Effects of Plastic Pollution on Freshwater Ecosystems and Their Biota 2022 Simone

Abstract:

In the relatively short time since plastics were first manufactured, they have become a principal pollutant in aqueous ecosystems around the globe. This paper seeks to address the growing issue of plastic pollution in freshwater ecosystems. Through a global analysis, plastic pollution has been shown to increase in magnitude over time through both micro- and macroplastics. Plastic pollution of lakes exhibits variation based on each lake’s unique environment. Through a study conducted on the Amazon River, plastic pollution of rivers and streams was found to be included in food web transference. It is of no surprise, then, that the plastic pollution of lakes and rivers/streams has proved to be a non-localized, worldwide problem. Even so, freshwater ecosystem interactions with both aquatic and terrestrial biota — including humans — can have both negative and positive effects, with the former holding the majority in various cited studies. With plastic now being an integral component in these systems, practical suggestions on managing plastics in freshwater ecosystems can provide a real solution to the results found throughout this paper.

Introduction:

From your car to your packed lunch container, plastics are inescapable. Though there have been some major efforts to reduce plastic waste, plastics are still managing to find their way into many different wildlife ecosystems as trash. While much of the scientific and environmentalist focus on plastic pollution is usually concentrated in marine ecosystems, freshwater plastic pollution is a topic needing just as much (if not more) attention, as both rivers and streams are vital components in carrying mismanaged terrestrial-based plastic waste into the ocean (Kasavan et al. 2021). In fact, an estimated 1.15 to 2.41 million tons of plastic waste is transported from inland rivers to marine systems annually (Stovall and Bratton 2022). This type of pollution is nothing new: plastic pollution has plagued the world since small-scale plastic manufacturing began in the 1950s (Kasavan et al. 2021). Though plastics are highly regarded for their convenience, durability, and versatility, much of what is produced is solely single-use items and packaging products that ultimately becomes unusable waste (Stovall and Bratton

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2022). Further, up to 99% of plastics are created from nonrenewable resources, such as coal, oil, and natural gas (Kasavan et al. 2021). With polymer production scaling more than 300 million tons produced each year, plastic occurrence is certainly of great importance (Azevedo-Santos et al. 2021).

However, in order to determine how to fix plastic pollution in freshwater ecosystems, there must first be an understanding of plastic. Thus, the broad term of “plastic pollution” must first be dissected and organized into usable information. A categorization of plastics is necessary, and scientists have chosen to do this by dividing them into four different sizes: nanoplastics (<100 nm), microplastics (<5 mm), mesoplastics (5–25 mm), and macroplastics (>25 mm) (Kasavan et al. 2021). This paper will focus specifically on microplastics and macroplastics. Microplastic examples include plastic fibers, pellets, and fragments, whereas macroplastics include plastic bags, bottles, and fishing nets.

With an accurate understanding of the categorization of microplastics and macroplastics, it is now necessary to determine how extensive this plastic pollution is. Though most studies on plastic pollution in freshwater ecosystems are conducted in Western countries, it is certainly a global problem requiring broader investigations (Kasavan et al. 2021). Regarding the worldwide scale, the U.S., Japan, and other European countries are “moderately good” at managing plastic waste, even though they produce significant amounts of such waste (Kasavan et al. 2021). While many plastic materials can be either reused or recycled, most materials must be discarded after only one use (Azevedo-Santos et al. 2021). These plastics can either be disposed of and carried by rainwater to water bodies or directly discarded into freshwater environments (Azevedo-Santos et al. 2021). This leads to little or no control over production or disposal chains, thereby leading to discarded plastic polymers inevitably reaching aquatic ecosystems (Azevedo-Santos et al. 2021). Such addition of plastics — whether micro or macro in size — then leads to an enormous compromise in water quality creating a far-reaching ecological imbalance (Qadri et al. 2020).

It is indisputable, then, that plastic is one of the fastest growing sources of pollution (Xu et al. 2020).

