International Journal of Marine Science, 2025, Vol.15, No.5, 245-254 http://www.aquapublisher.com/index.php/ijms 248 "hidden" in the deep-sea environment (Tsuchiya et al., 2023). At the same time, river flow into the sea and coastal hydrological processes (such as upstream and vortex) will also shape the regional microplastic distribution pattern. 3.2 Cumulative characteristics of sediments and marine organisms Microplastics in the ocean not only float in water, but a large number of particles eventually deposit on the seabed or are ingested by biological organisms and enriched in the body. The study found that the surface microplastics on the ocean surface only account for a small part of the total, and a large amount of microplastics enter the marine sediment bank through settlement. Microplastic abundance in nearshore beaches and seabed sediments is often higher than adjacent water concentrations because microplastic density or particle size increases more easily and accumulates in a relatively stable deposition environment (Cau et al., 2024). In addition, the seabed topography and ocean currents also affect the sedimentary distribution. There may be less microplastics in strong current channels, while slow flow rates are prone to becoming microplastic enrichment centers. In addition to the inanimate environment, microplastics are also fed by a variety of marine organisms or transmitted through food networks and entered the organism. Filter or feeding methods such as zooplankton and shellfish make them a high-risk group for microplastic intake. The cumulative characteristics of microplastics in organisms vary with biological species and feeding properties. Filter-feeding invertebrates often accumulate fibers and debris, while particles and film-based plastics are common in the gastrointestinal tract of fish (Parolini et al., 2023). Research also shows that there is a biological amplification phenomenon in marine food networks. Microplastics intake from low-trophic biological organisms can be transmitted to predators, and various types of plastic fragments are detected in top predators such as seabirds and sea beasts. These findings show that microplastics not only settle as environmental particles on the seabed, but can also move continuously along the food chain, affecting organisms of different ecological niches. 4 Effects of Microplastics on Marine Ecosystems 4.1 The hazards of microplastics to plankton and low-trophic organisms Low-trophic organisms such as phytoplankton and zooplankton are the basis of marine food webs, but are susceptible to invasion and harm from environmental microplastics. Phytoplankton may be hindered from growth due to microplastic particles blocking light and adsorbing nutrients. Studies have found that exposure of polystyrene microbeads with higher concentrations can cause a decrease in photosynthesis rate and changes in community structure in some algae (Yu et al., 2020). Zooplankton, such as copepods, krill, etc., will consume microplastics similar to the size of food particles when filtered and fed. Some copepods are stimulated and blocked after ingesting microplastics, and no longer eat real algae, resulting in a decrease in the survival rate of individuals under high exposure. In addition, microplastics stay short but pass through high frequency in zooplankton, which can interfere with their intestinal microbiota and metabolic functions. For low-trophic benthic organisms such as filterfeeding shellfish, the harm of microplastics cannot be ignored. In field investigations, considerable amounts of microplastics were detected in mussels, oysters and other bodies. Some individuals had more than hundreds of them. These plastic particles could trigger stress responses and inflammation in the body. For example, after exposing Mediterranean mussels to high concentrations of polystyrene particles, their digestive gland tissues showed inflammation and increased cell damage markers, and their filter feeding rate and energy reserves decreased significantly (Trestrail et al., 2021). Microplastics can also accumulate in shellfish gill tissue, causing impaired respiratory function. 4.2 Synergistic effects of microplastics and heavy metals/organic pollutants In addition to causing physical harm on its own, microplastics often have synergistic effects with other pollutants in the environment, aggravating the toxic effects on marine organisms. Due to its large specific surface area and strong hydrophobicity, plastic particles are prone to adsorbing hydrophobic organic pollutants and heavy metal ions in seawater. Microplastics can thus act as a "carrier" for these pollutants, concentrating them and carrying them into organisms (Wang et al., 2022). Studies have shown that when marine organisms ingest microplastics
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