Coastal bays serve as reservoirs for microplastics from East China: insights
from a mass budget model based on sedimentary findings

Over the past two decades, research and concern regarding micro- plastics (MPs, < 5 mm) in the marine environment have continued to escalate (Auta et al., 2017; Alimi et al., 2018; Rochman, 2018a). MPs can be either primary (originating from plastic production processes), or secondary, resulting from the degradation and fragmentation of larger plastics (Alimi et al., 2018; Dissanayake et al., 2022). MPs in marine ecosystems mainly originate from waste streams, inadequate plastic waste disposal management, and at-sea activities via various pathways, including rivers, precipitation, stormwater runoff, and weather events (Ali et al., 2024; Arif et al., 2024). The global oceans contain anestimated range of 15 to 51 trillion MPs dispersed throughout (Uddin et al., 2021), with approximately 70 %–90 % of these particles tending to settle and accumulate in sediments across various physiographic domains, including remote deep-sea sediments, shallow coastal areas, and intertidal zones (Brandon et al., 2019; Chen et al., 2020; Martin et al., 2020; Yuan et al., 2022). Bottom sediments are increasingly recognized as a potential long-term sink for MPs. The particle size dis- tribution of MPs found in ediments (predominantly <1 mm) corresponds closely with the size fraction of buoyant MPs that exhibit apparent depletion in surface water. This provides critical evidence that vertical transport and subsequent sediment sequestration may explain a sub- stantial proportion of the missing MPs in the global budget (Thompsonet al., 2004; Kane and Clare, 2019; Martin et al., 2022).
Sedimentary MPs buried on the continental shelf are vital for the global MP cycle by connecting terrestrial, oceanic and atmospheric reservoirs (Zhang et al., 2022). However, caution should be exercised when comparing surface sediment MPs across distinct spatial domains. Surface MPs exhibit dynamic redistribution behaviors by vertical migration through particle settling (driven by MP properties and hy- drodynamic forcing), whereas sediment disturbances can induce parti- cle resuspension. These mechanisms create spatially heterogeneous MP distributions that complicate interregional comparisons under varying oceanographic processes. (Van Cauwenberghe et al., 2015; Yang et al., 2021).
In environments with weak hydrodynamic conditions, such as enclosed water bodies, shallow waters, estuaries, and bays, the water flow is slow, and the degree of water exchange with the outside is low. A large amount of MPs input from the land are likely to accumulate here, resulting in a relatively high concentration of MPs (Fischer et al., 2016; Wu et al., 2020). In deeper environments far from point sources, MPs are mainly slowly input through means such as atmospheric deposition and oceanic circulation, and the water exchange is relatively active, making it difficult for MPs to accumulate, and the concentration is often rela- tively low (Van Cauwenberghe et al., 2013; Zhang et al., 2020, 2020). In these two types of environments, the main sources and accumulation mechanisms of MPs are significantly different, and through comparison, we can better understand the influence of different factors on the dis- tribution of MPs. The incorporation of MPs into bottom sediments de- pends on multiple factors, including proximity to point sources, hydrodynamics, bathymetry, sediment grain size, the characteristics of MP themselves (polymer types, density, shape, size), and other biotic processes (Kowalski et al., 2016; Harris, 2020; Karkanorachaki et al., 2021; Xu et al., 2022; Goral et al., 2023; Stead and Bond, 2023). Nevertheless, the timing and mechanisms of MP accumulation in marine sediments over recent decades remain insufficiently understood, creating a knowledge gap for assessing, predicting, and controlling plastic pollution (Chen et al., 2020; Chen et al., 2024).
Several previous studies have estimated the mass budget of MPs in the environment, primarily relying on modeling inputs (Zhang et al., 2022) and field measurements (Martin et al., 2017; Ryan et al., 2020; Kim et al., 2023). Studies in inland cities, watersheds, and lakes have aimed to quantify different sources of MP contamination by establishing transmission networks within a mass balance system or estimating in- puts based on measured concentrations across various pathways (Boucher et al., 2019; Chen et al., 2022). In comparison to inland en- vironments, offshore estuary-bay systems are more prone to forming MP hotspots due to natural accumulation, hydrodynamics, and increasing anthropogenic influences (Yonkos et al., 2014; Hope et al., 2021). However, investigations that solely evaluate individual compartments or focus on unidirectional outputs may overlook interdependencies among compartments and superimposed influences from variables. This can result in underestimations or probabilistic errors in quantifying the continuity and variability of MP masses during transmission processes, leading to uncertainties regarding quantities, behaviors, and fates of MPs within bay systems (Chen et al., 2022). Therefore, tracking the mass budget of MPs while identifying contributions from different pathways and exploring the structure and flow patterns associated with MP transmission is crucial for a comprehensive understanding of MP dy- namics in marine ecosystems.
In this study, we investigated the occurrence and distribution of MPs in the sediments of three typical bays (Hangzhou Bay, Sanmen Bay, Wenzhou Bay) in Zhejiang Greater Bay Area, China. Our research aims to determine the respective contributions of different sources and pathways by establishing a MP mass budget model. By providing initial estimates of annual inputs, outputs, and inventories of MPs, our findings offer novel insights into the fate of MPs. This is essential for directing management efforts towards reducing MP budgets in the future and for proposing targeted mitigation strategies to address risks associated with key transmission nodes.