Furthermore, plastic pollution is not only a problem that is spreading to all parts of the Earth, but it is also a problem that is getting worse over time. To increase the complexity even more, micro and macroplastic pollution is not only affecting aqueous ecosystems — it has also been shown to affect terrestrial interactions within these aqueous ecosystems. Birds, fish, invertebrates, reptiles, and mammals are all affected by freshwater plastic encounters (Blettler and Mitchell 2021). The most common plastic encounters in the terrestrial realm are through biota ingestion, entanglement, and even usage (such as through nesting) (Blettler and Mitchell 2021). Biota ingestion of plastic can lead to intestinal blockage, internal injury, and suffocation, whereas entanglement can reduce mobility and even cause strangulation (Blettler and Mitchell 2021). Many studies have shown that birds are the most affected terrestrial animal by macroplastic pollution in freshwater ecosystems (or at least the animal that is most reported on).

While the majority of recorded plastic encounters throughout all animal species are negative, there are some instances of “beneficial” plastic encounters. Some recordings of macroplastics being used by freshwater animals as places of refuge from predators and even places of settlement have been observed in freshwater ecosystems (Blettler and Mitchell 2021). However, even with these beneficial observations, the overall consequences of plastics in freshwater ecosystems remain poorly known, with few studies addressing the issue in freshwater ecosystems as opposed to the many focused on marine ecosystems (Azevedo-Santos et al. 2021). Thus, this paper seeks to find answers to the following questions: How is plastic pollution affecting freshwater ecosystems, and are these trends seen around the world or only in one spot/area?

Origins:

Freshwater ecosystems are the main destination of many different types of pollutants released in a watershed, as freshwater aqueous environments are naturally located in valleys and lower elevation terrains (Azevedo-Santos et al. 2021). Whether micro or macro in size, each type of plastic pollution is common in both rural and urban areas. Local human use and runoff are two important factors contributing to the amount of plastic pollution seen in watersheds (Stovall and Bratton 2022). Once plastic reaches a waterbody, it can then be trapped by instream structures, thus rendering it unable to leave the freshwater system (Azevedo-Santos et al. 2021). Once plastics are trapped, physical and/or chemical weathering can begin the fragmentation process of the polymers, thereby increasing the number of particles in the freshwater system as the plastic continually gets smaller and smaller (Azevedo-Santos et al. 2021). Based on this process, plastic pollution is known to have three origins: microplastic fragmentations, wear and tear of plastic polymer products, and intentional origins (i.e., exfoliants, industrial abrasions, etc.) (Dusaucy et al. 2021).

Lakes:

Out of these three varying origins, microplastic (MP) fragmentation is considered to be one of the main sources of microplastics in lakes (Dusaucy et al., 2021). Microplastics usually come from macroplastics through fragmentation. Environmental conditions, such as UV irradiation, pH, or temperature, play crucial roles in plastic fragmentation (Dusaucy et al. 2021). However, the intensity of these environmental conditions varies across lakes, as differences in factors such as altitude and nutrient output shift fragmentation effectiveness. This is also true for the relative contribution of each source of MPs to a certain lake, as each catchment’s unique features are the determinant when it comes to nature and intensity (Dusaucy et al. 2021).

As of 2021, only 98 lakes have been studied worldwide with regards to microplastic (MP) pollution — 20 of those lakes being rural and 78 being urban (Dusaucy et al. 2021). Out of those 98 lakes, the most commonly identified polymer forms were fibers and fragments (Dusaucy et al. 2021). Nevertheless, within these studied boundaries, it is naturally assumed that urban lakes would have higher MP pollution than rural lakes. However, this is not the case: in some instances, rural lakes actually have a higher MP pollution than urban lakes (Figure 1). This is made possible by the variation seen in each catchment’s unique features, as discussed before. Each lake is thus affected by its surroundings.

