae (water mites) were recorded only in S2 during the investigated year. A total of 6.9 MP particles were extracted from 200 adults. In summer, the highest number of MPs was recorded (0.042 ± 0.015 particles/ind) followed by winter (0.034 ± 0.013 particles/ind), and the lowest number was in spring (0.02 ± 0.004 particles/ind) (Fig. 5B). Statistical analysis showed a significant seasonal difference in MP loads per individual (F = 9.4, P = 0.02).

The fibers were the only MPs ingested by Hydrachnidiae adults (100%) (Fig. 6A). The lengths of fibers ranged from 165 to 315 µm with an average of 258 ± 73 µm. Also, the microplastics in the size range of 165-500 µm accounted for the highest proportion (96%) (Fig. 6B), and the most colors were red (73%) (Fig. 6C). The polymer in Hydrachnidiae adults was polyester (100%) (Fig. 6D).

Discussion

This study is the first report on the seasonal and annual abundance of microplastic concentrations and characterization extracted from surface water, Coroxidae, Notonectidae, and Hydrachnidiae (water mites), and the first observation of the effect of the water mites’ parasitism on the number of MPs ingested by aquatic insects in two wastewater sites in Sohag Governorate, Egypt. The results revealed that MPs were observed in all samples obtained from the two sites of wastewater, which indicates that MPs are widespread in different aquatic systems. These results support the earlier studies showing that MPs are ubiquitous in aquatic systems all over the world (Horton and Dixon 2018; Mani and Burkhardt-Holm 2020; Woodall et al. 2014).

Wastewater treatment plants remain poorly managed in developing countries (Mema 2010), and wastewater discharge from these systems is considered an important source of MP pollution in aquatic ecosystems (Browne 2015; Chang 2015). Large amounts of MP particles are discharged from these systems into the waterway daily (Eriksen et al. 2013; Mason et al. 2016; Murphy et al. 2016). S1 and S2 are situated near several treatment plants whose effluents carry MPs into these sites. MP abundance was higher in S1 than in S2, and the density of MPs in these sites varied in different seasons. The variation might be associated with the amount of sewage effluent discharged to the basins from the WWTPs. A significant increase in surface water MP abundance was recorded in S2 compared to S1 across all seasons. This may be attributed to the fact that S2 receives the waste of many factories including textile mills. S1 had lower MP abundance compared to S2 despite its contamination with agricultural plastic products. This confirms that the relative abundance of MPs is closely correlated with the presence of spatial sources of pollution (Irfan et al. 2020).

Fibers are among the dominant shapes of MPs observed in aquatic systems ranging from sea beds to small inland freshwater lakes (Cesa et al. 2017). The present results confirmed the previous study, where the microfibers represented a higher proportion in S1 and S2 throughout the year. The dominance of fibers in surface water suggests that the source of MPs is more related to direct effluents from garment washing processes. Napper and Thompson, (2016) reported that a single wash of about 6 kg of clothes can release nearly 500,000 fibers in its waste effluent, resulting in higher fiber particles in the environment. Moreover, Mintenig et al. (2017) reported that data on treated wastewater in Germany revealed that fibers dominated in more than 80% of the samples. Furthermore, Luo et al. (2019) reported that urban effluent, such as domestic pollution, was considered a key factor contributing to microfiber abundance. They also demonstrated that fibers are more likely to be observed close to the shoreline, where effluents are discharged. This may be an additional explanation for the dominance of fibers in the present results. In addition to domestic effluents, they can directly receive airborne fibers. This process can increase the concentrations of fibers in the environment (Brahney et al. 2020; Luo et al. 2019). The second frequent shape in the present study was fragments. S2 water samples contained a higher proportion of fragment particles compared to S1. This may be due to site-specific factors that may affect MP densities in each site (Eerkes-Medrano et al. 2015; Klein et al. 2015).

