Investigation of the Migration, Distribution, and Influence of Microplastic Leakage on Tuanjishan Island in Zhoushan Using Mike

Abstract

Amidst the rapid expansion of cage aquaculture and marine ranching in Zhoushan, safeguarding the marine environment has emerged as a matter of paramount concern. Microplastics (MPs), recognized as a burgeoning environmental hazard, have been shown to imperil marine organisms by inducing intestinal damage, disrupting feeding patterns, impeding nutrient absorption, and stunting growth when present in sufficiently high concentrations. Tuanjishan Island, once uninhabited but transformed into a landfill site in the 1980s, now faces burgeoning garbage production and gradual saturation of its landfill capacity owing to the city's continuous development. The intricate landfill milieu, characterized by high salinity, fluctuating temperatures and pH, gas emissions, physical pressure, and biodegradation, fosters the breakdown of plastic waste, leading to MP production. This transformation makes Tuanjishan Island a potential source of MP pollution that harbors environmental threats. Mismanagement leading to MP leakage can substantially impact marine aquaculture, impair the fishing economy, and significantly endanger the marine ecosystem. Therefore, predicting the aftermath of MP seepage is imperative. The evolution and sophistication of numerical simulation techniques have emerged as pivotal tools for unraveling and predicting the complex pathways and behaviors of marine pollutants. In light of these advancements, our study meticulously amalgamated the cutting-edge Mike 21 FM hydrodynamic module with a meticulously designed particle-tracking module to investigate the intricate migration trajectories of MP subsequent to their release from Tuanjishan Island. The delineation of the study area leveraged an unstructured mesh encompassing an expansive geographic range, with coordinates extending from 120°E to 124°E and 28.5°N to 33°N. Precise water depth metrics sourced from comprehensive nautical charts and topographical data rooted in the 1985 National Elevation Datum formed the foundational elements of this model. The intricate grid framework of our model, meticulously crafted using surface-water simulation software, comprised an assemblage of 62, 614 nodes and 118, 040 grids, ensuring a robust representation of the study area. The resolution of this grid was meticulously tailored, standing 400 m near the shoreline and 34, 500 m in the expansive outer sea. This meticulously formulated numerical model captured and replicated the complex three-dimensional hydrodynamic flow field across our study area, which was validated through rigorous verification exercises encompassing tide levels and velocities. A meticulously structured sequence of continuous experiments extending across the seasonal spectrum of spring, summer, autumn, and winter was orchestrated to examine the temporal distribution patterns of MPs subsequent to their inadvertent release from Tuanjishan Island. Our findings underscore the intricate migration pathways of MPs, which are intricately intertwined within the proximate surroundings of the Zhoushan Islands and are deeply influenced by the dynamic interplay of ever-changing seasonal ocean currents and the force of monsoons. This delineation highlighted distinctly nuanced shifts in distribution dynamics and migration characteristics across different seasons. Notable concentrations of MPs were detected within the confines of the deep-water channel and northern peripheries surrounding Zhoushan Island, coupled with discernible accumulations observed in various aquaculture locales. The concentration distribution of MPs within the northern region of Zhoushan Island fluctuated between 9%, 10%, 8%, and 6% during spring, summer, autumn, and winter, respectively. Meanwhile, within the deep-water channel bridging Zhoushan Island and Ningbo, MPs exhibited variations of 38%, 36%, 44%, and 42% during spring, summer, autumn, and winter, respectively, indicating inherent seasonally dependent fluctuations. In the hypothetical case of an MP leakage event, an intensified surveillance focus is imperative along the deep-water channel and adjacent coastal sectors in the northern expanse of Zhoushan Island. Alarmingly, predictive models suggest a potential migration of up to 10.11% of the leaked MPs toward aquaculture zones, representing an imminent threat to offshore aquaculture ventures, particularly in the regions situated northwest of Tuanjishan Island. Projections for the winter months anticipated an MP infiltration rate of nearly 2% into Xiangshan Bay, which would affect net cage aquaculture in the area. These intrusive MPs have demonstrated a disruptive capacity within marine ecosystems, disrupting the delicate marine food chain and posing a threat to its delicate balance. Furthermore, our study revealed a dominant concentration of MPs released in varied seasons within coastal precincts along 122.5°E, remaining conspicuously distant from the East China Sea and potentially attributable to the intricate oceanic dynamics. Therefore, while MPs released from Tuanjishan Island may not directly ingress into the Zhoushan fishing grounds, their inevitable impact on offshore net cage aquaculture is definitively substantiated. This comprehensive study represents a pivotal contribution to advancing our understanding of MP pollution within coastal areas and serves as a cornerstone for establishing a robust framework aimed at controlling and mitigating the adverse impacts of MP contamination. However, it is imperative to underscore the limitations of the current model, which regrettably overlooks the pivotal processes integral to comprehending the full scope of MP pollution. The absence of consideration for critical elements such as MP adsorption, biological pollution, sedimentation dynamics, and temporal alterations in the physical and chemical attributes of MPs signals an area demanding meticulous exploration in future research endeavors. Addressing these complexities will pave the way for more comprehensive and nuanced strategies for combating the multifaceted challenges posed by MPs in marine ecosystems.

Keywords

• microplastics
• tuanjishan island
• marine fisheries
• environmental risks
• numerial simulation