Therefore, in order to begin pollution repair, the source(s) of MPs must first be identified in each respective lake’s area. Factors such as wastewater treatment plants, runoff, and dumping are major sources of plastic pollution that affect lakes differently (Hou et al. 2021). Even atmospheric fallout is a source of microplastic pollution (Figure 1). Adding to the list of varying contributors, MP pollution is also shown to intensify from climatic conditions (Figure 2). Increasing variability within environmental transport pathways, MP suspension, and retention in lakes can further concentrate and increase MP pollution (Dusaucy et al., 2021). Whether rural or urban, lakes carry a great economic, recreational, and ecosystem value, and yet, each type is plagued by MP contamination. Even with the 98 currently studied lakes, this research is only scratching the surface in terms of quantifying MP lake pollution, as there is a lack of integration when considering the whole ecosystem — a crucial piece of the puzzle (Dusaucy et al., 2021).

Rivers/Streams:

We will now focus on an example seen in the pollution of the lower Amazon River. The Amazon (including its related freshwater ecosystems) receives more than 150,000 tons of synthetic polymers each year (Azevedo-Santos et al. 2021) (Figure 3). Herbivorous, carnivorous, and omnivorous species have all been documented as being affected by this freshwater plastic pollution. In fact, all three types of trophic levels in the lower Amazon River were recorded to ingest fragmented plastics as a food source (Figure 4). Aquatic algae and plants have also been shown to absorb tiny plastic particles, increasing the risk of plastic availability for secondary consumers (Azevedo-Santos et al. 2021). Many field studies of this interaction are poorly examined, but there are some laboratory experiments available that have shown negative impacts (Azevedo-Santos et al. 2021).

Another way of plastic transference within river and stream food webs is by filtering zooplankton consumption. Zooplankton have been recorded to ingest and retain MPs, which are then able to be transferred to higher trophic levels when the zooplankton are consumed by larval fish and other planktivores, steadily increasing in trophic position (Andrade et al. 2019). Even variation in colors of plastics were shown to attract different species of fish, as some plastic debris has a similar appearance to their natural food items, thus serving as an attractant to those fish (Andrade et al. 2019). An example of this occurrence was seen in planktivorous juvenile fish who preferentially ingested black plastic fragments, which are presumed to have similar associations with their natural prey. Interestingly, the three different trophic guilds (herbivorous, carnivorous, and omnivorous) were not shown to be significantly different from each other in terms of plastic frequency and/or ingestion magnitude.

Even with the findings in the Amazon and its related freshwater ecosystems, more studies are needed to investigate the effects of smaller plastic particles within other parts of freshwater biota, as these particles can be transferred from the digestive tract to other organs and muscles (Andrade et al. 2019). Increasing studies focused on different tributaries in unrelated parts of the world are also needed for a more holistic view on the pervasiveness of plastics in freshwater river/stream ecosystems. Furthermore, the focus of plastics found in aquatic fauna needs to be broadened in terms of anatomy. While it seems like the digestive tract is the best point of reference when investigating macroplastic and microplastic pollution, this is certainly not the only way since plastics can travel to other parts of the body. Though plastic pollution has already been shown to impact river/stream aquatic fauna, there is another unexplored potential: if these effects are impacting human health and food security (Andrade et al. 2019). This potential is yet to be seriously explored.

Effects on Biota:

In many studies conducted in the field and in the lab, plastic pollution has been shown to affect freshwater animals in a predominantly negative way with regards to ingestion and encounter frequency. In the first study conducted on MPs in freshwater ecosystems, it was found that chronic exposure to fragmented fibers (even in a low concentration) significantly decreased growth and reproduction in the freshwater amphipod studied (Chae and An 2017). Another study conducted a few years later found that ingestion of MPs actually dulled olfactory function in freshwater fish, thus leading to increased mortality from predation (Chae and An 2017). In fact, freshwater fish are the aquatic group with the highest amount of recorded plastic ingestion (Azevedo-Santos et al. 2021). Fish have been shown to ingest plastic and travel within their aquatic habitat, thereby mobilizing and even transferring plastic materials across food webs, including terrestrial ones (Azevedo-Santos et al. 2021).