The present result indicated that small particles were dominant and constituted a larger proportion of the MPs in our samples. Surface water samples from the two wastewater sites contain small particles (500–2000 µm). The small-size classes represented 77.6% of the total surface water samples. A significantly higher proportion of small MPs in the wastewater sites suggests that the MPs have been subjected to degradation mechanisms due to physical and chemical stresses from the environment (Wicaksono et al. 2021a; Zhang et al. 2016), as well as higher rates of biodegradation coinciding with increased microorganisms in these sites (Hoellein et al. 2017). Additionally, the closed environment of wastewater basins may increase the degradation of MPs through long-term accumulation (Eerkes-Medrano et al. 2015). The dominance of small-sized particles in the sampling areas may also be due to the high abundance of fiber particles in the samples. These findings have been recorded in many freshwater systems such as Laurentian Great Lakes USA (Driedger et al. 2015), Hövsgöl Lake, Mangolia (Free et al. 2014) Taihu Lake, China (Su et al. 2016), and Rhine River, Europe (Mani et al. 2019). Generally, an inverse relationship between MP size and concentration was a common finding in freshwater MP studies (Battula et al. 2021; Fan et al. 2021; Schmidt et al. 2018). However, larger plastic particles (1501–2500 µm) only accounted for the low ratio (24%) in the present samples, respectively. This trend of size distribution was more or less similar to that of other water systems, where the highest frequency of size was 250–1000 µm followed by (2000–3000 µm) (Nguyen et al. 2020). Furthermore, Barrows et al. (2018) found a global prevalence of MPs less than 1.5 mm (77%) throughout the ocean waters.

Eight different MP colors were identified in the present samples. Abundant colored MPs have been recorded in many studies (Gallitelli et al. 2021; Luo et al. 2019; Yuan et al. 2019). A great variety of colored plastic products are frequently used in modern life; and therefore, large quantities of plastic waste are produced and consumed. After degradation, these wastes can finally result in colored MPs (Yuan et al. 2019). Microplastic color can give information to speculate on the source and weathering process (Wicaksono et al. 2021a, b). For instance, the yellowish color of MPs can indicate the photooxidation and weathering processes of MPs (Liu et al. 2020). Moreover, the lower abundance of white color from the obtained samples indicates the low period of particle placements in the present sites because, by increasing MP placement time in aquatic systems, the effects of sunlight lead to the change of their colors to white (Rasta et al. 2021). Also, data obtained from research conducted by Singh et al. (2021) indicated that white-colored particles are mainly related to PS foam. This finding supports our data showing that foam particles are absent from wastewater sites. The most common MP color found was blue, accounting for 38% of the total samples in surface water. The second most frequent colors in our study sites were red and black, represented by 24% and 22.5%, respectively. Previous studies showed that the dominant species typically included blue (Barrows et al. 2018; Lahens et al. 2018; Strady et al. 2021), black (Qin et al. 2020; Sang et al. 2021), and red (Nguyen et al. 2021; Wicaksono et al. 2021a). However, the presence of a low percentage (1%) of yellow MPs indicates that the degradation of plastic waste does not appear where yellow MPs are highly degrading products (Liu et al. 2020).

The chemical composition of the polymer reflected the different sources of plastic pollution (Ballent et al. 2016). Chemical analysis of MP revealed that the proportion of PES was the highest in the two sampling sites. Domestic sewage derived from point sources plays an important role in MP pollution. For example, effluent from WWTPs contributes to high levels of MP in surface water (Conley et al. 2019), which could explain the higher abundance of polyester in both sites. PE and PP were found at the two sampling sites in different proportions according to the different seasons. They are used worldwide as material for various disposable items such as plastic bags, drinking straws, and bottle caps, which are used in large amounts every day and subsequently accumulate in aquatic environments after being thrown (Andrady 2011; Antunes et al. 2018). Moreover, PP is the most applied plastic in urban, industry food packaging, and agriculture (Shabaka et al. 2022). Therefore, they could be extremely observed in aquatic ecosystems (Zhao et al. 2015). From the previous results, it can be concluded that MP characteristics (physical and chemical) could be considered important factors in pollution fingerprinting (Luo et al. 2019).

