Impact of Microplastics on Aquatic Ecosystems

1. Introduction

Microplastics, referring to plastic particles < 5 mm in diameter, have become ubiquitous contaminants in ecosystems worldwide [1]. Due to their ubiquitous presence, persistence, complexity, and ever-increasing accumulation, microplastics have become a significant environmental issue that poses a growing threat to both planetary sustainability and human health [1,2]. Understanding the behavior, fate, and consequences of microplastics in various environments has become crucial for mitigating their impacts [1].

Aquatic ecosystems, which are home to a rich diversity of species and provide essential ecosystem services, face particular threats from microplastic pollution [3,4]. These ecosystems act as both sinks and sources of microplastics, with particles entering through various pathways such as runoff, wastewater discharge, and atmospheric deposition [5]. Microplastics, along with the chemicals and microorganisms they carry, have the potential to disrupt water quality, alter biogeochemical cycles, and affect the health of aquatic organisms [6,7]. Upon ingestion by aquatic organisms, microplastics elicit toxicological responses and exhibit a propensity for trophic-level bioaccumulation [8]. The presence of microplastics in aquatic environments can also jeopardize food safety and contribute to broader environmental concerns such as global climate change [9,10]. As microplastics continue to accumulate in water bodies, understanding their impacts on aquatic ecosystems is essential for the development of effective management and mitigation strategies.

Therefore, we launched this Special Issue, titled “Impact of Microplastics on Aquatic Ecosystems”, with the aim of bringing attention to the pressing need for research on the complex interactions between microplastics and aquatic environments, shedding light on the urgent need for interdisciplinary solutions to this global challenge. We finally included two research articles and three review articles in this Special Issue. In the next section, we briefly describe these contributions, which cover topics from the distribution characteristics and sources of microplastics to their impacts on aquatic algae, macroinvertebrates, food webs, and human health.

2. Main Contributions of This Special Issue

Comprehending the distribution, sources, and ultimate fate of microplastics within inland wetland ecosystems is essential for devising scientifically sound mitigation strategies. A heterogeneous suite of inland wetland classes—riverine, constructed, and lacustrine—was surveyed to quantify microplastic contamination signatures [contribution 1]. The results indicated that the abundance of microplastics exhibited significant variation across different types of inland wetlands, with values ranging from 532 to 4309 items/kg. Within the study area, the predominant color, shape, and polymer type of microplastics were transparent, fibrous, and polyethylene terephthalate (PET), respectively. The lake may serve as one of the potential sinks for microplastic pollution within the inland wetland ecosystem, attributed to its relatively closed environmental characteristics. Analysis of microplastic characteristics indicated that aquaculture, agricultural activities, sewage discharge, and household waste are the primary sources [contribution 1]. This study characterized microplastic pollution in a unique inland wetland ecosystem comprising riverine, constructed, and lacustrine wetlands. These findings clarified the occurrence patterns and provenance of microplastics in inland wetlands, offering critical data to inform effective pollution mitigation and ecosystem restoration strategies.

Microplastic pollution is recognized as a significant threat to both freshwater and marine organisms, with potential impacts likely varying among different feeding guilds. Microplastics were detected in benthic macroinvertebrates, namely filter feeders, carnivores, and grazers, from two adjacent yet hydrologically distinct rivers within South Africa’s Kruger National Park [contribution 2]. The study detected microplastics on and within macroinvertebrates from both rivers, with internalized particles localized primarily in digestive tracts due to ingestion. Among trophic guilds, filter feeders accumulated the greatest microplastic loads, predators displayed intermediate burdens, and grazers showed the lowest. Microplastic morphotypes were present in macroinvertebrates of both rivers in approximately equal proportions, indicating that biological selection is likely based on morphotype rather than microplastic color or size [contribution 2]. Microplastics in freshwater ecosystems within nature conservation areas demand heightened attention, given the high biodiversity in these regions. The presence of microplastics in macroinvertebrates highlights the urgent need for stricter policies aimed at reducing single-use plastics and other sources of microplastic pollution [contribution 2].

Aquatic organisms face substantial risks from microplastic contamination. However, the influencing factors and mechanisms underlying the toxic effects of microplastics remain unclear. Therefore, this review examined the toxic effects of microplastics on aquatic animals and algae in freshwater environments, as well as their potential impacts on human health [contribution 3]. The extensive review indicates that the impact of microplastics on algal growth is contingent upon both the type of microplastics and the species identity of the algae involved. Additionally, a significant correlation existed between the size of microplastics and the magnitude of their toxic effects. Microplastics can influence the photosynthetic processes of freshwater algae and induce oxidative stress. In some cases, they can even interfere with the transcription of intracellular genetic information. The toxic effects of microplastics on aquatic animals are multifaceted, including intestinal blockage, disruption of liver metabolism, impairment of digestive function, induction of stress responses, reduction in predatory performance, adverse impacts on growth and reproduction, and in severe cases, mortality. Moreover, microplastics may ultimately enter the human body via bioaccumulation in aquatic plants and animals, thereby causing adverse health effects [contribution 3].

Beyond organism-level bioaccumulation, microplastics undergo trophic transfer, facilitating biomagnification through food chains. The transfer of microplastics through food chains and the associated risks have been extensively reviewed [contribution 4]. This review synthesized microplastic characteristics and ecological impacts in freshwater ecosystems, emphasizing bioaccumulation across trophic levels (plankton to fish) and associated human health risks via the consumption of contaminated fish. The ingestion of microplastics by aquatic organisms can induce physical damage, such as entanglement, as well as chemical toxicity, including oxidative stress and the accumulation of hazardous substances. Trophic transfer could amplify microplastic burdens in apex consumers, posing direct human health risks via dietary exposure. The review further outlined prioritized interventions and regulatory approaches to reduce microplastic contamination [contribution 4].

Nanoplastics (NPLs), which fall under the broader category of microplastics, are defined as plastic particles with sizes less than 100 nm. Nanoplastic toxicity amplification stems from two mechanisms: (1) size-dependent surface-area-to-volume ratio enhancement and (2) surface chemistry modifications, both augmenting reactogenicity beyond conventional microplastics. The reduction of NPLs to nanometric dimensions can increase their likelihood of ingestion by lower trophic level biota. Additionally, the inherent mobility of NPLs facilitates their widespread distribution throughout the environment. Given their significant environmental implications, NPLs demand heightened research priority. This review quantifies NPL risks across freshwater and marine systems by the following: (1) characterizing exposure pathways, (2) assessing ecological impacts, (3) conducting preliminary risk screening, (4) comparing risk thresholds, and (5) prioritizing high-concern NPL subtypes [contribution 5]. This study further identified methodological challenges in standardizing risk assessments due to heterogeneous experimental parameters, environmental variables, and insufficient data. A multi-tiered risk assessment matrix was subsequently established to prioritize polymers based on their ecological hazards to aquatic ecosystems [contribution 5].

3. Conclusions

This Special Issue includes five papers that, through multidimensional perspectives and innovative research methodologies, comprehensively examine the distribution characteristics, migration patterns, ecotoxicological effects, and potential threats to human health of microplastics across diverse aquatic environments. It is important to reduce microplastic emissions, monitor their presence and dynamics in freshwater, and reveal their possible effects. Further research is warranted to assess the scope of microplastic pollution in freshwater systems and evaluate its ecological and public health implications. Moving forward, we anticipate increased multidisciplinary, cross-sectoral, and interregional collaborative research to collectively pool intellectual and technical resources for tackling microplastic pollution, preserving the global ecosystem, and safeguarding human health.