One study documented the occurrences of microplastics in the Nile River in Egypt. Over 75% of fish sampled in the Nile River contained MPs in their digestive tract (Khan 2021). The researchers then compared their findings to other similar studies from both marine and freshwater environments. The results showed that a 75%+ level of MP ingestion was rarely found throughout the literature, and the fish sampled from the Nile

River (tilapia and catfish) were potentially in the most danger of consuming MPs worldwide (Khan, 2021). Like many other studies, fragments were the most abundant type of microplastic found in the fishes’ digestive tract. The MPs found had various polymer originations, thus making it difficult to trace a specific plastic product from which they were all fragmented from (Khan, 2021). However, another cross study done on the Mississippi River in the United States noted how temporal and seasonal events, such as flooding, can change MP concentration and abundance in sampling (Khan 2021). Thus, the investigation of MP pollution in freshwater systems being impacted by changing conditions is an area of active research.

Regarding MP’s effects on fish, there have been recorded instances of inflammatory responses and alterations of fish intestinal health through their microbiome resulting from MP exposure (Khan 2021). In one study, with just 10 days of MP exposure, zebrafish in a lab setting were both physiologically and genomically verified to have markedly increased levels of oxidative stress (Khan 2021). Microbiome imbalance through increased oxidative stress by the MP exposure was also shown to possibly correlate with more susceptibility to disease (Khan 2021). This discovery of MP exposure disrupting fish microbiomes is certain to have important implications for humans in the future, as MP contamination is present in a variety of foodstuffs (Khan 2021). However, yet again, this area of research is still needing much more investigation.

Another underestimated freshwater interaction problem is avian usage of synthetic polymers for nesting, as this interaction makes freshwater birds and their young even more susceptible to contamination, ingestion, and entanglement (Azevedo-Santos et al. 2021). Even at the cellular level, microplastic exposure induces multiple adverse effects and stress responses in freshwater organisms (Xu et al. 2020). Examples of this include inflammatory responses in freshwater crabs and abnormalities in intestinal epithelial cell structure in freshwater shrimp (Xu et al. 2020). There have even been documented changes seen in gene expression when exposed to microplastics (Xu et al. 2020). Each ensuing study has chosen to focus on different species when exposed to different types of plastics. Even so, questions about the long-term effects of these small plastics on aquatic organisms have yet to be answered.

Another study conducted in 2021 focused on an annelid named Enchytraeus crypticus. The annelid was subjected to soil with green bottle cap microplastics mixed in. In each test, the annelids chose to be in the soil without MPs, or in an area with lower MP concentration (Khan 2021). Contact with the MPs was shown to increase oxidative stress even when the annelids did not ingest any plastics (Khan 2021). The researchers explained that the MP avoidance behavior observed by the annelids was due to the fact that the microplastics inherently changed the properties of the soil, and that the worms could sense it (Khan 2021). Yet again, further studies on freshwater worms are needed to link intracellular responses to behavioral and/or physiological changes, as shown in this terrestrial examination.

Impact on Humans & Plastic Through Time:

Concerning the freshwater plastic pollution effects on humans, the results are also quite devastating. Interestingly enough, the World Health Organization (WHO) has stated that nearly 80% of all human disease is waterborne (Qadri et al. 2020). Even further, a portion of the water contamination problem is solely attributed to plastic pollution. With recent studies, it appears that nearly all waterborne diseases represented by the WHO are preventable, whether by vaccine or industrial/personal choice (Qadri et al. 2020).

Regarding the effects of plastic through time, in a study conducted by Hou et al. (2021) microplastics in the digestive tissue of museum specimens were measured from the years 1900- 2017 in order to quantify the microplastic concentrations seen in freshwater fish. Prior to 1950, there were no MPs detected in any of the museum specimens. However, after 1950, it was discovered that MP concentrations in digestive tissue drastically increased (Hou et al. 2021). Interestingly enough, all detected polymers were fibrous in nature (Hou et al. 2021). Such ingestion of MPs is known to cause hepatic stress and loss of digestive function in fish (Hou et al. 2021). It is of no doubt then that plastic pollution throughout time can be regarded as “…one of the most serious problems worldwide.” (Chae and An 2017).