PLI is used to determine the extent of MP contamination (Pan et al. 2021). PLI had the greatest and lowest values in winter and spring, respectively, and the highest values of PLI were recorded in S2, these findings attributed to PLI values increasing with increasing MP abundance, and PLI values were connected to the number of MPs up to now. Similar findings were observed in a runoff study at Bushehr Port (Hajiouni et al. 2022). Because the MPs problem is becoming more serious, evaluating MPs risks will help to a more thorough understanding of the state of MPs pollution, migratory routes, and potential harm to human health, and aid in improving ecosystem safety (Xu et al. 2018). MP polymers’ hazardous effects are determined by their chemical constituents (Lithner et al. 2011). Polymers found in MP particles have been linked to environmental risks (Xu et al. 2018). MPs have been consumed by a variety of taxa, including fish (Lusher et al. 2013), prawns (Daniel et al. 2020), crabs (Watts et al. 2014), mussels (Von Moos et al. 2012), and aquatic insects (Khedre et al. 2023d). The current findings indicate that MPs leaching from wastewater have the potential to cause environmental contamination and should be regarded as a possible issue. The findings of this study agreed with those of studies conducted in India on coral reef ecosystems and beach sediments in the Gulf of Mannar (Patterson et al. 2022). The highest H of MPs was found in autumn at S1 and in winter at S2, which is most likely because of the season’s greater percentage of polyethylene and higher hazard score for polyethylene than for other detected polymers. Seasonal variations in MP types can be attributed to several variables, including industrial activity, the amount of sewage effluent that WWTPs dump into the basins, and agricultural cultivation. As a result, different MP polymer types can have an impact on pollution levels (Wang et al. 2021).

Our results showed that backswimmer notonectid adults accumulated MPs. Notonectidae are important predators dominating the food webs (Dettner and Ecology 2019; Papáček and Soldán, 2008) and can play a major role in shaping the structure and the abundance of the zooplankton population in several freshwater environments (Hampton et al. 2000). They have a broad diet that includes several aquatic organisms such as crustaceans, tadpoles, and aquatic insects (Fischer et al. 2012, 2013; Jara et al. 2012; Khedre et al. 2024a, b). In our study, 75% of all notonectid samples contained MPs in S1. However, in S2 this proportion increased to 85% during the investigated period. This finding suggests that the absence of MPs in samples indicates decreased contamination of the site in a specific area (Iannilli et al. 2023; Mistri et al. 2022). This may be explained by the fact that a high concentration of MPs in an aquatic environment leads to a high MP encounter rate resulting in an increased feeding rate (Scherer et al. 2017). Moreover, MP load/individual was significantly higher in S2 compared to S1, which was consistent with higher MP levels in S2. In this regard, Simakova et al. (2022) reported that a higher number of MP particles was consumed by aquatic insects where the concentration of MPs was higher. Furthermore, no correlation between water MP loads and notonectid MP loads (P > 0.05) was observed in the different seasons. This might be attributed to the different prey that the notonectid adults had fed on, which not only included aquatic but also terrestrial organisms trapped on the water surface, such as bees, ants, and mosquitoes adults (Quiroz-Martínez and Rodríguez-Castro 2007).

Corixidae (Cenocorixa sp.), which belong to the Heteroptera order presented one of the most dominant taxa in the wastewater sites throughout the investigated period. They are commonly described as predators; their feeding strategy differs from that of notonectid adults as a result of the absence of the standard piercing beak present in Notonectids. They ingest living materials (diatoms, algae, protozoans, small insects) when they stir up debris, and microscopic organisms in the bottom of the water body (Kriska 2023). The differences in their mouthparts and consequently their feeding mechanism, in addition to the differences in the size of adults of both species may explain the variation in both the qualities and quantities of MPs and the mean MP load/individual between corixid and notonectid adults. Huang et al. (2022) and Watts) mentioned that the anatomy of mouth parts and the physiology of organisms likely affect the ingestion of MPs. It is important to note that Corixidae are eaten by fish, birds, and humans (Srayko et al. 2022). Therefore, they may act as vectors that introduce MPs to food webs and another aquatic system since the adults can fly and disperse widely, and rapidly invade new habitats (Souty-Grosset et al. 2016).

