4.1 Source and distribution of MPs
There may be a close relationship between human activities along the Yangtze River and the different types of MP particles found in our sampling (Wang et al., 2017; Yang et al., 2021). Different factors may contribute to the spatial differences observed in MP contaminants along the river. We frequently observed discarded foam floats and containers, plastic drums and bottles, fishing nets, woven plastic bags and food wrappers on the banks of the Yangtze River and its main tributaries and on the lakeshores during field sampling, all of which are potential sources of MP particles.
Flake particles predominated in the upstream Yangtze River samples. These may have originated from gradual damage to discarded plastic woven bags. Polystyrene foam particles were found mainly in the Yangtze River bank samples; these originated primarily from aquatic product packing boxes and discarded foam floats, as reported by Free et al. (2014) and Lee et al. (2015). Fiber particles were widely distributed in the Yangtze River bank samples, mainly due to the large use of wind fences in the economically less developed areas of the upper reaches of the Yangtze River, and to the fishery that has developed in the lower reaches of the river. Film particles originated mainly from the degradation of discarded plastic bags and agricultural mulch (Sintim et al., 2017). The main sources of fragment particles were discarded objects that resulted from human lifestyles and the industrial production of hard plastic products; over the long term, hard plastic products break and produce fragments (e.g., broken plastic tubs, buckets and bottles). Film and fragment particles were widely present in samples throughout the survey area, with a relatively high distribution in more populated areas that further illustrates the close relationship between MPs and human activities. Pellet particles were found in the middle and lower reaches of the Yangtze River bank samples, which we mainly attributed to improperly discarded industrially manufactured primary materials. More pellet particles were found in lakeshore sediments because a lake is an important destination for pellet particle migration in the drainage basin. Sponges are widely used in industry and commerce for shock absorption, packaging, and heat insulation, and they increase pollution by PEU MPs (Zhou et al., 2018).
4.2 Microplastic contamination along the Yangtze River
MP particle concentrations in different reaches of the Yangtze River were dissimilar because they were influenced by different environmental conditions and had different sources (Feng et al., 2021). Previous studies have suggested that MP concentrations in aquatic environments were mainly driven by population density or economic development (Lahens et al., 2018; Peng et al., 2018; Eriksen et al., 2013; Yonkos et al., 2014; Liu et al., 2019). We found that the abundance of MP particles along the Yangtze River banks was relatively high in major urban watersheds, but there was no significant correlation between abundance and local population size or economic development indicators. We conclude that MP particles in shoal sediments have a complex of sources, including local discarding of plastic products and continuous input from upstream rivers, and that local hydrological conditions maybe have an important effect on plastic deposition. The abundance of MPs in riverbank sediments is also a consequence of long term accumulation. Over time, different types of plastic products and MPs are weathered by various factors, a process that continuously increases MP particle abundance (Song et al., 2017).
In contrast to previous studies of MP accumulation, we found that the abundance of MPs in the sediments of the main Yangtze River basin is at an intermediate level (Ding et al., 2019; Lin et al., 2018; Klein et al., 2015). The Yangtze River has long been considered to be the largest source of MPs entering the ocean, but previous studies ignored the retention of MPs in shoal sediments, leading to overestimates of the flux of MPs from rivers into the ocean (Siegfried et al., 2017).
We found only particles of a single shape category and a low abundance of MP particles in the upper reaches of the Yangtze River. We attribute this to relatively little human disturbance in the area, thus reducing the number of possible sources and therefore the abundance of MP particles. In the middle and lower reaches of the river, the abundance of MP particles was greater. Greatest abundance was found around Wuhan, and the particles were mainly flakes. This may be related to local long term flood defenses and remediation. There were more foam and fiber particles in the lower reaches due to the developed fishery and transportation of aquatic products. The significant positive correlation between the abundance of foam particles and the total abundance of MP particles implies that PS plastic products are the main sources of MPs in the sediment samples of the main sampling areas of the Yangtze River. MP particle abundance was relatively high in the Yangtze estuary, which indicates that urban estuarine river input was an important source of MPs entering the ocean (Rech et al., 2014; Zhao et al., 2015). Tides may also be an important factor in the accumulation of MPs in estuaries (Xiong et al., 2015).
4.3 Microplastics in the tributaries of the upper Yangtze River
In the tributaries of the upper reaches of the Yangtze River, where human activities are less relevant, the abundance of MP particles is nonetheless relatively high. This is related to the types of polymer; polypropylene (PP) and polystyrene (PS) fragment easily in natural conditions (Song et al., 2017). The significant positive correlations between the abundance of flake particles and the total abundance of MP particles in tributary riverbank samples indicate that flake particles are predominant.
In the upper reaches of the tributaries, which are undeveloped, woven plastic bags and netted wind fencing discarded during the construction and maintenance of water conservancy and hydropower facilities produced a high abundance of MP particles. Particle shapes were mainly flake and fiber, and flake particles were predominant.
Agriculture and other human activity is more frequent in the lower reaches of each tributary, and both overall abundance and the number of shape categories in our samples increased. We found that flake particles decreased significantly in the lower reaches of each tributary. This observation suggests that a dam on the upper reaches of a tributary may have trapped considerable amounts of MPs that originated upstream of the dam (Zhang et al., 2015; Zhang et al., 2015; Xiong et al., 2019).
Runoff is believed to contribute to microplastic pollution (Gasperi et al., 2014; Dai et al., 2013). Significant correlation between MP particle abundance and mean annual runoff was found for each tributary of the upper Yangtze River. This suggests that increased runoff transports more surface microplastic waste into the rivers, and, as a result, the abundance of MP particles remaining in riverbank sediments may be increased by waves or wind. These particles will, over time, flow into the upper reaches of the Yangtze River and continue to be transported downstream, possibly into the oceanic system, thus becoming an important source of marine MPs.
4.4 Characteristics of microplastics contamination in lakeshores
The sediment MP particle concentrations found in lakeshore samples were high in comparison with some other studies, such as of the Caofeidian reclamation area (Zhou et al., 2016), Halifax Harbor in Nova Scotia, Canada (Mathalon et al., 2014), the Poyang Lake system (Jian et al., 2020), Dongting Lake (Wang et al., 2020) and Taihu Lake (Su et al., 2016).
Particles in all seven shape categories were found in all lakeshore sediment samples. Abundance of MP particles in lakeshore samples was significantly greater than in riverbank sediment samples because particles in rivers are mainly transported downstream, whereas particles in lakes are mainly deposited and thus remain in place. Abundances in the lake outlet samples were significantly less than those found at inlets. This is likely because the lake water outflow rate was low; still water provides good conditions for a large number of MP particles to be trapped in lake and lakeshore sediments (Xiong et al., 2019). Our findings imply that previous models may have overestimated the flux of MPs from land to ocean because they did not take into account the retention of MPs in river and lake systems.