Conclusions:

In closing, throughout the many scientific studies discussed, both macroplastic and microplastic pollution is shown to affect both lakes and rivers/streams in a mostly adverse sense through all types of bio-interactions. Further, this type of pollution is not localized in just one part of the world — it has spread to encase the entire globe. However, there are effective ways for humans to combat both macroplastic and microplastic pollution in freshwater ecosystems. Some courses of action include preventing plastic buildup by banning single-use plastic items, making recycling bins more available for public use, funding awareness campaigns, imposing financial penalties for non-reusable packaging, and so much more (Dusaucy et al. 2021). Water is an indispensable resource for all living organisms, especially freshwater. Proper action is necessary to protect freshwater ecosystems and their biota — whether terrestrial or aquatic. While plastics in all their forms may never fully go away, with the appropriate (and consequently prompt) action, plastic pollution in freshwater ecosystems can be managed.

Khan, F. R. 2021. Prevalence, fate and effects of plastic in freshwater environments. MDPI, Basel, Switzerland.

Qadri, H., R. A. Bhat, M. A. Mehmood, and G. H. Dar. 2020. Freshwater pollution: effects on aquatic life and human health. Pages 15–26 in R. Qadri and M. A. Faiq, editors. Freshwater pollution dynamics and remediation. Springer Singapore Pte., Singapore, Republic of Singapore.

Stovall, J. K., and S. P. Bratton. 2022. Microplastic pollution in surface waters of urban watersheds in Central Texas, United States: a comparison of sites with and without treated wastewater effluent. Frontiers in Analytical Science 2:1-10.

Xu, S., J. Ma, R. Ji, K. Pan, and A.-J. Miao. 2020. Microplastics in aquatic environments: Occurrence, accumulation, and biological effects. Science of The Total Environment 703:1–14.

Literature Cited

Andrade, M. C., K. O. Winemiller, P. S. Barbosa, A. Fortunati, D. Chelazzi, A. Cincinelli, and T. Giarrizzo. 2019. First account of plastic pollution impacting freshwater fishes in the Amazon: Ingestion of plastic debris by piranhas and other serrasalmids with diverse feeding habits. Environmental Pollution 244:766–773.

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Figures

FIGURE 1: Graph from Dusaucy et. al 2021 showing the amount of atmospheric fallout (wet and dry) of microplastics between different geographical localizations including France, China, Germany, and England. London, England is shown to have the highest mean concentration of atmospheric fallout whereas Paris, France, (Suburban) is shown to have the lowest mean concentration of atmospheric fallout.

FIGURE 2: Graph from Hou et al. 2021 showing microplastic concentration as the number of particles per individual in four different fish including the Largemouth bass, Sand shiner, Channel catfish, and Round goby, through 1900-2018. There is no occurrence of microplastic concentration in any species prior to 1950. Post-1950, microplastic concentration occurs with an increasing visual trend.

FIGURE 3: Photographic examples from Azevedo-Santos et al. 2021 displaying plastic pollution such as buildup of plastic bottles, plastic fishing nets, and micro, meso, and macro plastic fragments, in various South American freshwater ecosystems including (but not limited to) the Amazon basin, Rocha River, and Paraná River.

FIGURE 4: Figure from Andrade et al. 2019 showing a summarized trophic network with plastic intake pathways in the lower Xingu Basin. Plastic sources such as single-use plastic bags and drinking bottles are shown to be fragmented and then able to enter the trophic network. While the trophic network’s focus is mainly aquatic, a terrestrial connection is also shown by an avian species ingesting a plastic-contaminated carnivorous serrasalmid.