Our results showed that water mites also accumulated MPs. Water mites are found globally in a great variety of freshwater habitats. They have a complex life cycle that has co-evolved with important freshwater insect groups (Smith et al. 2010). Adult water mites are predators of different prey including copepods, mosquito larvae, chironomid larvae, and ostracods (Pozojević et al. 2019). Larval water mites are common ectoparasites of aquatic and semiaquatic insects including water bugs, dragonflies, mayflies, and water beetles (Smith 1988). The mean MP load per individual detected in our samples was 0.03 ± 0.01 particles/ind. This suggests that water mites may accumulate MPs either through feeding on chironomid larvae (López-Vázquez et al. 2022) that have previously ingested MPs or by consuming MPs by mistake if the particles resemble their prey.

Moreover, our results suggest that the presence of water mites affects the number of MPs ingested by aquatic insects that live in the same site. MPs were significantly higher in S2 in two insect taxa (Corixidae and Notonectidae) that are known to be infected by larval water mites (Smith et al. 2010). In contrast, the number of MPs was lower in S1, where water mites were absent. These differences may have two possible explanations. First, the presence of higher MP concentrations in S2 than in S1 may affect the MP loads/individual (Scherer et al. 2017). Second, the presence of parasite infection by water mites in S2 has been linked to MP contamination (Dixon 2016; Hernandez-Milian et al. 2019; Parker et al. 2021).

In our study, larval water mites were detected in adult corixids. This suggests that higher parasite loads may increase the susceptibility of individual hosts to have higher MP loads, or influence the ingestion of MPs (Parker et al. 2020, 2022). In this regard, Ponton et al. (2011) reported that compensatory changes in feeding were observed in infected insects and are likely to involve physiological mechanisms modifying the quantity of ingested food. For instance, increased food uptake by infected insects can compensate for the extra energy required or nutrient costs imposed by growing parasites. Further research is needed to confirm our suggestion, the effect of parasitism by mites on MP ingestion in the host.

Regarding the shape of MPs detected in the different taxa, fibers were the main MP type and accounted for 85–100% of the total MP. This was caused by the higher abundance of fibers observed in surface water samples in the two sampling sites compared with the fragments. This could be related to the fact that fibers have the smallest width as reported in the present study and the previous data (Pirc et al. 2016). This was similar to the previous findings which indicated that fibers were the most dominant MPs in most freshwater invertebrates, for instance, Naji et al. (2018) reported that MP fibers represented more than 50% of the total MPs in molluscs from the northern region of the Persian Gulf. Hurley et al. (2017) reported that 87% of the MPs ingested by freshwater annelid (Tubifex tubifex) were microfibers. Akindele et al. (2020) and Akindele) demonstrated that fibers were the most abundant MPs in both abiotic and biotic samples. (Bertoli et al. 2022) recorded fibers in 48.5% of their investigated taxa including different insect orders, e.g., Odonata, Heteoptera, Coleoptera, Diptera, Trichoptera, and Plecoptera. The abundance of fibers in the different taxa has been attributed to their diverse sources, e.g., washing domestic wastewater, fisheries, and other anthropogenic activities (Mason et al. 2016; Murphy et al. 2016).

Besides MP shape, the size of MPs is another characteristic that influences the interaction between MPs and aquatic taxa (Scherer et al. 2017). The present results showed different size ranges of MP (< 500-2500 µm) in the different taxa. Small microfibers (< 500-1000 µm) were the predominant size in most examined insects. Statistical analysis showed that MPs in both size classes < 500 µm, and 501-1000 µm were significantly higher than in their surrounding MPs water (P < 0.05). Small microfibers < 500 µm represented a high proportion of the total MPs in Hydrachnidiae (96%). However, Notonectidae ingested MP particles in the upper size range (1000-1500 µm). The reason for the MP size variability in these taxa may be the significant variability in the body size of Notonectidae compared with Hydrachnidiae. A similar finding was reported by Scherer et al. (2017) who demonstrated that MP size preference depends on the size of specimens where larger organisms ingested larger MPs since the morphology of mouthparts may allow the ingestion of large MP particles that pass through the alimentary canal. As previously reported for freshwater mussels, more MPs of smaller sizes were ingested in both field and laboratory samples, which indicates that the ingestion of MPs is size-dependent (Qu et al. 2018). It is interesting to note that some MPs extracted from the examined taxa were smaller in size than those detected in water samples. This suggests that MP size reduction may occur inside their gut because of biodegradation. Likewise, Yang et al. (2014) reported that some bacterial strains, such as Bacillus. and Entrobacter alsuriae break down polyethylene MPs in the gut of wax worms (Plodia interpunctella). Moreover, Immerschitt and Martens, (2021) stated that mechanical fragmentation of MPs may take place in the gizzard of Odonata nymphs by their chitinous teeth.

To our best knowledge, smaller particles of MPs may cause more harmful effects than larger ones particularly to lower trophic organisms as these organisms capture any food source of appropriate size without selection (Nguyen et al. 2020).

Regarding the MP colors detected in the examined taxa, the most abundant colors were blue followed by red and black. One possible explanation is that the dominance of blue-colored MPs in the surrounding environment increases the possibility of their ingestion by different insects. An alternative explanation is that the organisms prefer, or are attracted to this color, which may look like their food. This is supported by the fact that blue-green algae are considered food resources for collector-gatherer groups (Parker et al. 2022), which may lead to the accumulation of high loads of blue MPs within the present taxa. The high proportion of red microfibers in water mites (70%) in S2 suggests that these organisms could actively select red MPs like their prey. Previous studies reported that chironomid larvae (characterized by red color and filament shape) are one of the best prey for water mites (Vasquez et al. 2022). Ory et al. (2017) and Wicaksono et al. (2021b) confirmed that aquatic biota may feed on MPs that are like their prey in color and shape. The prevalence of blue, red, and black MPs in the present taxa was also observed in other freshwater invertebrates (Bour et al. 2018; Lenz et al. 2023; Parker et al. 2022).

The chemical composition of MPs must be well characterized to offer a better understanding of the negative impact of MPs on aquatic biota (Bagheri et al. 2020). Dominant polymer types observed in the aquatic fauna of specific regions reflected the abundance of these polymer types in the water of the same region (Munari et al. 2017). In the present study, PES was the most common polymer in the examined taxa.

Notonectid seemed to accumulate three polymer types (PES, PP, and PE) corresponding to the polymers in the wastewater. This suggests that notonectid could be best employed as MP qualitative bioindicators in freshwater.

Conclusion

As far as we know, this is the first study that provides data about the seasonal abundance, distribution, composition, and risk assessment of MPs in surface water, aquatic insects (Coroxide and Notonectidae), and water mites (Hydrachnidiae) in two of the most polluted wastewater sites in Sohag Governorate, Egypt. As well as the effect of water mites’ parasitism on the number of MPs ingested by aquatic insects. Our results suggest that the presence of water mites affects the number of MPs ingested by aquatic insects that live in the same site. Also, our results showed that water mites accumulated MPs. However, further research is needed to confirm our suggestion. Furthermore, the current study can serve as a warning sign for MP buildup in all compartments of the aquatic environment. We give a unique possibility for reaching conclusions on the relationship between pollution levels and interior microplastics using aquatic insects. This is critical and beneficial for policymakers in developing a more comprehensive strategy for microplastic mitigation, especially when certain species or ecosystems are vulnerable to microplastics. Additional research on the bioaccumulation of MPs and associated contaminants (such as heavy metals and persistent organic matter) within various higher trophic levels in various aquatic systems should be carried out to comprehend the implications and risks of MPs as well as the toxicity caused by the adsorption presence of these contaminants in freshwater biota.